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Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 
Edited

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Does the rotor plates are positioned in the exact center of the stator ones, or not ??

Please take a picture !

?

73, Jacques, VE2JFE in Montreal

?

De?: [email protected] <[email protected]> De la part de Emanuele Girlando via groups.io
贰苍惫辞测é?: 16 février 2025 16:29
??: [email protected]
Objet?: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

?

On Sun, Feb 16, 2025 at 10:11 PM, Emanuele Girlando wrote:

May be grease deposited on the plates over time (at eye I don't see anything)?

Sorry, that is not true. Looking carefully I can see a thin deposit on the plates:

The outer part of the plate looks cleaner as I passed my finger on it..

?

--

Emanuele (IU1KNR).
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 
Edited

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Emanuele,

Fantastic pictures, but I do not believe that the C7A tuning cap can be at fault.

Obviously the coil have too much inductance, so it’s core have to be moved OUT of the coil (to the top, or to the bottom, I do not know which one will be best).

Normally, the trimmer cap should be set at it’s center setting, then the core of the coil have to be adjusted to obtain a peak at the low frequency specified in the alignment instructions, then the response should be peaked with the trimmer at the high frequency specified in the alignment instructions.

Then go back to the low frequency with the core, then to the high frequency with the trimmer, and doing this several times until no improvement is measured.

?

But Ah… I understand: no alignment instructions in the manual for the CORES of the coils, like if nobody had touched those since the day of the manufacture !!!

AARGH !

?

Find a plastic driver and MOVE those cores while observing what happen with your Spectrum Analyser, PLEASE !

BC band: low side at 600kHz (L3 core ??), High side at 1400 kHz (C4 trimmer), then loop again.

?

Is it just me, or the two lower band inductors are connected in reverse (one for each other) in the S85 schematic ???

?

?

73, Jacques, VE2JFE in Montreal

?

De?: [email protected] <[email protected]> De la part de Emanuele Girlando via groups.io
贰苍惫辞测é?: 16 février 2025 16:12
??: [email protected]
Objet?: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

?

I think it's time to narrow? the investigation down and tackle one problem at a time.

As the "v1 / V2 coupling" and the "not C62" problems are really intriguing but seem loosely related to the "trimcaps don't peak" initial problem, I would put them temporarily a part.

In order to reduce complexity, I would also remove from the table the fact that the problem occurs in both the antenna circuits and the mixer circuits.

I would start the analysis again considering the antenna circuit only.

There will be no shortage of surprises though.

?

Since the circuit is difficult to understand, and in fact it seems it shouldn't even work as it is, let's temporarily consider it as a black box and examine its operation as if it were a pure unknown DUT.

The setup is as follow:

The receiver is turned OFF. Band switch on band 1.

BS dial fully CW (-> C5-A fully open that is at minimum capacity.

Tracking generator output set at -40dBm.

Absolute vertical readings are only qualitative measurements.

Relative vertical readings can make some sense.

START 500KHz / STOP 1800KHz - SPAN=1300KHz - 130KHZ/DIV.

?

Here is what I get:?

Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity)

The peak is a little below 500KHz (in fact not visible) - C4 has no visible effects.

Turning the main dial toward higher frequencies, I see the peak moving right.

At the end:

Band 1: main tune at 1.6MHz - the top of band 1? (C7A fully open - minimum capacity) - C4 fully close (max capacitance)

I expected to see the peak to move up to 1.6MHz. It doesn't happen! It stops a 960KHz (!!) without even reaching the alignment frequency of 1400KHz.

?

Band 1: main tune at 1.6MHz - top of band 1 (C7A fully open - minimum capacity) - C4 fully open (min capacitance)

So, turning C4 fully open the peak moved right by about 180KHz reaching 1140KHz, still far away from 1.6MHz (and even below 1400KHz!).

At 1400KHz (the alignment frequency) I cannot never ever have a peak.

?

Considerations:

  1. the peak, while correctly starting from the bottom of the band, never ever reaches the top, even when the mail dial is at 1.6MHz; it doesn't even reach the alignment frequency of 1400KHz. This is the problem.
  2. C4 (that I confirm is part no. 44-191) moves the peak left and right of about 180KHz; it would be good if the peak would be within a 180KHz span from 1400MHz but it is not.
  3. Given this data C4 will never ever "peak" at 1400KHz as required by the alignment procedure.

If the peak went up to 1.6MHz, then I would be able to get the peak around 1400KHz as per service procedure and carefully looking for it tweaking C4.

Another possibility is: the span C4 provides is not sufficient. It should be in the order of 450KHz, too much in my opinion.

?

