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:

?

?

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:

?

?

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.

?

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.

?

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

?

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

?

?

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

?

?

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.

?

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.

?

73, Jacques, VE2JFE in Montreal

?

?

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…

?

?

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.

?

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.

?

?

--
don??? va3drl

?

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.

?

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.

?

?

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

?

?

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.

?

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.

?

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

?

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

?

?

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

?

?

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.

?

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.

?

73, Jacques, VE2JFE in Montreal

?

On 03/05/2025 11:09 AM PST Jim Whartenby via groups.io <old_radio@...> wrote:

?

?

Tom

Jacques is right on the money, with a positive triode grid voltage on pin 5 of the 6K8, just about all of the other voltage readings will be off except for perhaps the heater voltage.? It is worth noting that the 117 volt convention for the power transformer primary voltage is actually an average expected voltage.? The tolerance at a specified 117 line volts is about +/- 7.5%.? For 115 line volts convention, the tolerance is approximately +/- 10% and the modern convention for a 120 volt line voltage tolerance is +/- 5%.? In all of the above conventions, the maximum line voltage is about 126 volts.? The only voltage that has actually changed over all these years is the lower voltage limit.

?

The ohms-per-volt loading of a circuit by a VOM is essentially how much current is required to deflect the meter pointer to full scale deflection.? The 1kohm-per-volt rating implies that the meter movement is 1mA since 1 volt / 1kohm is 1mA.? Early VOMs were inexpensive so they used less sensitive meter movements.? In like manner, 20kohms-per-volt has a meter movement that requires 50 microamps for a full scale deflection.? I have seen VOMs with 50kohm-per-volt ratings.

?

Most VOMs and some VTVMs have an accuracy of +/- 3% at full scale deflection.? This implies that the meter accuracy is worse (actually twice the FSD) when the meter pointer is below mid range.? At the same time, the ohms-per-volt loading of the VOM on the circuit under test is dependent on the range multiplier.? For instance, the Simpson 260 has a 20kohms-per-volt rating so on the 250 volt range, the shunt resistance of the meter on the circuit is 5 megohms.??

?

This is sufficiently close to the reported shunt impedance of a VTVM or DVM of approximately 10 megohms.? This shunt impedance is independent of the range switch and is only dependent on the voltage amplifier in the VTVM and the range multiplier circuit or the method of conversion used in the DVM.? Some VTVMs have even higher loading resistances such as the Keithley 601 which is 100 trillion ohms with an accuracy of 1%.? Truly amazing for a mechanical meter movement.

?

Getting back to the line voltage measurement, if the 117 volt primary sees 120 volts, the error is ((120v - 117v) / 117v) =? +/- 2.6%.? Since the VOM or VTVM has a +/- 3% full scale error, the combined tolerance is ((3^2) + (2.6^2)) ^1/2 = +/- 4%.? When you add in the line voltage tolerance of +/- 7.5% the measurement error is increased to +/- 8.5%.??

?

When the increased error caused by less than half scale deflection of a meter movement is added to the mix, the reading error can exceed +/- 10%.? So when comparing the manufacturers voltage measurement with your meter reading, if you are below a 10% disagreement then you are golden.??

?

The remaining issue is that hallicrafters didn't include an expected voltage reading at the tube sockets in the S-20R manual which is at the heart of this problem.? In this case, the tube manual will give you reasonable voltage levels for the tube in question. But remember, the tube manual ratings are bogey numbers that are, in fact, more average values.

Regards,

Jim

?

?

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

?

?

On Tuesday, March 4, 2025 at 02:41:02 PM CST, Allthumbs via groups.io <btse1@...> wrote:

?

?

Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty—the LO is not oscillating.

?

I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I’m posting the readings in the chart below. FYI—I took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.

?

?

If it helps, here's the mixer section:

If you’re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I’m posting will point to a solution that would mean I won’t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)

?

Any thoughts? Thank you,

?

Tom

On 03/05/2025 11:09 AM PST Jim Whartenby via groups.io <old_radio@...> wrote:

?

?

Tom

Jacques is right on the money, with a positive triode grid voltage on pin 5 of the 6K8, just about all of the other voltage readings will be off except for perhaps the heater voltage.? It is worth noting that the 117 volt convention for the power transformer primary voltage is actually an average expected voltage.? The tolerance at a specified 117 line volts is about +/- 7.5%.? For 115 line volts convention, the tolerance is approximately +/- 10% and the modern convention for a 120 volt line voltage tolerance is +/- 5%.? In all of the above conventions, the maximum line voltage is about 126 volts.? The only voltage that has actually changed over all these years is the lower voltage limit.

?

The ohms-per-volt loading of a circuit by a VOM is essentially how much current is required to deflect the meter pointer to full scale deflection.? The 1kohm-per-volt rating implies that the meter movement is 1mA since 1 volt / 1kohm is 1mA.? Early VOMs were inexpensive so they used less sensitive meter movements.? In like manner, 20kohms-per-volt has a meter movement that requires 50 microamps for a full scale deflection.? I have seen VOMs with 50kohm-per-volt ratings.

?

Most VOMs and some VTVMs have an accuracy of +/- 3% at full scale deflection.? This implies that the meter accuracy is worse (actually twice the FSD) when the meter pointer is below mid range.? At the same time, the ohms-per-volt loading of the VOM on the circuit under test is dependent on the range multiplier.? For instance, the Simpson 260 has a 20kohms-per-volt rating so on the 250 volt range, the shunt resistance of the meter on the circuit is 5 megohms.??

?

This is sufficiently close to the reported shunt impedance of a VTVM or DVM of approximately 10 megohms.? This shunt impedance is independent of the range switch and is only dependent on the voltage amplifier in the VTVM and the range multiplier circuit or the method of conversion used in the DVM.? Some VTVMs have even higher loading resistances such as the Keithley 601 which is 100 trillion ohms with an accuracy of 1%.? Truly amazing for a mechanical meter movement.

?

Getting back to the line voltage measurement, if the 117 volt primary sees 120 volts, the error is ((120v - 117v) / 117v) =? +/- 2.6%.? Since the VOM or VTVM has a +/- 3% full scale error, the combined tolerance is ((3^2) + (2.6^2)) ^1/2 = +/- 4%.? When you add in the line voltage tolerance of +/- 7.5% the measurement error is increased to +/- 8.5%.??

?

When the increased error caused by less than half scale deflection of a meter movement is added to the mix, the reading error can exceed +/- 10%.? So when comparing the manufacturers voltage measurement with your meter reading, if you are below a 10% disagreement then you are golden.??

?

The remaining issue is that hallicrafters didn't include an expected voltage reading at the tube sockets in the S-20R manual which is at the heart of this problem.? In this case, the tube manual will give you reasonable voltage levels for the tube in question. But remember, the tube manual ratings are bogey numbers that are, in fact, more average values.

Regards,

Jim

?

?

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

?

?

On Tuesday, March 4, 2025 at 02:41:02 PM CST, Allthumbs via groups.io <btse1@...> wrote:

?

?

Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty—the LO is not oscillating.

?

I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I’m posting the readings in the chart below. FYI—I took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.

?

?

If it helps, here's the mixer section:

If you’re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I’m posting will point to a solution that would mean I won’t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)

?

Any thoughts? Thank you,

?

Tom