Restoration of RCA 8TS30 #C061765

[ChrisW6ATV]

Hello-

After realizing my CT-100 needs quite a bit of restoration of its own, I decided to work on my 8TS30 to get better experience with a full recap/test/alignment job first. When I first started collecting early TV sets about 1979-80, I was just beginning to learn TV repair (and knew nothing about doing a complete restoration), so I only did the repairs I learned or knew were needed on the sets I did work on, before essentially halting all of the work on my old TVs for 25 years as I followed my still-going career that includes video-display repairs itself. So, despite 30+ years of TV and electronics service work, this one will be my "first" real restoration. Besides plenty of useful experience, I have a good selection of test equipment (often chosen based on what works well at my job), as well as plenty of ideas learned from all of the great people here on Videokarma (and the Antique Radios forum).

I tested the RCA's CRT, and it started almost dead but went into the lower part of the Good scale on my B&K 467 after only a few minutes running in the Clean/Balance mode that boosts the filament voltage, so it is probably good enough to get going, if not plenty bright. It is an RCA 10BP4A from 1950, so it is already a replacement. The chassis also has two 5U4GBs from 1957-58, and a 6BG6GB horizontal output from 1960, so it was kept going at least that long. No obvious problems on the chassis, except a fuse that has had a piece of thick wire-type solder melted to both ends, and a 100 ohm 2 watt resistor that is cracked.

[ChrisW6ATV]

I checked all the tubes and resistors. Three weak tubes, and the 5V4 was replaced with a 5U4. A few resistors too high in value, and of course a chassis full of wax capacitors and bad electrolytics. I had most of the replacements for the wax caps in my kit from Just Radios, picked up the resistors and a few caps locally, and ordered the rest from Digi-Key tonight. I also picked up one tube locally and stole a couple of others from a beat-up 630TS I have as a some-day project here. The fuse is odd; it is in line between one filament connection and another, in this parallel-filament set. Who knows why it would have been added?

I will start the re-capping in the next few days. Here is the "before" picture:

[ggregg]

keep posting, I've got one that I should probably get to pretty soon.

[Phil Nelson]

Originally Posted by ChrisW6ATV

I will start the re-capping in the next few days. Here is the "before" picture:

Plenty of elbow room under there. Recapping should be straightforward.

Phil Nelson

[ChrisW6ATV]

I saw another picture of an 8TS30 chassis that had the same fuse, so it must be original. Now, I am going to trace that part of the circuit to figure out why it would be installed there. And, of course, I will replace it with a proper-value fuse.

Phil, I will use your Web site for inspiration!

[Penthode]

Originally Posted by ChrisW6ATV

I checked all the tubes and resistors. Three weak tubes, and the 5V4 was replaced with a 5U4. A few resistors too high in value, and of course a chassis full of wax capacitors and bad electrolytics. I had most of the replacements for the wax caps in my kit from Just Radios, picked up the resistors and a few caps locally, and ordered the rest from Digi-Key tonight. I also picked up one tube locally and stole a couple of others from a beat-up 630TS I have as a some-day project here. The fuse is odd; it is in line between one filament connection and another, in this parallel-filament set. Who knows why it would have been added?

I will start the re-capping in the next few days. Here is the "before" picture:

You cannot replace the 5V4 with a 5U4 in this set. The 5V4 is the damper diode and the indirectly heated 5V4 will have the appropriate characterisics for high frequency rectification. The directly heated 5U4 will not. In fact the 5U4 may help destroy your flyback transformer.

[Penthode]

Originally Posted by ChrisW6ATV

The fuse is odd; it is in line between one filament connection and another, in this parallel-filament set. Who knows why it would have been added?

Fuse? To which filaments is it connected?
I do not recall seeing a filament fuse on this set.

[ChrisW6ATV]

The 5U4 was in the socket when I got the set; I have a correct 5V4 that I will install after the chassis work.

The picture with the same fuse I have is from an 8TS30 that Steve Pielock restored and described on the Antique Radios forum. Here is the picture link:

http://i249.photobucket.com/albums/g...CIMG6905-1.jpg

I hope the flyback has not already been damaged from having that 5U4 in there...

[Reece]

You may be able to extend the leads for the fuse topside so it would be easier to get to.

[ChrisW6ATV]

Originally Posted by ChrisW6ATV

The fuse is odd; it is in line between one filament connection and another, in this parallel-filament set. Who knows why it would have been added?

Originally Posted by ChrisW6ATV

I saw another picture of an 8TS30 chassis that had the same fuse, so it must be original. Now, I am going to trace that part of the circuit to figure out why it would be installed there. And, of course, I will replace it with a proper-value fuse.

Oops, a complete brain-fart on my part... The fuse is NOT in the filament wiring at all. It is in the boost circuit off pin 8 of the 5V4 damper. There is a part I have not yet identified nearby that is in the filament wiring, and I confused that part with the fuse.

[ChrisW6ATV]

I cannot believe it was six months ago that I last posted about this set. I did the recapping on and off as time was available, with the electrolytics only "restuffed" minus the can tops for now. Last night, I reinstalled the tubes. My Variac must be buried in my shed, so plan B was to plug the chassis into a Kill-A-Watt device and monitor the current. I fired it up without the rectifier tubes and it drew less than an amp, then I added the 5U4s and it read about 2.7 amps, and I got hiss from the speaker. A bit of adjustment and I had a raster on my 5AXP4! But... Then I saw smoke from near the transformer-oh, no! I saw there is a covered area behind the transformer, and I opened it up and see it has power resistors, covered with dust. I cleaned them and reapplied power, and no more smoke (whew!). Next, I connected a Channel 4 signal from a digital tuner, and I have good sound as well as video, but the horizontal is off-frequency and the hold pot does not change it. Still, I am 95% successful already. I posted some pictures below. First, the "after" recapped chassis, next is the dust-coated resistors, last is the raster on the 5AXP4.

[ChrisW6ATV]

The horizontal frequency problem got fixed simply by adjusting the "Synchrolok" horizontal discriminator control on the back panel, so this set basically came up working on the first try. I do still have a dirty contrast control, though, so maybe another few drips of Deoxit will help.

[miniman82]

Way to go, put the 10" tube back in and see what it looks like!

[Phil Nelson]

Lookin' good! That cabinet is already in pretty good shape. Looks like you're just about done, apart from touching up the cabinet finish and reinstalling the CRT.

Phil Nelson

[ChrisW6ATV]

Thanks, guys. I found out the "dirty contrast control" is probably something more-I tested it and its action seems good but the IF/video gain fluctuates rapidly most of the time, sometimes showing a high-contrast picture even with the control all the way down, and even shorting its center terminal to the near-ground end does not affect the problem.

The plate voltages on the four video IF tubes are higher than what is on the schematic, and the last one especially changes along with the flickering picture. (I suspect that is a symptom of the problem though, not the cause, but I will need to do more tests.) Just putting the voltmeter probe on them calms the problem down a bit (and reduces the gain as well). This might be a good excuse to test and fire up the B&K 1077B I got a while back from a generous member.

Any suggestions are welcome as well, thanks.