Changing figures, the same happens on band 2, 3 and 4 (the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band.

?

?

So the question is: why does the peak stop at 1140KHz without ever reaching the top of the band?
In my opinion the only possible reasons are: "C7-A defective: too high capacitance" or "a coil is seriously defective".

Another alternative: C4 is faulty not allowing the due frequency span,
This is why I started thinking I have to disassemble the ANT unit and check the wiring and all the components one by one.

C7 is my preferred suspect. If for some reason it has too much capacitance, this would affect all bands the way we see.

?

Thoughts?

How can I check C7 without destroying it?

May be grease deposited on the plates over time (at eye I don't see anything)?

May be it needs cleaning? can I use IPA? Oil? gasoline?

--

Emanuele (IU1KNR).
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 
Edited

On Sun, Feb 16, 2025 at 10:11 PM, Emanuele Girlando wrote:
May be grease deposited on the plates over time (at eye I don't see anything)?
Sorry, that is not true. Looking carefully I can see a thin deposit on the plates:
The outer part of the plate looks cleaner as I passed my finger on it..
?
--
Emanuele (IU1KNR).
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 
Edited

I think it's time to narrow? the investigation down and tackle one problem at a time.
As the "v1 / V2 coupling" and the "not C62" problems are really intriguing but seem loosely related to the "trimcaps don't peak" initial problem, I would put them temporarily a part.
In order to reduce complexity, I would also remove from the table the fact that the problem occurs in both the antenna circuits and the mixer circuits.
I would start the analysis again considering the antenna circuit only.
There will be no shortage of surprises though.
?
Since the circuit is difficult to understand, and in fact it seems it shouldn't even work as it is, let's temporarily consider it as a black box and examine its operation as if it were a pure unknown DUT.
The setup is as follow:
The receiver is turned OFF. Band switch on band 1.
BS dial fully CW (-> C5-A fully open that is at minimum capacity.
Tracking generator output set at -40dBm.
Absolute vertical readings are only qualitative measurements.
Relative vertical readings can make some sense.
START 500KHz / STOP 1800KHz - SPAN=1300KHz - 130KHZ/DIV.
?
Here is what I get:?
Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity)
The peak is a little below 500KHz (in fact not visible) - C4 has no visible effects.
Turning the main dial toward higher frequencies, I see the peak moving right.
At the end:
Band 1: main tune at 1.6MHz - the top of band 1? (C7A fully open - minimum capacity) - C4 fully close (max capacitance)
I expected to see the peak to move up to 1.6MHz. It doesn't happen! It stops a 960KHz (!!) without even reaching the alignment frequency of 1400KHz.
?
Band 1: main tune at 1.6MHz - top of band 1 (C7A fully open - minimum capacity) - C4 fully open (min capacitance)
So, turning C4 fully open the peak moved right by about 180KHz reaching 1140KHz, still far away from 1.6MHz (and even below 1400KHz!).
At 1400KHz (the alignment frequency) I cannot never ever have a peak.
?
Considerations:
  1. the peak, while correctly starting from the bottom of the band, never ever reaches the top, even when the mail dial is at 1.6MHz; it doesn't even reach the alignment frequency of 1400KHz. This is the problem.
  2. C4 (that I confirm is part no. 44-191) moves the peak left and right of about 180KHz; it would be good if the peak would be within a 180KHz span from 1400MHz but it is not.
  3. Given this data C4 will never ever "peak" at 1400KHz as required by the alignment procedure.
If the peak went up to 1.6MHz, then I would be able to get the peak around 1400KHz as per service procedure and carefully looking for it tweaking C4.
Another possibility is: the span C4 provides is not sufficient. It should be in the order of 450KHz, too much in my opinion.
?
Changing figures, the same happens on band 2, 3 and 4 (the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band.
?
?
So the question is: why does the peak stop at 1140KHz without ever reaching the top of the band?
In my opinion the only possible reasons are: "C7-A defective: too high capacitance" or "a coil is seriously defective".
Another alternative: C4 is faulty not allowing the due frequency span,
This is why I started thinking I have to disassemble the ANT unit and check the wiring and all the components one by one.
C7 is my preferred suspect. If for some reason it has too much capacitance, this would affect all bands the way we see.
?
Thoughts?
How can I check C7 without destroying it?
May be grease deposited on the plates over time (at eye I don't see anything)?
May be it needs cleaning? can I use IPA? Oil? gasoline?
--
Emanuele (IU1KNR).
Re: SX-28A Hum

 
Edited

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Don, Richard:

I do not agree.

CH2 is listed as a 4Hy choke in the parts list, and the C43 across is a 5100pF (0.0051?F).

