Rca ctc-5

[DaveWM]

finally got off my rear and replaced the RG static convergence pots. They were really fouled up, once I got them out of the circuit, one side was open and both had drifted a lot. there are some more pots in there, but I would rather not have to replace them as they are on a mount that is inside the chassis, with the dynamic coils mounted to the same. It would be a major pita. Since the chassis is so easy to pull I am just going to see how it works now.

[DaveWM]

I decided to open up the old pot, there was a break between the tap and one side, the pot wiper would complete the circuit when placed directly over the break, its hard to see but must be a crack.

[old_tv_nut]

Exactly the failure mode of the pots in mine. I think a poor design, too much current and pushing the technology to the limit to make a pot of only 100 ohms with a 25 ohm tap.

[DaveWM]

yep both pots opened in the 25 ohm section. Hopefully the new pots will hold up, I know I will not be putting the kind of hours on them they used to see when this tv was in regular service.

[old_tv_nut]

Got my fingers crossed for you - maybe they suffered from high contact resistance after getting dirty, so the new ones will last for a while?

[DaveWM]

I noticed one of the speaker plug in mounted to the fiber board on the speaker had come loose, so I pulled the speaker and fixed that. while I had it out I checked the 8mfd cap that is the crossover to the tweets. It checked at about 1 ohm on my simpson 260, both ways, so I dug around and found a sprauge atom 10uf 50v NP. I piggy backed it to the existing cap, clipped on lead so the old would be out of the circuit. the ne cap would quickly go to 700k ohms on the 260. I figure it prob will make no difference, but as long as I had it out I wanted to fix it. I also noted one of the two tweets has a lead broken where it mounts so I will have to pull it and see if I can fix. if not I have a few spares.

[DaveWM]

been slow to move on this, but I did replace the cap on the crossover and did fix the speaker fiberboard, and put it all back in. I have the chassis on the bench just need to get off my duff and put it back in the set and do a setup.

I am missing the two top pulls for the doors, a close up pic of them may help me find something that will be correct, those who have this set pls post a pic of the top pulls.

[Penthode]

Curious: I had exactly the same failure on the red static convergence pot on my CTC5. There was a crack about a half way along the low resistance element end.

I do not think the pot suffered from electrical stress. I think the problem was with manufacturing and shrinkage of the carbon element over time. The break always seems to occur between the two rivets on the lower resistive portion to the tap simply because there are physically close together.

This evening I tried my fix: I went to "Canadian Tire" and picked up a rear window defroster repair kit. The kit includes conductive paint. I cleaned the pot element where the break occurred and painted a narrow band right over the crack.

After the paint dried, I reassembled the pot and it seemed to work fine. I checked the resistance of the previously broken section and it now measured 35 ohms, which is slightly low but about right. I then reinstalled the pot in the set and the static convergence adjustment now works okay.

Overall, I like the static adjustment on the front panel. But the pots are a pain. The design isn't bad: even though the static convergence voltage is derived from horizontal output plate current, any variations in current are mostly compensated for so that the convergence does not vary. It's just those bad pots. I would be just as happy to substitute the convergence clover leaf assembly with one which has permanent magnet adjustment, but that would be non-original.

I suspect the repaired pot should last a while until I can think of a better solution.

[Penthode]

I made an interesting discovery today. I was troubled by the low grid bias on the horizontal output tube. This pointed to a slight reduction of horizontal drive.

I examined the circuit of the 6CG7 synchroguide horizontal oscillator. Comparing with the CTC7 and CTC9, the plate voltage of the oscillator was a bit lower.

To increase the drive, I thought I could raise the plate voltage a little. I discovered that R604 and R603 form a voltage divider. Further, I discovered my set had an 82K 1watt resistor for R603 where the schematic showed 100k. I tried substituting a higher resistance which raised the drive a bit. The offshoot is that the unregulated HV has gone up (good) and the grid bias on the horizontal output is now correct (very good).

Then after this, I read in the "Series 700 Setup-Service Manual" (available on at the ETF website), where, on the page 71 pictorial of the horizontal oscillator circuit board, there was a little note to the side of figure 132. It reads "Note: R603 may be reduced in value if less drive to horz. sweep output is desired".

So whadaya know?

[DaveWM]

Nice catch I will check that before I take the chassis off the bench

glad to hear about the pot fix, i was wondering about how that conductive paint may work. I saved the bad pot after taking it apart, think I will try it as well.

[DaveWM]

reviewing the schematics I can see the ctc-5 has the 82k and the ctc-5N has the 100k for R603. I have a ctc-5N and it has the 82k, hmmmm.

the ctc-7 has a different setup, with a pot for drive adj, and a cap in series with the 100k/pot.

Question for Pentode, did you check the cathode current before and after the switch from 82k to 100k?

I had no evidence of a drive bar on my set so I may try changing the resistor if for no other reason to see if I can get that cathode current down. I still have to try it with with the new convergence pots so I can get a good reading in the cathode voltage of the horz out tube. It was high before, but I think that was just due to the increased resistance of the old pots. IIRC the called for cathode voltage is 7.5 v and I was at 12v. So will base line it with the new pots, get a revised cathode voltage, then experiment with the R603.

I did check the current (200ma) but the RCA schematic did not specify what it should be, just gave the cathode voltage. I suspect 200ma was about right but would like it to be a low as possible. I did adj the eff coil for the dip.

[miniman82]

Don't bother worrying about it. The horizontal trans in my Wingate runs cool as a cucumber after hours of play time, and that's after I added pots for drive and HV to try and boost HV output. Most I ever saw out of it was around 22kv, I suspect the transformer itself and the way it's wound are holding back any more output potential. I think mine was around 200ma as well, but like I said it's not like it gets hot anyway.

[dieseljeep]

Originally Posted by miniman82

Don't bother worrying about it. The horizontal trans in my Wingate runs cool as a cucumber after hours of play time, and that's after I added pots for drive and HV to try and boost HV output. Most I ever saw out of it was around 22kv, I suspect the transformer itself and the way it's wound are holding back any more output potential. I think mine was around 200ma as well, but like I said it's not like it gets hot anyway.

Nick! Did you move all those sets out east with you.

[Penthode]

Originally Posted by DaveWM

Question for Pentode, did you check the cathode current before and after the switch from 82k to 100k?

I ended up selecting 110k. This yielded -40 volts on the Horizontal Output grid as per the schematic. The cathode current measured 210mA.

I did check the current (200ma) but the RCA schematic did not specify what it should be, just gave the cathode voltage. I suspect 200ma was about right but would like it to be a low as possible. I did adj the eff coil for the dip.

With 7.5 volts on the cathode, and the cathode resistance to ground at 23.5 ohms (the 82ohm in parallel with the static convergence 100ohm pots), this equals 320mA.

I now measure 5.0 volts for the 210mA. I increased the drive well below the appearance of the drive line. The HO tube runs a bit cooler and I have better regulated HV.

[miniman82]

Originally Posted by dieseljeep

Nick! Did you move all those sets out east with you.

I only had room for the CT-100, 21-CT-55 and CTC-4. The rest are at my folks' house awaiting a chance for me to rescue them.