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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!
¿ªÔÆÌåÓýAnd the Band Spread variable cap C5A is in minimum position (plates fully unmeshed) all this time ? ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de Emanuele Girlando via groups.io ? Sorry: L3 has no core inside, as per schematic. About C7 I am reconsidering the idea: after all when at the top of the bands it is fully open.. so the dirt can interact only minimally... ? -- Emanuele (IU1KNR). |
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Re: SX-28A Hum
¿ªÔÆÌåÓýJacques that¡¯s not fair, you are doing your own arithmetic.? And much of what you say must be right, but nobody mentioned what C42 is doing. In my quick re evaluation after peaking at the manual, without the choke and its cap the output is not what I would call normal/ flat, instead C-42 is pulling down the response[plate AC volts] as frequency goes up. I suppose that can be base boost, but usually base boost does not mean ??kill the mids and really kin the highs. I will leave it to you to propose exactly how the response works when the out works, I¡¯m gonna cheat and look in the book. I had to draw stuff, maybe this will help some others doodle around.. ? Image End image From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 3:31 PM To: [email protected] Subject: Re: [HallicraftersRadios] SX-28A Hum Importance: High ? 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 -- don??? va3drl |
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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!
Sorry: L3 has no core inside, as per schematic.
About C7 I am reconsidering the idea: after all when at the top of the bands it is fully open.. so the dirt can interact only minimally...
?
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Emanuele (IU1KNR). |
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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!
¿ªÔÆÌåÓýDoes 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 ? On Sun, Feb 16, 2025 at 10:11 PM, Emanuele Girlando wrote:
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). |
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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!
¿ªÔÆÌåÓýEmanuele, 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 ? 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:
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? Another alternative: C4 is faulty not allowing the due frequency span, 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). |
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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!
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..
?
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Emanuele (IU1KNR). |
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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!
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:
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). |
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Re: SX-28A Hum
¿ªÔÆÌåÓýDon, 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 ? Richard ,agree again Re ¡°I am pretty sure the markings on the schematic are reversed.???? ??Agree again ¡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
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Re: SX-28A Hum
¿ªÔÆÌåÓýHi 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 ? Yes, hum is present ¨C at the same level ¨C 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 |
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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!
¿ªÔÆÌåÓýJacques,? 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 |
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Re: SX-28A Hum
¿ªÔÆÌåÓýRichard ,agree again Re ¡°I am pretty sure the markings on the schematic are reversed.???? ??Agree again ¡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 -- don??? va3drl |
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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 ¨C at the same level ¨C 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 ? 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 ¨C 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 ? 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 ? 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 ?
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Re: SX-28A Hum
¿ªÔÆÌåÓýRichard, 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 -- don??? va3drl |
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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 ¨C 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 ? 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 ? 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 ?
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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!
¿ªÔÆÌåÓýHi 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 |
Jacques_VE2JFE
don Root
Emanuele Girlando
Jim Whartenby