Both form a parallel resonant circuit centered at 1114 Hz.

So when they are both “in circuit” loading the plate of V12, a maximum stage gain will be developed at this frequency and almost nothing way below or way over that.

Not sure HOW that can be considered a “bass boost”.

For me, the maximum bass response is developed when the V12 plate is loaded only with the R37 resistor.

On the IN position of the switch (according to the schematic).

?

73, Jacques, VE2JFE in Montreal

?

De?: [email protected] <[email protected]> De la part de don Root
贰苍惫辞测é?: 16 février 2025 14:36
??: [email protected]
Objet?: Re: [HallicraftersRadios] SX-28A Hum

?

Richard ,agree again

Re “I am pretty sure the markings on the schematic are reversed.???? ??Agree again
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+.”??? perhaps if the choke resistance is high?

…in the ?Halli “OUT” position, at mid frequencies, won’t the choke resistance determine the gain? ?

?

After a gazillion hours, here is ?how Don sees the base boost switch. ?.. comments?

Image here

End image and all

From: [email protected] [mailto:[email protected]] On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 5:31 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

The B+ goes to the plate of the first audio through R-37, when the
Bass switch is in the IN position (as shown on the schematic) the choke
is bypassed, the B+ to the plate through R-37, when the switch is in the
OUT position, as shown on the diagram R-37 is bypassed and the B+ now
from R-38 goes to the choke and thence to the plate. So, in one position
the plate load is R-37 and in the other position its the choke and
condenser. I am pretty sure the markings on the schematic are reversed.
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+. If I am seeing it right either R-37 or the resonant
choke is put out of the circuit by the Bass switch. It is 2:30AM and I
have no business being on line. ?


--
don??? va3drl

Re: SX-28A Hum

 
Edited

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Hi again Tom,

Maybe some will find the following approach brutal, but here is what I will try with that SX-28A.

Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis.

Power-up the set again.

If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s).

?

BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced.

Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within.

OR there is shorted turns in the winding of the primary on one side.

OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other.

?

I just hope that this makes sense…

?

?

?

?

?

73, Jacques, VE2JFE in Montreal

?

De?: [email protected] <[email protected]> De la part de thoyer via groups.io
贰苍惫辞测é?: 16 février 2025 08:37
??: [email protected]
Objet?: Re: [HallicraftersRadios] SX-28A Hum

?

Yes, hum is present – at the same level – regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot.

?

CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner.

?

Tom
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 

开云体育

Jacques,? maybe look at “scope” sweeps in here and some in just after posts I think.

I didn’t get far, and misinterpreted scope face info.?? I have to copy then and add my own marks or I will get things wrong, so I did not get thru them, and ?I / we somehow went down another road [logging road] and got stuck, and it has rabbit holes too. ?

In case you are looking, I think all images in this thread are pasted in, [not attachments]

I’m going to look at tank sweeps again . or whatever they are called these days.

/g/HallicraftersRadios/message/31577

From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Saturday, February 15, 2025 10:50 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

?

Hi Don,

I followed this case more or less since Emanuele posted it, so be kind if I misunderstood anything about.

I remember that Emanuele complained about a complete loss of sensitivity in the lower part of the Bands 1 and 2: is it right ?

Does somebody already asked him to check the amplitude of the Local Oscillator at the low end of those two bands ?

My doubt is: this seems to be an alignment problem, but what will be the effect of a “dying” LO on the low end of the BC band, for example ?

That can happen if the Q of the oscillator coils is lower than they should be, or if the 6SA7 is end-of-life (not enough gain).

?


--
don??? va3drl
Re: SX-28A Hum

 
Edited

开云体育

Richard ,agree again

Re “I am pretty sure the markings on the schematic are reversed.???? ??Agree again
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+.”??? perhaps if the choke resistance is high?

…in the ?Halli “OUT” position, at mid frequencies, won’t the choke resistance determine the gain? ?

?

After a gazillion hours, here is ?how Don sees the base boost switch. ?.. comments?

Image here

End image and all

From: [email protected] [mailto:[email protected]] On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 5:31 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

The B+ goes to the plate of the first audio through R-37, when the
Bass switch is in the IN position (as shown on the schematic) the choke
is bypassed, the B+ to the plate through R-37, when the switch is in the
OUT position, as shown on the diagram R-37 is bypassed and the B+ now
from R-38 goes to the choke and thence to the plate. So, in one position
the plate load is R-37 and in the other position its the choke and
condenser. I am pretty sure the markings on the schematic are reversed.
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+. If I am seeing it right either R-37 or the resonant
choke is put out of the circuit by the Bass switch. It is 2:30AM and I
have no business being on line. ?


