On 03/07/2025 11:32 AM PST Jim Whartenby via groups.io <old_radio@...> wrote:

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I wonder if the 6K8 is a little gassy?? It seems to me that the plate potential could overwhelm the high impedance of the triode grid circuit if some gas is present.? Is a known good 6K8 available to substitute?

Jim

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

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On Wednesday, March 5, 2025 at 10:31:49 PM CST, Jacques_VE2JFE via groups.io <jacques.f@...> wrote:

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Hi Don.

Any tube having a “floating” control grid, a heated cathode and a positive plate voltage cannot pass any current, because the electron flow will charge the grid highly negative and this will prevent any plate current going thru.

If a hi-impedance voltmeter (say having 10Meg ohms input) is connected between that grid and GND (or cathode), it will show a negative grid voltage.

There is tubes such polarised in some audio circuits: cathode directly connected to GND and a hi value of grid to GND resistor (typically 10Meg).

For that reason, measuring a positive voltage from a “floating” grid respective to GND is not possible.

In the 6K8 case, as the LO is not working and R7 is ± 47k ohms, the grid is practically at the same positive potential than the cathode.

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73, Jacques, VE2JFE in Montreal

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Hi Jacques? Your words seem good to me until “IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all”…

I don’t understand that. ??“could not be measured at all”… ?means zero or negative volts only?? .

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So what is setting the grid volts? As I see it, R7 is the only path from the grid to to B+ or B-minus. ?

I will have to try again to redraw the circuit.. and read some tube theory again.. ???

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From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Wednesday, March 5, 2025 7:47 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice

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Hi Don, simple: the 6K8 is made as a triode + hexode, sharing the same cathode, so the cathode bias resistor (R8) voltage depends on both the triode and hexode sections.

So even if the triode section does not work as intended, there is still some current coming from the hexode…

That being said, when the LO works normally from the triode section, the triode grid voltage have to be negative because the triode works in class C, and the grid current, charging the coupling capacitor (C37) results in a negative MEAN voltage to the grid.

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In the Table of voltages below, the grid is about at the same voltage than the cathode, which is an unmistakable sign that the oscillator is not working.

And as the triode section is drawing all the current it could, this result in lower triode plate voltage and also in a lower screen grid voltage on the hexode part, and etc.

?

IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all…

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73, Jacques, VE2JFE in Montreal

--
don??? va3drl

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Hi Don, simple: the 6K8 is made as a triode + hexode, sharing the same cathode, so the cathode bias resistor (R8) voltage depends on both the triode and hexode sections.

So even if the triode section does not work as intended, there is still some current coming from the hexode…

That being said, when the LO works normally from the triode section, the triode grid voltage have to be negative because the triode works in class C, and the grid current, charging the coupling capacitor (C37) results in a negative MEAN voltage to the grid.

?

In the Table of voltages below, the grid is about at the same voltage than the cathode, which is an unmistakable sign that the oscillator is not working.

And as the triode section is drawing all the current it could, this result in lower triode plate voltage and also in a lower screen grid voltage on the hexode part, and etc.

?

IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all…

?

--
don??? va3drl

On Mar 6, 2025, at 11:53?PM, Scott WA9WFA via groups.io <whitebear1122@...> wrote:

?Hi, I’m looking to buy a Hallicrafters SR-500 Tornado transceiver and PS-500 in good physical and preferably working condition. ?If I can’t find an SR-500, I would consider buying an SR-160/PS-150.

In 1970 a friend borrowed me an SR-160 when I first got my General, and I had a wonderful time working 75, 40, 20 meter SSB with it until I left for military duty after high school graduation. Here I am some 54 years later looking for some of that wonderful nostalgia of the SR-160/SR-500. ?Thanks. ?

73, Scott WA9WFA

On 03/06/2025 11:18 AM EST Anthony W. DePrato via groups.io <wa4jqs@...> wrote:

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Bob:

good luck with the sx 100 you have the next to last run if i recall had so many that i collected over the years it is hard to recall hihi.

but i can say this the sx 100 is a great receiver after i restored mine had a ham drop by to look at another receiver and then talked me out of mine.

73 Tony WA4JQS

On 3/5/2025 11:22 PM, Bob via groups.io wrote:

Tony, thanks for the reply, will remove all of them also. Bob w8exv

On 03/05/2025 3:54 PM EST Anthony W. DePrato via groups.io <wa4jqs@...> wrote:

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NO do NOT leave them in the radio they like to go off like an M80

73 Tony WA4JQS

On 3/5/2025 12:38 PM, Bob via groups.io wrote:

My sx-100mk2a has several black cat caps, woud it be ok to leave these in

or replace them also.

