Testing a CTC-11 horizontal hold control

[Kamakiri]

Originally Posted by DavGoodlin

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.

I've got the 270 at the hold coil. There's only so many parts left here that it could be

[Electronic M]

Originally Posted by Kamakiri

I've got the 270 at the hold coil. There's only so many parts left here that it could be

Is that 270 AC like the post you quoted is looking for? If so try to follow it with a scope towards the output grid.

[old_coot88]

To tell if it's AC or not, put a cap (any random value .001 - .01 mf) for DC isolation in series with the test lead, and put the meter on AC.

[Kamakiri]

The 270 was DC.....measured with my meter on the hold coil.

Now, here's something interesting. DavGoodlin was kind enough to show me how to get the sawtooth on my scope, got nothing. But it was also the first time I'd tested it under full line voltage....I'd been running it on a variac at about 85V. Just me being cautious.

Since I got nothing, I went back to test the voltage at the same point on the horizontal hold coil with the set plugged in at full line voltage. It started out at about 280 VDC, and over the course of 30 seconds dropped off and ended up leveling off at 65 VDC. . No AC voltage shown.

I didn't have time before work this morning to re-check pin 6 of the 6CG7, but I will later.

[miniman82]

Sagging there indicates the second plate on your 6CG7 is being dragged down, in other words the tube is being turned on hard which lowers plate voltage. You have no waveform, the oscillator is not running. I would expect grid voltage on the same triode section to be high, confirming the stuck 'on' diagnosis. You need to have a close look at the components around that tube; drifted resistors, leaking caps, bad grounds, the works. Synchroguide circuits are fiendishly simple, which paradoxically makes them difficult to troubleshoot at times because there is so little to go wrong.

[old_coot88]

Have you verified with an ohmmeter check that L32 (the H.osc plate coil) has continuity to all three legs? there should be 25 ohms between pins 1 and 3, and 5.5 ohms between pins 2 and 3.

The 25 ohm section is good apparently, since you're getting full B+ to the plate of the 6CG7. But if the 5.5 ohm section were open, it would kill the oscillator completely.

[Kamakiri]

The 25 ohm I've got. The other sections I've got 48 and 60 ohm, not 5.5......

[old_coot88]

On blowing up the schematic for a closer look, that 5.5 is actually 55 ohm. My mistake. At least the coil has continuity.

[Kamakiri]

Roger that, thanks

On to the resistors....

[DavGoodlin]

Originally Posted by Kamakiri

Roger that, thanks

On to the resistors....

Hope that is closer to the trouble since you replaced the caps. Try varying the coils just to see if its off so far it quit.

[Kamakiri]

Made it about 3/4 of the way through the resistors....so far so good. Board grounds are fine.

Part of what makes this repair difficult for me is that I have to set aside a lot of the methodology I use to fix early b/w sets. It's been my experience with color sets that trying to component test at random just doesn't work the way it does with early black and white, which is what I'm used to.

For example, if you have a vertical sync problem with an early b/w, just start testing and replacing resistors in that whole section, because chances are that even if none are wildly off, most of them will *be* off by enough that it will destabilize the whole circuit. So far I haven't found a single resistor that's made it out of its 10% tolerance.

[Kamakiri]

Originally Posted by DavGoodlin

Try varying the coils just to see if its off so far it quit.

How?

That's kind of what I'm thinking is going on....that over time techs have used the "golden screwdriver" technique to compensate for weak components, and now I've got something that's fresh and totally out of whack.

Nick was spot on in his earlier post. There's not a lot that *can* go wrong here, which constantly leaves me thinking I'm barking up the wrong tree.

[DavGoodlin]

These RCA's can be undone by stupid stuff, most heat-related. I just got done with a CTC16 where a wire run on top of PW700 next to a 6GU7 tube socket pin, was arcing only under full AC power and smoking the cathode resistor. Visually, the resistor was the prime suspect until seeing the little flashing arc under the socket. The resulting potpourri of reeked of classic RCA fly arcing and made the set's owner think his NEW flyback just burned up.

So all looks well on that oscillator PW, even the tube and socket? Its just not normal for an synchroguide oscillator to NOT run. The coils could not have been adjusted out so far as to shut it down I suppose.

Maybe Im grabbing at straws here but see what is external to the circuit, causing issues DC voltages are coming *TO* the stakes. on the osc PWB. Start with resistance at D, to see if maybe a shorted AFC diode (which are over on the vert PWB) should be very high like 780 K ohm.

H-275 vdc, A -386 vdc, F-6.3 vac, D - zero dc and 40v p-p ac, G and E both close to 260 vdc. If any cap gets leaky like C604, C605 and C606 its curtains.

If I was not out of VK memory, Id post the schematic for clarity.

[Electronic M]

Originally Posted by DavGoodlin

If I was not out of VK memory, Id post the schematic for clarity.

Perhaps it is time to get a free image hosting account on a site like Flickr or Photobucket.

How are the traces? I've seen open and or crappy traces on that era of board...If they look bad or voltages are not getting places I tend to reflow/add new solder to the traces.

[kvflyer]

FYI, I have a Photobucket account. It is now rubbish, full of delays, popups, begging for money etc. Many times I just cannot even use it.

Just sayn' (but I guess I get what I am payn' for...)