--
don??? va3drl
Re: SX-28A Hum

 

Tom
OK, does R35 have any affect on the hum?

Can you scope the terminals of CH2 when the Bass switch is in both positions?? You most likely will have to do either a differential measurement using channel B minus channel A because of the B+ or you can disconnect one end of R37 to remove B+ from CH2.? You might also adjust R35 to see if it has any affect on the o'scope display.? Testing so far seems to indicate that CH2 is the source of the hum in the audio amplifier.??

Have you reset the mounting screws for both CH1, CH2 and the power transformer?? Somewhere there is a path for magnetic flux to affect the audio amplifier.? 120 cycle hum seems to indicate that the source is after the rectifier but the only center tap secondary winding in the power transformer is in the high voltage winding so I also suspect the power transformer.

The ripple you see on the plates of the 6V6 appear to be what is expected.? 7 volts of ripple amounts to 2.5% of the B+ which indicates adequate power supply filtering so the filter cap C49 is doing it's job.
Sticky wicket!
Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy


On Sunday, February 16, 2025 at 07:36:57 AM CST, thoyer via groups.io <thoyer1@...> wrote:


Yes, hum is present – at the same level – regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot.

?

CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner.

?

Tom

?

From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, February 16, 2025 3:26 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Tom

When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level?

?

The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking?

?

Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux.

Regards,

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

?

?

On Saturday, February 15, 2025 at 07:54:33 PM CST, thoyer via groups.io <thoyer1@...> wrote:

?

?

I unscrewed CH2 from the chassis and let it float, no change.

?

From: HallicraftersRadios@groupsio <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Ok, tried two different 6V6’s, no change.

?

On the plates of the 6V6’s there is 6.8Vrms of 120hz ripple. If I remove the 6v6’s it drops to 4.7Vrms and the hum is no longer audible (as expected).

?

With the 6V6’s back in (Russian tubes BTW), switching the bass “IN”, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple.

?

These measurements are made with my o’scope.

?

I did some poking through my “inventory” of parts and do not have a suitable choke to swap in for CH2 – still thinking there may be some leakage going on there……?

?

Fun stuff huh?

?

Tom

W3TA

?

From: [email protected] <[email protected]> On Behalf Of don Root
Sent: Saturday, February 15, 2025 6:19 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Hi Jacques, I just received 1.5 cents from you.

Your proposal may well be, but how do you explain the difference ?the switch setting makes?

From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Saturday, February 15, 2025 5:57 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

My two cents:

If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise.

Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause.

?

73, Jacques, VE2JFE in Montreal ?


--
don??? va3drl

Re: SX-28A Hum

 

开云体育

Yes, hum is present – at the same level – regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot.

?

CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner.

?

Tom

?

From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, February 16, 2025 3:26 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Tom

When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level?

?

The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking?

?

Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux.

Regards,

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

?

?

On Saturday, February 15, 2025 at 07:54:33 PM CST, thoyer via groups.io <thoyer1@...> wrote:

?

?

I unscrewed CH2 from the chassis and let it float, no change.

?

From: HallicraftersRadios@groupsio <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Ok, tried two different 6V6’s, no change.

?

On the plates of the 6V6’s there is 6.8Vrms of 120hz ripple. If I remove the 6v6’s it drops to 4.7Vrms and the hum is no longer audible (as expected).

?

With the 6V6’s back in (Russian tubes BTW), switching the bass “IN”, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple.

?

These measurements are made with my o’scope.

?

I did some poking through my “inventory” of parts and do not have a suitable choke to swap in for CH2 – still thinking there may be some leakage going on there……?

?

Fun stuff huh?

?

Tom

W3TA

?

From: [email protected] <[email protected]> On Behalf Of don Root
Sent: Saturday, February 15, 2025 6:19 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Hi Jacques, I just received 1.5 cents from you.

Your proposal may well be, but how do you explain the difference ?the switch setting makes?

From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Saturday, February 15, 2025 5:57 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

My two cents:

If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise.

Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause.

?

73, Jacques, VE2JFE in Montreal ?


--
don??? va3drl

Re: SX-28A Hum

 

开云体育

6SC7, sorry.

?

Thanks for the input, yes more poking is needed. Hopefully later tody.

?

Tom

?