? ? ? thanks in advance Bob w8exv

--
Anthony W.DePrato WA4JQS CQ DX HALL OF FAME #39 DXCC HONOR ROLL CW PHONE RTTY CALLS HELD VP8SSI VP8BZL V31SS 3Y0PI ZS8JQS WA4JQS/KC4 WA4JQS/ZS1

Virus-free.

--
Anthony W.DePrato WA4JQS CQ DX HALL OF FAME #39 DXCC HONOR ROLL CW PHONE RTTY CALLS HELD VP8SSI VP8BZL V31SS 3Y0PI ZS8JQS WA4JQS/KC4 WA4JQS/ZS1

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Hi Don.

Any tube having a “floating” control grid, a heated cathode and a positive plate voltage cannot pass any current, because the electron flow will charge the grid highly negative and this will prevent any plate current going thru.

If a hi-impedance voltmeter (say having 10Meg ohms input) is connected between that grid and GND (or cathode), it will show a negative grid voltage.

There is tubes such polarised in some audio circuits: cathode directly connected to GND and a hi value of grid to GND resistor (typically 10Meg).

For that reason, measuring a positive voltage from a “floating” grid respective to GND is not possible.

In the 6K8 case, as the LO is not working and R7 is ± 47k ohms, the grid is practically at the same positive potential than the cathode.

?

??

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On 03/06/2025 3:36 PM PST don Root <drootofallevil@...> wrote:

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An aside from ongoing topic ??S-20R mixer cct. ?…

While trying to “see” how the LO circuit works? things got me in a big knot.

C38 seems to run from some osc tanks back to the mixer RF tank area.

When I follow that I get to? the common [not ground] of the coils/transformers RF tanks? and found them to be floating = strange to me.

I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS] ?

IN ORDER TO separate lines so I could see I had to shift stuff and here is where I got.?? Maybe I got something wrong so need somebody to check my “re-draughting”

?

--
don??? va3drl

Tom
Depending on your model of GDO you can either use it to generate a
signal and look for a dip in "grid" voltage at resonance as power is
absorbed by the LO tank circuit or you can use it as a "receiver" and
monitor whatever frequency is generated by the LO.? In either case the
GDO coil has to be closely coupled to the oscillator coil under test.

If the LO tuned circuit is good then you should be able to verify that
the LO tank circuit is adsorbing power form the GDO.? If you have not
verified this then I would first suspect that the distance between the
GDO coil and the LO coil is too great so no coupling is taking place.? I
have a GDO but have never used it.? I prefer to use a working receiver
as a tester since it is much more sensitive.

There is also the slight possibility that when you replaced components,
you may have mis-wired something or the dreaded previous owner did the
deed.? This is something that we have all done at one time or another,
especially if we have taken a break halfway through the process.? I
believe Jacques asked if you had double checked the LO circuit wiring
but I don't remember your answer.

An open LO coil is easy to ohm-out but an open tank or coupling
capacitor is a bit more of a problem.? Then there is the issue of a
broken or shorted wire to the tuning capacitor oscillator section or a
faulty band switch segment.? I would closely eyeball things in the LO
before resorting to any more digging into the chassis.? Even verify the
replacement component values.? I have sometimes transposed resistor
stripes so it helps to verify resistors values with the VOM or DMM.
Regards,
Jim

An aside from ongoing topic ??S-20R mixer cct. ?…

While trying to “see” how the LO circuit works? things got me in a big knot.

C38 seems to run from some osc tanks back to the mixer RF tank area.

When I follow that I get to? the common [not ground] of the coils/
transformers RF tanks? and found them to be floating = strange to me.

I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS]

IN ORDER TO separate lines so I could see I had to shift stuff and here
is where I got.?? Maybe I got something wrong so need somebody to check
my “re-draughting”


--
don??? va3drl

On 03/06/2025 3:36 PM PST don Root <drootofallevil@...> wrote:

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An aside from ongoing topic ??S-20R mixer cct. ?…

While trying to “see” how the LO circuit works? things got me in a big knot.

C38 seems to run from some osc tanks back to the mixer RF tank area.

When I follow that I get to? the common [not ground] of the coils/transformers RF tanks? and found them to be floating = strange to me.

I am using Richard’s posted Handbook [ ohh thanks? RICHARD, GOOD SCANS] ?

IN ORDER TO separate lines so I could see I had to shift stuff and here is where I got.?? Maybe I got something wrong so need somebody to check my “re-draughting”

?

--
don??? va3drl

?

Hi Don.

Any tube having a “floating” control grid, a heated cathode and a positive plate voltage cannot pass any current, because the electron flow will charge the grid highly negative and this will prevent any plate current going thru.

If a hi-impedance voltmeter (say having 10Meg ohms input) is connected between that grid and GND (or cathode), it will show a negative grid voltage.

There is tubes such polarised in some audio circuits: cathode directly connected to GND and a hi value of grid to GND resistor (typically 10Meg).

