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Re: S-20R mixer cct.

 

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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