From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 12:08 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Do you mean the 6SA7 or 6SC7? The 6SC7 is the dual triode that acts
as the first audio and phase splitter. Pulling that leaves the 6V6's
active. CH-2, the bass boost resonator, connects from the grid of V-14.
one of the output tubes, and one plate of the 6SC7, the side that's used
for the audio pre-amp. Just lift one side of the choke. I don't think
its the choke. See what the Bass switch is actually doing. According to
the diagram its shorting the choke when in the IN position, Since it is
resonating at the grid I think it should be the other way. In any case
disconnecting it should prove one way or the other. If it turns out its
boosting the bass in the OUT position it would explain why the hum gets
greater, simply because the gain goes way up, but not the source of the
hum. Also, does the volume control have any effect on the hum? If so
what effect? Also note the phase splitter grid comes from the junction
of the two 6V6 grid resistors at R-39. At this point it is getting the
unbalanced audio from the output of the two halves of the 6SC7 and
generates a balancing signal. This is the out of phase signal that dries
the other 6V6. It is also what puts the effect of the tone control and
bass boost on both sides. I have forgotten the name of this type of
phase splitter but its very common. Also, look at C-44 for ripple. There
should be very little. If the boost circuit works as I think any ripple
or even a heater to cathode leak in the 6SC7 will be magnified by the
resonant boost in the grid of the first audio and in the phase splitter.
A couple of minutes with the scope should tell you. BTW, did you
change the 6SC7, if its got a leaky heater it could be the cause of the
hum. More poking is needed.

On 2/15/2025 2:33 PM, thoyer via groups.io wrote:

“When it comes to the switch area, that schematic is spinning my head”

Glad it is not just me…… ?

I have not had time to get back to this, maybe tonight.

I believe it is in the 6V6 area because when I pull the 6SA7 the symptom
remains. Maybe the choke has some leakage to ground? I was going to pull
it last night and check it on my Sencore inductance tester. Got side
tracked and didn’t get there. I’ll try tonight.

Tom

W3TA

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

Re: SX-28A Hum

 

The B+ goes to the plate of the first audio through R-37, when the
Bass switch is in the IN position (as shown on the schematic) the choke
is bypassed, the B+ to the plate through R-37, when the switch is in the
OUT position, as shown on the diagram R-37 is bypassed and the B+ now
from R-38 goes to the choke and thence to the plate. So, in one position
the plate load is R-37 and in the other position its the choke and
condenser. I am pretty sure the markings on the schematic are reversed.
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+. If I am seeing it right either R-37 or the resonant
choke is put out of the circuit by the Bass switch. It is 2:30AM and I
have no business being on line.


On 2/15/2025 7:15 PM, don Root wrote:
It seems that the way to look at this switch is that in one position the
plate load is provided by the choke while in the other the load is
switched to R37, and as an aside the choke gets shorted ? Any votes on
this? ?just for fun try the thumbs ?voting machine, I won’t look.

**


--
don??? va3drl
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
Re: SX-28A Hum

 

开云体育

Richard, we agree about the switch position labels on the schematic being reversed.

?

From: [email protected] [mailto:[email protected]] On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 5:00 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

I am curious about that switch. I would interpret IN as meaning the
bass was boosted but the switch shows it the other way. Also the
frequency response chart shows the bass boosted with switch "IN". +++++++++


--
don??? va3drl
Re: SX-28A Hum

 

I am curious about that switch. I would interpret IN as meaning the
bass was boosted but the switch shows it the other way. Also the
frequency response chart shows the bass boosted with switch "IN".
The TONE control is a simple RC high end roll off so should not
affect the hum. I have at least two SX-28 handbooks, one the military
handbook, all are the same.


On 2/15/2025 6:10 PM, thoyer via groups.io wrote:
Richard,

When the bass switch is “in” it boosts the bass. Going by the schematic
it shows that it shorts the choke / cap. Audibly it definitely boosts
the bass, I need to confirm the wiring of the switch vs the schematic.
Wouldn’t be the first error I’ve found.

Yes, the tone adj and the bass boost are independent.

You suggested shorting the choke – but isn’t that effectively what the
switch is doing?

And I agree, I had most of this radio torn apart so it is a good
possibility there is a miswire somewhere in that circuit – wouldn’t be
the first time………..

Thanks,

Tom

W3TA

*From:*[email protected] <[email protected]> *On
Behalf Of *Richard Knoppow via groups.io
*Sent:* Saturday, February 15, 2025 7:15 PM
*To:* [email protected]
*Subject:* Re: [HallicraftersRadios] SX-28A Hum

I have not read all the posts on this thread so may be repeating
something. Tom, I do not have an SX-28, In normal operation what does
the bass switch do? Does the bass increase or decrease when the switch
is in the IN position? The handbook is confusing. The schematic suggests
the BASS switch boosts the bass by adding a resonant choke on the output
of the first audio amplifier. The schematic shows the switch cutting off
the choke in ON, that looks backward to me. The additional TONE control
is just a conventional high roll off. It appears that both of these tone
controls are independent, is that correct.
It seems to me the bass boost should INCREASE hum.
The plate transformer of the output stage connects to the input of
the B+ filter. This is not raw AC since there is a fairly large cap
across it. The balanced circuit should remove any residual hum. The
rest of the amplifier is fed by filtered B+ and seems unlikely to have
significant hum on it. Try shorting out the tone control choke, if its
getting induced hum that should eliminate it.
Since you practically rebuilt the receiver perhaps some wiring
error was made or some new part is actually bad. Worth going over again.
It is always frustrating to me to try to trouble shoot problems
where I can't just jump in an make measurements.

