Today's Mystery Guest (RCA CTC-4)

[old_coot88]

Originally Posted by Phil Nelson

...I hope even more that the flyback hasn't failed. Besides checking components around the oscillator, I'll see what I can learn by testing resistances on the flyback.

Don't you have a B&K Analyst (or other source of horz grid drive)? If so, you could lift the 'upstream' end of C115 and inject there, just to verify everything downstream of that point is kopusetic.

[Phil Nelson]

Yes, I have a B&K Analyst 1077B that can produce a horizontal grid drive signal.

I assume you mean injecting that signal with the HOT and damper tubes back in place. I will keep a close eye on things and shut down immediately at any sign of trouble.

Phil Nelson

[Phil Nelson]

Here is the TV Analyst's horizontal grid drive signal. Very stable.



I disconnected C115 at its junction with R138/R139 and injected the signal there. With tubes back in place, the screen grid voltage at pin 1-8 dropped from 350V+ to around 125V (160V expected) and the HOT did not red-plate. The voltage at grid pin 4-5 was -75V instead of the expected -38V.

Out of curiosity, I reconnected C115 and tried the set without the injected drive signal. The HOT red-plated and I shut down immediately.

Which suggests something fishy upstream of the HOT, I guess.

This was done on the workbench. I suppose I could set the chassis behind the cabinet and connect to the CRT with this setup, if it's important to look at the screen and judge whether the flyback is still alive. I don't have extension cables for the yoke/convergence/CRT, but the original cables reach if I put the chassis on a big box. It's just hard on the back to crouch down on the floor to measure things

Phil Nelson

[old_coot88]

I think you can safely presume the fly is OK and the problem lies entirely in the osc. area. I would concentrate on getting the osc running right as the first order of bizness.

[Phil Nelson]

I will check things around the oscillator tube. The tubular caps and resistors were eliminated long ago. That leaves a handful of mica caps + the various coils.

Phil Nelson