Today's Mystery Guest (RCA CTC-4)

[old_coot88]

Dang. That swatch of carbonization looks like it involved a half dozen turns or more. That could sure play hobb with the 'Q' factor as well as alter the inductance.

Re. the sweep test, what you'd wanta do is leave coil and cap in parallel, with the lower end grounded. Then connect both genny and scope to the top (with their cable shields grounded, of course).

A parallel-resonant LC tank (which this is) exhibits highest impedance at resonance, grounding out all other frequencies.
But a series-resonant tank (L and C in series) exhibits lowest impedance at resonance, making it useful as a wave trap to ground out an unwanted signal.

[Phil Nelson]

OK. I visited the local surplus store and found a couple of 12muh inductors in a bin. Later today I'll try the sweep test on old & new parts and see what transpires.

Phil Nelson

[old_coot88]

To get the sharpest blip, you'd probably wanta use a small coupling cap from the gen to the tank. Maybe 10 pf or so.

[Phil Nelson]

Is this what you're talking about -- 270pf cap in parallel with the inductor, and 10pf on the end?



Seems to do nothing around 3.58 mc, but when I went lower, I could get a waveform showing up between about 2.3 to 2.7 mc, peaking in amplitude around 2.55 mc.



Out of curiosity, I made another little circuit using the old L42 and two more caps (10 and 270 pf). The response was basically the same.



So, either the original L42 was good after all, or I still don't have things set up correctly. (Or it could be some evil combination of antique signal generator, middle-aged scope, and antique brain )

Meanwhile, I installed a new inductor in place of the old L42 in the TV and reconnected things to the oscillator tube. No change or improvement. Still seeing 0 volts at pin 6 of the oscillator.

Phil Nelson

[DaveWM]

some obvious stuff here, is the tube socket good (use a test socket to make sure the tube pins are connected to the circuit. Different tube used? correct tube used? just trying to cover the bases.