CT-100 restoration questions

[Tim]

I have been working on my CT-100 on and off for about 4 months now. Life keeps getting in the way of this important process. . Anyway, I was reading on Pete's site that it is recommended to replace the HV filament windings on the flyback and I am wondering what the experiences of the group were on this topic. Is this a common failure point? If so, does anyone have a recommendation as to where to get about 3 ft of flexible HV wire? The 2nd anode stuff I have is too thick and stiff for this application.

Secondly, when replacing the Selenium stacks with diodes, has anyone found the need to add additional resistance to compensate for the voltage drop that was present with the seleniums? I have not applied power yet so have not made any measurements myself. Wondering what others have found.

I'm hoping to have the old girl running by Christmas.

Thanks in advance.

[Steve K]

Hi Tim:

I have restored four CT-100s and have not replaced the filament windings on any of them. Perhaps I have been lucky that the originals were good and continue to work. As far as the rectifiers, I just sub in the new ones without any resistors. Of course I keep the originals in place but just disconnect them from the circuit.

Steve

[Steve McVoy]

I have also restored 4 CT-100s and a couple of 21CT55s, and have never replaced the filament windings. I've also kept the original seleniums in every one of the sets. I think these sets were operated for so few hours that the seleniums are still good.

I did add additional fusing - especially in the cathode of the horizontal output tube. However, I have never had a flyback failure, so I think they are pretty tough.

[Tim]

Thanks guys. I think I will leave the fly alone.

I have added the extra fusing. I attached a fuse holder for the cathode under the chassis near the HO tube. My approach to the additional B+ fusing though was to make use of a 3/8" thick phenolic block I have had laying around and use the disconnected seleniums as a mounting shelf. I drilled and tapped the block and mounted the fuse blocks and diodes on it. This keeps the set looking original and the B+ protected by the cage while allowing acces by just removing a few screws.

I used two 3A 1000V diodes in series to replace each stack. Overkill but it should be trouble free.

I have attached a photo.

[John Folsom]

Tim,

I have replaced the selenium rectifiers with diodes, no dropping resistors needed. I restored a CBS RX90 which subsequently developed an arc over from the filament winding of the HV rectifier to the flyback core, so now when I restore a set I replace the filament wires out of paranoia. I use that very flexible silicone wire rated at 20KV or better. I can send you some if you need it.

I would recommend adding fusing downstream of the purity control and the field neutralizing control. I have had more than one set develop a B+ fault which burned up one or both of these hard to find pots. I just seem to be unlucky.

Good luck!

[Tim]

Thanks John. Much appreciated. Paranoia is a good enough reason for me. I'll e-mail you my address in case you don't still have it.

I have already fused the pots.

[Sandy G]

How's the CRT on yr CT-100 ? That seems to be the deal-breaker on way too many of 'em...

[Tim]

Luckily, the CRT is good which is why I went ahead with the electronic rebuild of the chassis. I have attached a photo of my CRT being driven by another member's restored chassis. Nothing was set up or adjusted so convergence is off, but it works!

[Steve K]

Hi Tim:

Nice job with the fuse/rectifier block, it looks very professional.

One thing to watch out for on a CT-100, the convergence transformer. If it goes out it can take the focus pot along with it. It is not easy to find an original replacement for that pot. If your transformer is bad John sells replacements.

You probably already know this but the high voltage regulation on a CT-100 is nothing to write home about!

Steve

[Steve McVoy]

Along with what Steve says about the convergence transformer, be sure to change the HV capacitor from the focus pot wiper to ground. If it becomes leaky, it will destroy the focus pot, and they aren't easy to find.

[John Folsom]

I was not going to bring up the convergence transformer, but it has been my exprience that they are prone to failure, either going open, or worse yet, shorting secondary to primary or frame, which may burn up the focus pot.

I have seen most or the horror story failure modes in early 15" color sets, which is why I a bit paranoid and take extra precautions to try to prevent trouble before it happens.

[Tim]

Thanks for the comments guys. Yes, I have installed a "Folsom Special" transformer. John, you may recall I picked one up when I visited you.

I melted the potting tar and old transformer out of the housing and put the new one in. I left the tar out though. The old transformer secondary measured about twice what it was supposed to.

Steve, I was wondering about those HV "doorknob" caps but as they are typically good, I was going to leave them alone until I brought it up and see if there are any issues. Steve, or anyone, is there a source for the one connected to the focus pot?

[Steve McVoy]

Allied sells a good variety of high voltage tubular capacitors at reasonable prices.

[Tim]

Thanks Steve,

I have already changed that cap and have purchased from Allied in the past. For some reason, I thought you were writing about one of the doorknob caps.

[Steve McVoy]

The doorknobs seem to hold up well, and if they fail they don't damage other parts in the CT-100.