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

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

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.

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.

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. :headscrat . 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.

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.

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.

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

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

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.

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.

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.

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.

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.

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...)

My flickr account is bit flaky - I log on several different PC's. At work it acts one way, at home another, the laptop still another, and on the phone its very hard to deal with.

Flickr its great for posting numerous pictures of a restore and makes sense when you have lots of photos, instead of one-off posts like this, where it is kludgy to use.

Flickr acts the same on all my devices....VK's upload/image size limits prevent me from uploading %99.8 percent of the pictures I have, and a hosting site has proved easier for me to implement/use than resizing my images and hosting them on VK.



It is a bit less clunky if you run adblock plus in your browser (I still get popups occasionally). My photobucket account is effectively dead to me....Once I (rapidly) ran out of storage capacity and they started prompting me to buy more, I switched to flickr and have not looked back....Well I do occasionally go there to post an old photo still on the bucket.

Call me late to the party, but Dad's (incomplete) Sams for the CTC11 is marked up pretty good. There's this (not in Dad's handwriting) - HOT cherries, check Osc tube filament for 6V. Was half. No corrective action listed.

And plenty about contrast, swapping guns to check CRT, and other non-germane stuff.

PF Reporter also had an instance...

http://www.americanradiohistory.com/...rch=%226dq5%22

Me, no experience earlier than a CTC12, so no real help beyond that note..

Gotta wonder at this point if either of the windings might have shorted turn(s), dragging down the Q enough to keep the oscillator from starting.

All the horizontal circuits were mostly the same, even a 16 is easy to troubleshoot with a CTC-9 schematic if you know what to look for.

Ed stopped by. I think we nailed it. He said that the horizontal diodes are leaky. Since they're mounted on the vertical board, I didn't even think of it.....but I had a feeling that it was a garbage in/garbage out situation.

He said to replace them with germanium diodes. Any suggestions?

I've seen standard 1N4007s work fine as horizontal AFC diodes...I'd try those first if you've got em.

Just curious :), while the AFC dual diode can definitely cause loss of H sync, how would it being 'leaky' prevent the osc from running?

Not tryin' to be a smart arse, just thinking it could be a fault condition I never ran into before.

The actual AFC diodes are selenium according to SAMS. I don't know what the effect would be if you used a different type of diode. Most notably would be the forward junction voltage: germanium - 0.3V; silicon - 0.7V; selenium - 0.95V. I suspect that leaky AFC diodes might cause the first stage of the 6CG7 to drive the oscillator way off frequency to the point of shutting down the oscillator.

Of all the AFC diodes I replaced, the only problem I observed was lousy AFC operation, never an oscillator shutdown.

Not sure. You're totally correct in what you're saying, because my experience is the same. We were triaging this and my 5....he had the 11 chassis on the bench.

Honestly not quite sure, but it's a direction to go in.

I spied this on ebay, so I said what the heck, let's do this instead:

http://www.ebay.com/itm/262744695681...%3AMEBIDX%3AIT