RCA 6T75 Restoration (16GP4)

[bandersen]

When I bother cleaning the underside, I use odorless mineral spirits and q-tips.

[zenithfan1]

Great job! Your set looks nice! Like I said in my thread, a lot of those cans are made slightly different at the terminal end,(difference in disc material and the way the metal edge is knurled over) my method was used on mid 60s caps in a few Zeniths and an Admiral round color set. Terminals and fiber disc came right out every time. I'll have to try the cut the can off while on the chassis method myself, if I can get into a set with enough room to do so, hasn't happened yet. The Motorola I did used a very annoying material for the disc and almost all of the terminal was aluminum so I had to bring the leads outside of the cap and hide them.

[dobulee]

Wow -- a CTC-4, very impressive! Very nice page.

John, the missing parts Ive identified are as follows, using reference designators from the schematic you gave me for the AT chassis:

C131 -- 47pF in the 1&2 video amplifier at the 4.5MHz trap. (Sorry, just can't force myself to say mega cycle!)

The second/third seem less critical, but:

C220 -- 100pF
C219 -- 0.0015uF both in the cathode circuit of the 1st video amplifier

Alex

[John Folsom]

Alex, while I am not much for those nFarads, I am a Hertzian kinda guy. :-). Hard to know if those were just omissions, or production changes. RCA seemed to be in constant flux with the design of this set. I would try tuning the trap, and if it won't null at 4.5mhz, I would put the cap back. I would probably try the other missing caps after I got the set working and see what difference (if any) they make. Good luck, thanks for keepin' us posted.

[dobulee]

Well -- that's it for the Film Cap replacements -- save the Caps in the Damper circuit, and a few other 1kV parts that I had to order.

Other than that, its on to the electrolytics.

Alex

[dobulee]

Anybody have any tips for removing rusty tube shields from their tubes?

I have a few that won't give up the fight...

[AUdubon5425]

I've pried tube shields off in the past - of course those were the shields that were made of spring metal that just met or overlapped on one side. If they're solid, I'd try dripping a bit of Liquid Wrench or Maltby's on the tube and letting it sit for a few days. You may lose the markings though.

[Eric H]

I push them through with a nut driver over the tip, a little lube might help as mentioned.

[Dan Starnes]

How did I miss this thread? Great set and congratulations.

[sansui2000A]

That is the exact same chassis in my TV. I'm really having a lot of problems getting mine going.

http://www.audiokarma.org/forums/sho...d.php?t=226988

[dobulee]

Thanks for the tips -- Sansuii, you're way ahead of me in performance at this point!

Well, the set has now consumed 90% of my free time for 2 weeks, but I finished recapping the last 5 or 6 films, and all the electrolytics. I guess this is where the real fun begins...

I haven't removed the CRT or the speaker, nor have I made any long wiring harnesses yet, so I put the chassis back in the set, and brought it up slowly on the poor man's variac -- assorted series light bulbs, while monitoring the 375V Bus, as its the only one readily accessible on the picture tube when the chassis is in.

Once I got it up to full line, there was still not much going on, Not so much as static on the speaker. The 375V bus topped out at about 445VDC, seemingly indicating that there's an awful lot of something not drawing very much current. (incidentally, I have gone through most of the resistors I deemed critical -- the power supply chain seemed to check out reasonably well). I flipped the audio input switch to Phono, and hooked up an IPOD on an RCA cable. Still Nada.

Peering inside, it looked like the 6K6 audio output tube was showing some haze, like it might have gone gassy. So much for NOS. I poked around, but all I could come up with was a 6V6 -- close enough to try. I popped it in, and Voila! Sound. Albeit pretty faint, even at top volume. After some more digging, I did find another 6K6, and put it in. Moderately better results -- for about the next 25 seconds when the 5U4 gave up the ghost.

I can see right away that I should have stocked up about 5X more on all the tubes. So. There it is. No signs of raster, no signs of HV. Either that or the CRT is dead. The only up side is that the CRT filament is definitely not open.

I'd consider bringing the set up on silicon diodes at reduced line voltage to get the proper bus, but the death of the 5U4 seems to indicate something is pretty wrong. Plus, the filament voltages would be low. When I get some more 5U4's, I'll start by digging into the other bus voltages.

When you guys probe on a bench, do you always do it with the CRT attached? I was thinking of jumping out the CRT 'loopback' pins on the power supply, and just pulling the horizonal output tube to start looking at things. Whats your standard procedures?

Guess I'm taking hte weekend off, since new 5u4's wont get here for a few days.

Alex

[Eric H]

Alex, it seems odd that you would have 445 volts of B+ if there was a short, did the heaters on the 5U4 burn out?

It's a bit late to point this out now but I never recap an entire set all at once any more, there are too many chances to make a mistake. I usually do the main filter cap first and then try it, then if everything is OK I proceed to the next one.

Well any way lets work on what you have, it might be a good idea to use diodes and a 200 watt dim bulb tester while you check things out, it will keep you from having to replace any more 5U4's if there is a problem.

I'd look closely to make sure you haven't wired any electrolytics in backwards, if you have it'll soon show itself in any case when it vents.

There is a 5" CRT used for testing purposes, 5AXP4 I think it is, they show up on eBay now and then.

You don't have to hook up the CRT at all, you can leave the chassis out of the cabinet with the HV lead inside a glass jar to keep it from arcing, this will allow you to do some voltage checks and concentrate on getting your high voltage working.

I'm not sure what you mean by "loopback" pins? you should be able to just leave the CRT base connector unplugged while you work on it, it's not a series string chassis right?

[Tube TV]

Might be a long shot , but also check the transformer for shorts between the 5 volts windings and the B+ windings , the 445 volts is a bit interesting .
Could there be a missconnection when you changed the transformer wires ?

[sansui2000A]

I have been working on mine on the bench without CRT. You need to jump across two pins on the yoke connector to feed power to the rest of the set. I'm on vacation next week and will try to work on the set and post results.

[jeyurkon]

Does that mean that you were testing it without the yoke connected?
John