I've just finished working on an 8TS30 chassis but have hit one snag that's really a bear: Video gain is erratic and unreliable. I've been through the entire circuit over and over and have replaced all paper caps, electrolytics and a great number of resistors and micas. The quality of the video is excellent but keeps popping from one setting of the contrast control to another. I'm trying to get a new control from Moyers but haven't heard anything back yet. Also, it may be possible that there is an intermittent short in the CRT which otherwise tests excellent. Other than the possible problem with the CRT, has anyone had this problem with this chassis or its close cousin, the 630 chassis?

Ralph-

The problem may be bad mica coupling capacitors in the IF circuits. My 8TS30 had similar symptoms, and other collectors suggested replacing those caps. I think there are four or five of them, all the same value. Phil Nelson has a great Web site description of his 630TS restoration, including the IF capacitors.

http://antiqueradio.org/RCA630TSTelevision.htm

Here is the discussion of my 8TS30 restoration as well:

http://www.videokarma.org/showthread.php?t=251332

Thanks, Chris, for the note. I replaced the 270 pf IF coupling caps and the bum bleeder resistors right from the get-go as I am very familiar with Phil's recommendations. Perhaps some other caps in the IF are doing the dirty deed though the ones I changed out actually measured right on the nose. I'm suspecting the contrast control but it may be very difficult to find a reliable replacement. Anyone got any other ideas?

I would have been very reluctant to change the capacitors in the video/ audio IF because then you will need to realign the strip. Also I have never seen a bad mica in an RCA IF strip over many years and you should rarely if ever replace them and only when necessay.

Also lead dress is very important in high frequency RF/ IF sections and so the RCA notes should be followed explicitly.

Now that the micas have been replaced, what are the plans for realignment?

Long shot, but there might be a reference to a factory known issue here: http://oldtubes.net/library/free_manuals/TV_Trouble_Cures_Vol_4.pdf

Also - sometimes browsing through these triggers an idea...

Penthode: I was very careful to measure the capacitance of the IF coupling caps that I removed and found them to be within 1 or two pfs of what the schematic called for. I replaced them with silver micas that were also within that same spec. The IF and set performance seems to be exactly the same as it was before this change. The audio is good since it was not touched and it lines up exactly with the video which was changed. I'm easily getting clean video through 4 megahertz; so I doubt it needs a realignment at this point. Why that gentleman advised Phil to change them, I don't know. I agree about RCA's micas being pretty reliable.

Replacing those capacitors very definitely fixed the problem in my 8TS30.

Hey, Chris, I found a batch of 270 pF ceramics on eBay for 5 bux w/free shipping today! The photo was too blurry to make out a brand, but they look like the right shade of orange to be Cera-Mite. Thanks for bringing this up - now I have a stash to use on my 8TS30 resto...

Penthode: I was very careful to measure the capacitance of the IF coupling caps that I removed and found them to be within 1 or two pfs of what the schematic called for. I replaced them with silver micas that were also within that same spec. The IF and set performance seems to be exactly the same as it was before this change. The audio is good since it was not touched and it lines up exactly with the video which was changed. I'm easily getting clean video through 4 megahertz; so I doubt it needs a realignment at this point. Why that gentleman advised Phil to change them, I don't know. I agree about RCA's micas being pretty reliable.

Ralph, I appreciate the care you have taken. I just would like to point out the RF/IF sections are critical. Changing the micas in the IF strip is generally not required. One has to be cautious when changing parts, especially when it affects tuning in these areas.

Even though you have taken the care to ensure the same capacity, tuning will have drifted and the set would generally benefit from an alignment check.

I have try to convey the need for systematic restoration and troubleshooting, especially to beginners, otherwise things can get out of hand.

Saying that, I am happy for your success and congratulations getting the set working again.

Why that gentleman advised Phil to change them, I don't know.It has been over 10 years, and I don't recall if he gave any details beyond advising me to change them. He had decades of experience and I was a novice, so I did what he suggested.

Regarding your original query about erratic gain, my 630TS behaved the same way every once in a while, which I attributed to a funky manual gain control.

I generally don't replace micas unless I have a specific reason to suspect one. The troublesome ones that I can recall in recent projects were all in sweep circuits. I do seem to find more bad ones now than in days gone by . . . or possibly I'm just getting fussier.

Phil Nelson

I've had micas go bad in mostly audio circuits that use them as a high frequency filter to prevent oscillation at ultrasonic frequencies. I find they go noisey or lose value and the circuit starts to become unstable (oscillate).