On 2/13/2025 4:21 PM, thoyer via groups.io wrote:

I just finished pretty much a complete overhaul of a nice condition
(physically) SX-28A. Replaced just about every resistor and all caps.

Radio is working well except for a 120hz hum. When I switch the Bass
in,
the hum goes away.

Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that
is there independent of the bass switch position. The output side of
the
HV filter is clean, no ripple. I paralleled another 47uf 450v cap
across
the choke input cap and there was no difference.

I double checked the wiring against the schematic along with component
values. Is the schematic correct in this area? I found once schematic
error maybe there is another?

Thoughts?

Tom

W3TA

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
Re: SX-28A Hum

 

Tom
When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level?

The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking?

Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy


On Saturday, February 15, 2025 at 07:54:33 PM CST, thoyer via groups.io <thoyer1@...> wrote:


I unscrewed CH2 from the chassis and let it float, no change.

?

From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Ok, tried two different 6V6’s, no change.

?

On the plates of the 6V6’s there is 6.8Vrms of 120hz ripple. If I remove the 6v6’s it drops to 4.7Vrms and the hum is no longer audible (as expected).

?

With the 6V6’s back in (Russian tubes BTW), switching the bass “IN”, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple.

?

These measurements are made with my o’scope.

?

I did some poking through my “inventory” of parts and do not have a suitable choke to swap in for CH2 – still thinking there may be some leakage going on there……?

?

Fun stuff huh?

?

Tom

W3TA

?

From: [email protected] <[email protected]> On Behalf Of don Root
Sent: Saturday, February 15, 2025 6:19 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

Hi Jacques, I just received 1.5 cents from you.

Your proposal may well be, but how do you explain the difference ?the switch setting makes?

From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Saturday, February 15, 2025 5:57 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum

?

My two cents:

If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise.

Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause.

?

73, Jacques, VE2JFE in Montreal ?


--
don??? va3drl

Re: SX-28A Hum

 

Do you mean the 6SA7 or 6SC7? The 6SC7 is the dual triode that acts
as the first audio and phase splitter. Pulling that leaves the 6V6's
active. CH-2, the bass boost resonator, connects from the grid of V-14.
one of the output tubes, and one plate of the 6SC7, the side that's used
for the audio pre-amp. Just lift one side of the choke. I don't think
its the choke. See what the Bass switch is actually doing. According to
the diagram its shorting the choke when in the IN position, Since it is
resonating at the grid I think it should be the other way. In any case
disconnecting it should prove one way or the other. If it turns out its
boosting the bass in the OUT position it would explain why the hum gets
greater, simply because the gain goes way up, but not the source of the
hum. Also, does the volume control have any effect on the hum? If so
what effect? Also note the phase splitter grid comes from the junction
of the two 6V6 grid resistors at R-39. At this point it is getting the
unbalanced audio from the output of the two halves of the 6SC7 and
generates a balancing signal. This is the out of phase signal that dries
the other 6V6. It is also what puts the effect of the tone control and
bass boost on both sides. I have forgotten the name of this type of
phase splitter but its very common. Also, look at C-44 for ripple. There
should be very little. If the boost circuit works as I think any ripple
or even a heater to cathode leak in the 6SC7 will be magnified by the
resonant boost in the grid of the first audio and in the phase splitter.
A couple of minutes with the scope should tell you. BTW, did you
change the 6SC7, if its got a leaky heater it could be the cause of the
hum. More poking is needed.


On 2/15/2025 2:33 PM, thoyer via groups.io wrote:
“When it comes to the switch area, that schematic is spinning my head”

Glad it is not just me…… ?

I have not had time to get back to this, maybe tonight.

I believe it is in the 6V6 area because when I pull the 6SA7 the symptom
remains. Maybe the choke has some leakage to ground? I was going to pull
it last night and check it on my Sencore inductance tester. Got side
tracked and didn’t get there. I’ll try tonight.

