Testing a CTC-11 horizontal hold control
 

Have an issue with hot plating on the horizontal output tube of a CTC-11, and I'm pretty sure that I've traced it to a bad horizontal hold control.

This is described as a sine wave coil. I assume to test it properly the set has to be set up on a scope, but is there any way to test it for an open while out of circuit?

Not really familiar with the chassis, but looking at the SAMS, to ADJUST it properly requires a scope and adjusting the H osc simultaneously to keep sync.

Out of the circuit, you can check for open with an ohmmeter, but if the problem is a shorted turn instead, that would be hard to tell.

It looks to me like it's the only DC path to the H osc plate. If that is correct, and it's open, the H osc would die completely. Is that what you are seeing with hot-plating? It also means you could check for an open while in-circuit (with power off, of course [adding this comment in case someone less knowledgeable reads this in future]).

Sure. If your schematic lists a resistance for the coil you can check that is in that ball park with the set off.

It may be possible that the adjustments are far enough off that it may not be oscillating.

BTW you should not need to have the output tube plugged in while testing the horizontal oscillator circuit with the set on. All the output does for the osc. is provide a feedback that the phase /AFC system uses to pull the frequency/phase of the osc. into lock, so without that feedback it should free run (which some sam's H adjustment procedures have you force it to do anyway).

Right....the reason that it's hot plating is that I'm missing the -60V on the 6DQ5. Caps have been replaced and I'm halfway through checking the resistors but I'm really thinking that it's the horizontal hold control. Low hours set that's been moved a LOT in its life.

With the 6DQ5 removed*, check for plate voltage (pin 6) of the H osc tube. If it's missing, trace back component-by-component toward the 380V source.

*Even with the plate cap disconnected, the 6DQ5's screen grid (G2) will get cooked when there's no drive on G1 (the drive signal is what develops the negative bias voltage on G1).

Only thing I'm missing is the -60V (the RCA manual says -57...close enough) on pins 1 and 5. I have zero.

Just to confirm.. you do have 240V on the plate (pin 6) of the 6CG7, right?

Not sure. Since I've had the chassis on the bench I've just worked off tube resistances. I'll check it tonight and report back.

Have you verified that the ground stakes are good? These sets are known to break the solder between the PCB and the stake pins on the chassis.

First thing I did was check all the grounds :)

I'm not used to working with chassis out of the cabinet. Is this something that I can do with the 6DQ5 pulled and the chassis on the bench, or should I just jig it back up in the cabinet?

You should be able to bench it (assuming if there is a yoke plug B+ interlock that it does not break osc. B+).....B+ voltage may be off a bit with it out of the cabinet, but then again having the H. output not working correctly will affect the B+ anyway (and is the main factor in changing the B+ out of cabinet on a working chassis) so it should not matter...

As I understand it, the -60 is developed when a sawtooth of sufficient p-p amplitude hits the 6DQ5 grid. This is called contact bias. If the oscillator is way off frequency, the amplitude of the sawtooth drops sufficiently to make the grid LESS negative. Low bias will cause HOT to conduct and plate to glow in the center, no bias lights up the plate as you are experiencing, both kill HV. If not using an oscilloscope, use the peak-peak scale on a VTVM set to AC.

Go from the hold control coil (either terminal) all the way through the wave-shaping network of caps and resistors to pin 1 of the 6DQ5, which is not plugged in for this test.
If 270 volts P-P is not found at the hold coil, the oscillator is not running. If something close is found, see where it gets stopped or attenuated.

OK one more time.:) Yes/no, is there plate voltage on pin 6 of the 6CG7? Sams calls for 240V. Is it, or any fraction of it, present on pin 6?

With the variac at about 80V and the 6DQ5 pulled, I was showing ~270