Why Does My Varactor Diode Test Circuit Behave Differently on a Breadboard?

[piepermd]

Sorry, I was getting the trimmers confused with the tuning caps.
Replace 216 for my 218.All old VT electronics smell a bit when first powered after a break. Dust on the heated components such as valves and resistors. Gasses from oils or electrolytes used in paper and electrolytic capacitors.

It is possible that removal of a VT resulted in higher voltages across a capacitor.

You should repeat the experiment and see if it happens again. If it does, try to find the faulty

It should be OK.
If the +HT supply rises before those VT filaments warm, then they will appear to be open circuits for a while anyhow.

OK, I’ll give it another go.

 

[piepermd]

It was OK with the tubes removed but no test equipment, but when I set up for the tests it acted strangely when bringing up the Variac. Usually when I switch on the Variac that feeds the radio power supply the beginning voltage is about 0.4 volts and it increases very rapidly as I turn the dial. With the test equipment it starts at about .13 volts and increases slowly. I think the problem might be that the computer that USB- powers the Arduino unit with the Si5351 oscillator is on a different outlet. My radio power supply is connected to earth ground by the transformer center tap, and so is the radio chassis. When I disconnect the Arduino output from the LO coil the voltage behavior goes back to normal. I’m not sure if these two outlets are on a different breaker or if that matters. The HP signal generator is plugged into the same outlet as the power supply and causes no sagging of the voltage as I bring up the Variac.

 

[piepermd]

The HP unit provides the RF, and the Arduino the LO.

 

[Tom.G]

...the computer that USB- powers the Arduino unit with the Si5351 oscillator is on a different outlet. My radio power supply is connected to earth ground by the transformer center tap, and so is the radio chassis.

That description is a bit unclear, but the earth ground by the transformer center tap is suspicious.

With the radio and test equipment powered on but NOT connected to each other, measure the voltage between them; both between chassis and between signal lines of the two.

The measurement between chassis should show Zero volts.
If any voltage is measured, use an outlet strip to plug everything into the same outlet. Then repeat the measurement between chassis.

A) If there is still a voltage difference, something is either miswired internally or has failed. Fix it.

B) If the voltage between pieces has now disappeared, there is a problem with the wiring to one or both outlets. Try re-connecting the test equipment with the radio and power supply. If all is well, continue with your tests.

Cheers,
Tom