Tom

W3TA
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 

开云体育

Hi Jacques, last reply tonight

Ok Ill try. I guess I must be getting cranky!

?

I followed this case more or less since Emanuele posted it, so be kind if I misunderstood anything about.

I remember that Emanuele complained about a complete loss of sensitivity in the lower part of the Bands 1 and 2: is it right ? ???yes

Does somebody already asked him to check the amplitude of the Local Oscillator at the low end of those two bands ??? ??don’t think so

My doubt is: this seems to be an alignment problem, yes ?but what will be the effect of a “dying” LO on the low end of the BC band, for example ? I don’t know

That can happen if the Q of the oscillator coils is lower than they should be, or if the 6SA7 is end-of-life (not enough gain).

?

FYI He said the LO aligned well on all bands, the RF and mixer tanks align and work well on band 3,4 ??but the RF and mixer tanks ?of 1,2 don’t align apparent too much mmFd at minimum.?? He has an impedance sweeper of some sort, and they show resonance too low, say 1200 kc not 1400 but don’t quote me.?

I was trying to get him ?to sweep at 600kc dial position, because if that if way off no little trimmer will help. Im sure he did a lot before posting and knows a lot.

I felt that it is a common mode problem ?all four tanks, but ?if so it has not been found. ???If the LO was weak, wouldn’t it show up first on band 4? … a question.

But your though may well be IT.? I don’t know how he did the sweeps but I would think they would be independent of the LO, mystery continues< I quit for now. Please fix my spellink ??I’m too tired.

?

From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Saturday, February 15, 2025 10:50 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

?

Hi Don,

I followed this case more or less since Emanuele posted it, so be kind if I misunderstood anything about.

++++++++++++++++


--
don??? va3drl
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 

开云体育

Bob, Some of us are not bean counters, but want to know now things work and don’t work, and give them some love too.

You can go back to Stephah Gray in about 1750 who found out electric fluid moves fast and far .. and early telegraphs ?ideas arose and later Cook, and Morse ones and Mr. Baudot’s codes and stuff, ?transatlantic cables.. they didn’t quit. remember Marconi; he didn’t quit.

If we were in business there would be ?huge piles behind our houses and we would be calling Chinese amazon every day for a new dog to throw into the pile.

I would never make a bench Technician, ?but I had to startup new steel mills, with germanium transistors and big SCRs and where the dogs had to be tamed and trained out in minus 40 weather…not a couchy bench job.

Some of us just don’t fit into todays business society….Missfits.

Bob, we have a bit of snow but if you are in Toledo, your gonna need a big scale to handle all the fluff Mississippi is sending up. The radar has all kinds of colours showing up. We are ducking up here. Good luck

From: [email protected] [mailto:[email protected]] On Behalf Of Bob Dostall via groups.io
Sent: Saturday, February 15, 2025 9:52 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

?

I feel that the guy that had the original post many moons ago was correct. Get rid of it. The job of any good bench technician is to be able to recognize a dog and move on, not beat a $40 radio to death.

Bob Dostall

Senior Technician

Vasu Communications ?


--
don??? va3drl
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 

开云体育

Hi Don,

I followed this case more or less since Emanuele posted it, so be kind if I misunderstood anything about.

I remember that Emanuele complained about a complete loss of sensitivity in the lower part of the Bands 1 and 2: is it right ?

Does somebody already asked him to check the amplitude of the Local Oscillator at the low end of those two bands ?

My doubt is: this seems to be an alignment problem, but what will be the effect of a “dying” LO on the low end of the BC band, for example ?

That can happen if the Q of the oscillator coils is lower than they should be, or if the 6SA7 is end-of-life (not enough gain).

?

73, Jacques, VE2JFE in Montreal

?

?

Maynard , I’m gonna mess-up your fine writing! .. Sorry.


If there is stray coupling transferring signals at the top end of each
band, we might expect that it would see lower loss at the higher
frequencies, yes ?and that is what is happening, it seems. I’m on the fence ?And even though
there is signal flow at the high end of each band, it isn't very robust
if I recall the earlier messages correctly. I wonder, ?Emanuele was focused on alignment, and said the bottom end was very bad , and perhaps the top was useable, all as I recall, but can’t be accurate.

And we need to keep in mind that there may be two problems here and not
confuse them. Well Emanuele raised one problem, somebody else started into the other. ??

?

Richard is thinking about the circuitry between the
antenna terminals and V1, the RF amp and quite appropriately so. Yes but that only matters is you add an external wire to A2 and remove the link ??

