S-20R mixer cct.
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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 |
开云体育Hi Tom, The triode section of the 6K8 is effectively not oscillating, and that explain all the voltage readings you got. There should be an open coil somewhere, or an open capacitor in the oscillator path. Are all the bands defective (aka not working) ? If all the coils windings are OK when checked with an ohmmeter, either C33 or C37 (or both !) can be “open”. ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de Allthumbs via groups.io ? 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 |
开云体育Hello Tom Around the HF, especially the LO, I wonder if you can trust a DMM having only wires {no isolation probe } at some pins. It never did work while you have had it? ? Assume, you tried another tube ? ?and since you did not say..it ?must be dead on all bands? What all has been changed {or not] in that area… ?? De-Oxit on socket ? Do you have other test equipment?
-- don??? va3drl |
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Thank you, Jacques. The problem is on all bands. I have replaced all the resistors and capacitors in this set except for a large mica cap buried under the oscillator coil assembly. I was hoping not to have to lift that assembly.
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Tom
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Thank you, Don. No, the set was not operating when I acquired it last year. It is dead on all bands. Over the winter I through it and replaced all resistors and capacitors, upgraded the power cord to 3-conductor, and replaced the speaker with a PM type plus a choke. I have a spare, 4" electrodynamic type of speaker like the original but have not yet tried it. Yes, I tried cleaning the 6K8 socket pins with De-Oxit, and I tried a second tube. If that was the only problem, I don't think the voltages on the tube socket would still be "off."
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Tom
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开云体育Richard ?do your DVM’s have an isolation probe for capacitance decoupling back to the DOT? ??? Resistance of DVMs can be? very high, but ?test wires have capacitance that will ?mess with tuned circuits as the LO.? Halli sometimes indicated which pins can be measured with only a VTVM , but the S-20R on Bama has no chart and the schematic I see is mostly not readable. ? _ -- don??? va3drl |
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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 |
开云体育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 ? 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 |
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Thank you, Jim and Jacques. Frankly, I'm not concerned about DMM inaccuracies and their probable or possible causes. The voltage readings I reported are proof enough that something is wrong, especially the positive voltage on the triode grid. I didn't report doing so but I had also sniffed the LO cct with my trusty old GDO and it registered nothing.
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I guess I better face the music and lift the band switch unit and all its coils, at least enough to better expose the 6K8 socket. Not really looking forward to doing that for fear of not reconnecting everything correctly. But there's a big old mica capacitor thoroughly buried in the 6K8 circuitry that I had better check. And try to see where the apparent lack of continuity in the LO circuitry is.
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This S-20R is in pretty decent cosmetic condition and so I consider it worth the repair. Time to take a deep breath and dive in....
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Tom
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开云体育Tom, I guess that this “big old mica capacitor” is C7, which decouple the 6K8 cathode to GND, in parallel with C8 (0.05?F, 200V). IF it is a REAL mica cap (not the “postage stamp” PIO ones), it should still be good. And as such, not my prime suspect for a non-working LO. Because you wrote that you replaced all the PIO capacitors, maybe it is worth to carefully check all your work, just to be sure that they are all connected to the right places, especially in the LO section (C33 and C37) ! ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de Allthumbs via groups.io ? Thank you, Jim and Jacques. Frankly, I'm not concerned about DMM inaccuracies and their probable or possible causes. The voltage readings I reported are proof enough that something is wrong, especially the positive voltage on the triode grid. I didn't report doing so but I had also sniffed the LO cct with my trusty old GDO and it registered nothing. ? I guess I better face the music and lift the band switch unit and all its coils, at least enough to better expose the 6K8 socket. Not really looking forward to doing that for fear of not reconnecting everything correctly. But there's a big old mica capacitor thoroughly buried in the 6K8 circuitry that I had better check. And try to see where the apparent lack of continuity in the LO circuitry is. ? This S-20R is in pretty decent cosmetic condition and so I consider it worth the repair. Time to take a deep breath and dive in.... ? Tom
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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. 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 |
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I mentioned in a previous post that I sniffed the LO with a GDO. The LO is not working.
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Tom
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开云体育Jacques +++ Yes? that’s what I tried to say a bit too generally. It is probably only the grid[S] that cant reliably take the capacitance of a “wire probe” or whatever the term is, and mostly at higher frequencies, So it PROBABLY is a rather divergent consideration right now.. after the GDO test looking at the 6K8….. Hi?? cathode current, ?and hi Po current? … all due to high grid-O volts ?? …how does grid get so High …is ?that bias resistor R7 open?? little plate [p] current? ? ? [added ] Function??????? S???????? ???H?????????????? P??????????? GS??????????? Go??????????? Po??????????? H????????????? K????????????? G ?? remarks?????????? ????????? --????????????? --???????????? HI? ??????????LO?????????? V HI? ????????????LO????????? --???????????? ?HI ? ref ??from ??????? ?I prefer to try to understand what’s going right vs wrong before ?pulling things,, since I am an expert at making bad things go worse. From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Wednesday, March 5, 2025 2:45 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice ? 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 ?
-- don??? va3drl |
开云体育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… ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de don Root ? Jacques +++ Yes? that’s what I tried to say a bit too generally. It is probably only the grid[S] that cant reliably take the capacitance of a “wire probe” or whatever the term is, and mostly at higher frequencies, So it PROBABLY is a rather divergent consideration right now.. after the GDO test looking at the 6K8….. Hi?? cathode current, ?and hi Po current? … all due to high grid-O volts ?? …how does grid get so High …is ?that bias resistor R7 open?? little plate [p] current? ? ? [added ] Function??????? S???????? ???H?????????????? P??????????? GS??????????? Go??????????? Po??????????? H????????????? K????????????? G ?? remarks?????????? ????????? --????????????? --???????????? HI? ??????????LO?????????? V HI? ????????????LO????????? --???????????? ?HI ? ref ??from ??????? ?I prefer to try to understand what’s going right vs wrong before ?pulling things,, since I am an expert at making bad things go worse. ? From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Wednesday, March 5, 2025 2:45 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice ? 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 ? ?
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开云体育
This might help.
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1-4-5-1, RF BLOCKING PROBE
The inexpensive DVM’s and VOM’s work fine unless you are trying to measure a dc voltage with RF present, like the plate, grid or cathode of an oscillator or mixer. It is simple to make an RF blocking probe for an inexpensive meter. Install a 270uh to 1mh
choke in the barrel of a dc probe. It will work with oscillators and low power mixers. Don’t go messing about in the PA of a transmitter with one.
Walt Cates, WD0GOF
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开云体育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?? . ? 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.. ??? ? 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 ? 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… ? 73, Jacques, VE2JFE in Montreal -- don??? va3drl |
开云体育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. ? 73, Jacques, VE2JFE in Montreal ? 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?? . ? 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.. ??? ? 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 ? 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… ? 73, Jacques, VE2JFE in Montreal
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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? Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Wednesday, March 5, 2025 at 04:03:26 PM CST, Allthumbs via groups.io <btse1@...> wrote:
I mentioned in a previous post that I sniffed the LO with a GDO. The LO is not working.
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Tom
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Allthumbs
Jacques_VE2JFE
don Root
Jim Whartenby
waltcates