For that reason, measuring a positive voltage from a “floating” grid respective to GND is not possible.

In the 6K8 case, as the LO is not working and R7 is ± 47k ohms, the grid is practically at the same positive potential than the cathode.

?

On 03/05/2025 1:57 PM PST Jim Whartenby via groups.io <old_radio@...> wrote:

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Jacques & Tom

Not mentioned so far was the use of another working radio to see if the oscillator is alive.? The oscillator frequency should be either above or below the received frequency spaced by the IF.? It should be relatively easy to wrap a few turns of insulated wire around the 6K8GT and connect the far end to the antenna terminals of the radio being used as test equipment.? In the BCB, one could set the radio used as test equipment to 1000kc and tune the radio under test for any signs of LO life.

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As for the dequeuing resistor value, I have used as low a value as 100 ohms with success.? Back in the day when I was testing cell phone power amplifiers ICs, that resistor value would be good enough to isolate the DVM from the watt or so of RF.? 1k sounds like a decent value to try first, go higher if you like.? ?Wrap one lead around the DVM probe and cut the other end of the resistor short enough to contact the tube socket terminals.? 1k should have no noticeable affect on DVM accuracy.

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It would be interesting to know just how far the DVM impedance will force the LO to go off frequency.? Not sure if it would be the result of DVM itself or just because the lead is long and "functions" as an antenna and loads the LO?? If Tom and you are right and the DVM lead completely kills the LO, then that would be worth knowing too.

Regards,

Jim

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

?

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On Wednesday, March 5, 2025 at 01:44:41 PM CST, Jacques_VE2JFE via groups.io <jacques.f@...> wrote:

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Hi Jim, I agree with all you wrote, but apparently, Don doubted that the DVM connection to the LO of this S-20R could “kill” the oscillator.

I experienced that before: and this is why a 1Meg resistor was added in the Heathkit VTVM probes.

But there is more the probe shielded cable capacitance which can make trouble when the RF circuits were DC probed without that “isolating” resistor.

An similar kind of trick I use with my bench DMM is a probe with a 10Meg 1% resistor inserted within.

The displayed DC voltage is then half of the true value, but the very high impedance circuits such probed are -almost- not harmed.

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For the S-20R problem now: it was not a probing “incident” as all the DC values around the 6K8 triode section are off.

The oscillator is obviously not working, because when it does, the grid voltage have to be negative, as it operates in class C.

As I previously mentioned, something in the coupling capacitors is open, or as suggested by Richard, short in the variable capacitor LO section.

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73, Jacques, VE2JFE in Montreal

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Hi Don, simple: the 6K8 is made as a triode + hexode, sharing the same cathode, so the cathode bias resistor (R8) voltage depends on both the triode and hexode sections.

So even if the triode section does not work as intended, there is still some current coming from the hexode…

That being said, when the LO works normally from the triode section, the triode grid voltage have to be negative because the triode works in class C, and the grid current, charging the coupling capacitor (C37) results in a negative MEAN voltage to the grid.

?

In the Table of voltages below, the grid is about at the same voltage than the cathode, which is an unmistakable sign that the oscillator is not working.

And as the triode section is drawing all the current it could, this result in lower triode plate voltage and also in a lower screen grid voltage on the hexode part, and etc.

?

IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all…

?

--
don??? va3drl

On 03/05/2025 3:54 PM EST Anthony W. DePrato via groups.io <wa4jqs@...> wrote:

?

?

NO do NOT leave them in the radio they like to go off like an M80

73 Tony WA4JQS

On 3/5/2025 12:38 PM, Bob via groups.io wrote:

My sx-100mk2a has several black cat caps, woud it be ok to leave these in

or replace them also.

? ? ? thanks in advance Bob w8exv

--
Anthony W.DePrato WA4JQS CQ DX HALL OF FAME #39 DXCC HONOR ROLL CW PHONE RTTY CALLS HELD VP8SSI VP8BZL V31SS 3Y0PI ZS8JQS WA4JQS/KC4 WA4JQS/ZS1

Virus-free.

?

Hi Don, simple: the 6K8 is made as a triode + hexode, sharing the same cathode, so the cathode bias resistor (R8) voltage depends on both the triode and hexode sections.

So even if the triode section does not work as intended, there is still some current coming from the hexode…

That being said, when the LO works normally from the triode section, the triode grid voltage have to be negative because the triode works in class C, and the grid current, charging the coupling capacitor (C37) results in a negative MEAN voltage to the grid.

?

In the Table of voltages below, the grid is about at the same voltage than the cathode, which is an unmistakable sign that the oscillator is not working.

And as the triode section is drawing all the current it could, this result in lower triode plate voltage and also in a lower screen grid voltage on the hexode part, and etc.

?

IF the R7 were open, a positive grid voltage (quite close to the cathode one) could not be measured at all…

?