?I think that Hallicrafters changed the S-40A circuitry to eliminate one
coil in that part of the S-40B and it makes the circuitry in the
schematic look as if it is miswired but I don't think it is. Well Maynard, my speculation is that the 40A was not operating so well, possibly because the transformers had too much Q, but the various antennas swung the ?tunning off course, so they flattened the resonance .. all speculation because they kept mucking with band 1 and 2. ?We could start a conspiracy case!

To save that one coil, I think they gave up balanced inputs on Bands 1
and 2, but didn't revise the instructions in the manual to indicate that. Well, they don’t want to admit a problem

The other problem is with the tuned circuits between V1 and V2 and I
think that's where Emanuele's problem is.? It is a mystery to me so far, all I know is from what he said; all of the 4 tanks appear to need less capacitance when aligning, and the lower half of the bands are useless[my word]. ?Many many things have been checked and checked, yet nothing makes sense. BTW I watch “mayday” mostly about planes crashing, and long investigations to try to dig out the cause{s}. they try everything. ??

Thanks, Jim T., for the very nice pictures. Those are helpful and of
really good quality. Yes , very good of Jim ?to do all that work

Some thoughts:

1. I don't see any evidence of C62 or a gimmick in Jim T.'s receiver.
If it is working well without that coupling then my thoughts about this
are probably incorrect;? ?we will see

2. The S-85 and S-40B schematics seem to be identical in the circuitry
between V1 and V2. One of them must be incorrect for these various
circuits to work properly;? except for C62???

3. Someone might own and use an S-85 and, if they don't pay much
attention to the lower ends of Bands 1 and 2, they might have the same
difficulty as does Emanuele without observing it.? hard to believe,? but sometimes we buy stuff that works badly and are so disillusioned that it goes to the attic out of sight and some picker finds it 70 years later, and passes it around and around till it comes out here.


?


--
don??? va3drl

Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!

 

Oops! In my note number 2 I should have said "... except for C62."
Thanks for catching that, Don.

73,

Maynard
W6PAP


On 2/15/25 18:42, don Root wrote:
Maynard , I’m gonna mess-up your fine writing! .. Sorry.


If there is stray coupling transferring signals at the top end of each
band, we might expect that it would see lower loss at the higher
frequencies, *yes***?and that is what is happening, it seems.*I’m on the
fence *?And even though
there is signal flow at the high end of each band, it isn't very robust
if I recall the earlier messages correctly.*I wonder, ?Emanuele was
focused on alignment, and said the bottom end was very bad , and perhaps
the top was useable, all as I recall, but can’t be accurate. *

And we need to keep in mind that there may be two problems here and not
confuse them. Well Emanuele raised one problem, somebody else started
into the other.

Richard is thinking about the circuitry between the
antenna terminals and V1, the RF amp and quite appropriately so. Yes but
that only matters is you add an external wire to A2 and remove the link

?I think that Hallicrafters changed the S-40A circuitry to eliminate one
coil in that part of the S-40B and it makes the circuitry in the
schematic look as if it is miswired but I don't think it is. Well
Maynard, my speculation is that the 40A was not operating so well,
possibly because the transformers had too much Q, but the various
antennas swung the ?tunning off course, so they flattened the resonance
.. all speculation because they kept mucking with band 1 and 2. ?We
could start a conspiracy case!

To save that one coil, I think they gave up balanced inputs on Bands 1
and 2, but didn't revise the instructions in the manual to indicate
that. Well, they don’t want to admit a problem

The other problem is with the tuned circuits between V1 and V2 and I
think that's where Emanuele's problem is. It is a mystery to me so far,
all I know is from what he said; all of the 4 tanks appear to need less
capacitance when aligning, and the lower half of the bands are
useless[my word]. ?Many many things have been checked and checked, yet
nothing makes sense. BTW I watch “mayday” mostly about planes crashing,
and long investigations to try to dig out the cause{s}. they try
everything.

Thanks, Jim T., for the very nice pictures. Those are helpful and of
really good quality*. **Yes , very good of Jim ?to do all that work*

Some thoughts:

1. I don't see any evidence of C62 or a gimmick in Jim T.'s receiver.
If it is working well without that coupling then my thoughts about this
are probably incorrect; we will see

2. The S-85 and S-40B schematics seem to be identical in the circuitry
between V1 and V2. One of them must be incorrect for these various
circuits to work properly; except for C62???

3. Someone might own and use an S-85 and, if they don't pay much
attention to the lower ends of Bands 1 and 2, they might have the same
difficulty as does Emanuele without observing it. *hard to believe,? but
sometimes we buy stuff that works badly and are so disillusioned that it
goes to the attic out of sight and some picker finds it 70 years later,
and passes it around and around till it comes out **here.

*


--
don??? va3drl
521 - 540 of 32064