TV-37 vertical hum?

[miniman82]

Went through all ground points in DC and AC looking for drops, couldn’t find anything. Tried 3 times to attach a scope shot, but apparently the forum changed what it deemed acceptable and now I can’t figure it out.

[timmy]

Originally Posted by miniman82

Went through all ground points in DC and AC looking for drops, couldn’t find anything. Tried 3 times to attach a scope shot, but apparently the forum changed what it deemed acceptable and now I can’t figure it out.

What about a whisker heater cathode short in the crt. Put a diode across the 25z6

[miniman82]

Swapped CRT with a 3KP1, same result. I went as far as powering it on its own pure DC regulated supply, still did it. I feel like either the negative supply simply has too much ripple (unlikely given that another site showed exactly the same power supply conditions and his works fine), or that the vertical amplifier is being upset by the ripple when it shouldn’t be.

[Tom Albrecht]

Originally Posted by miniman82

Went through all ground points in DC and AC looking for drops, couldn’t find anything. Tried 3 times to attach a scope shot, but apparently the forum changed what it deemed acceptable and now I can’t figure it out.

You may have to forgive me for beating a dead horse, but I’m not sure that looking for voltage drops is the best way to find a bad ground, unless you are able to see mV level voltages reliably. If it’s an aluminum chassis, I no longer bother to try to measure anything — I just wire all the grounds together with fairly substantial wire, since aluminum chassis can be so problematic for grounding. But if you want to measure something first, I’d check resistances to chassis, looking for anything over 1 ohm. Even mV level signals on a cathode can end up getting amplified to problematic levels.

Another specific place I recall seeing a bad ground in a TV-37 is the tuner, where some coils are grounded to chassis (or was it the band switch?). Caused one of the two bands not to work well. In this case, hum wasn’t the problem; RF performance was. Bad grounds on aluminum chassis can create all kinds of problems.

If all your grounds are under 1 ohm, then you can move on to looking for other causes.