Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

Originally Posted by old_coot88

You indicated about 249V, which is down from the spec'd 280V, showing the tube is dropping about 30V across the 1K resistor (R250). That's .9 watt and the resistor would be running quite hot. If it is, you might want to sub a 2 watt resistor during the troubleshooting period.

I just remeasured those voltages. Today I measure +267.5V at the junction of R249 and R250. I measure +255V at pin 1 of the oscillator tube. R250 seems to be holding up.

The Sams and RCA schematics give somewhat different voltages at various points. In Sams the voltage at junction of R249 and R250 is given as +270V. In RCA it is given at +285V. Both give a nominal line voltage of +117VAC and I set my variac at approximately that level, but it's not a laboratory grade instrument and I'm using a DMM.

The Sams and RCA schematics agree in showing a 5V drop across R250 (either 270-265 or 285-280). Today it looks like I am dropping 12.5V (267.5-255).

I remeasured voltage at pin 2. It is +107V (Sams calls for +85V and RCA calls for +110V).

While I was away, I ordered a couple of new 6AZ8 tubes and tried subbing them out of superstition. No change.

I also installed two very short leads, coming out from pin 6 of the oscillator tube and ground, that I can use to substitute crystals more easily. I swapped all four of my crystals with no change. Still measuring zero volts at pin 6.

I removed the can from T115 (the 3.58 mc CW transformer) to replace the two 1000pf caps, C219 and C220. Now that I look closely, it looks pretty toasty.



The resistance on the primary should measure 6.8 ohms according to the Sams schematic. I measured it at 6.8 ohms. When the caps were off, I measured the other coils; both of them have continuity and they measure at .2 ohms. I just noticed that the Sams parts list has a note stating, "R-Y windings = .2 ohms."

I tried the TV again after those cap replacements and saw no change.

I'm not sure how much faith to put in those resistance measurements. I'd feel a lot better if these coils looked brand spanking new.

Phil Nelson

[Phil Nelson]

I have ordered a replacement for transformer T115. A while back (post #179), I did a test that seemed to show that T115 is at least partly functional, but I suspect that didn't tell the whole story, and those coils look they overheated in the past.

If the new transformer doesn't improve the situation, I'm not sure what else to try, except to replace the tube socket.

I wonder if there's a way to do a bench test of the old transformer after I take it out?

Phil Nelson