As you may know, I purchased the Motorola 19CT1 color TV set a while back. I limit its run time, of course, but have found that like a car, it works best to give it limited (and supervised) exercise. Running it about a half hour once a week or two seems to maintain the best picture. If it sits too long, the color washes out on me. Actually, the very first day I got the set I was wondering if it really was a color TV or not, since all I got was black and white. Later on I'd get a weak color. On the half hour a week exercise program, I've been getting normal looking color. Of course, it can't compete with a modern TV, but it's far better than I'd expect for such an old set.

This has been a good routine until last weekend. When I turned it on, I got no picture (not even a raster), and the sound was present (could hear the audio broadcast) but it was distorted (very static sounding and muffled). It does look like the previous owner did indeed have the original capacitors replaced, so my first assumption would be one or more bad tubes (I'm sure more than one can go at the same time by coincidence).

As luck would have it, I received an original SAMs photofact document on it last night. It's a little unclear which specific tubes I should try, so I thought I'd ask for a few suggestions. Of course, it could well be a poorly soldered or defective capacitor, but since I'm not up to replacing capacitors or resistors yet, I'll leave that to the experts. Since I can replace tubes (I have a stock of all the tubes it's supposed to take on hand), I thought I should give that a try first. I work for free. :D



Thanks,
Joe

Does your set use tubes for low voltage rectifiers such as 5U4's for the B+? If they are weak or bad, that would effect the entire set

It may. I know there's a small "cage" of tubes in it for the power rectification (there's at least one tube type with a topcap in there). The "cage" is a small vented metal box inside the chassis and I understand any high voltage tubes are placed there for safety. It looks like all power rectification (certainly the high voltage) is done via tubes.

My list of tubes is at home and I'm at work. I'll check tonight and let you know the complete list of what tubes the set uses.



Thanks,
Joe

That set uses selenium rectifiers. It is possible that the problem is a tube, but not likely. Those sets are terribly unreliable.

Here's the complete list of vacuum tubes used in the set, from the SAMs documentation:

1. RF Amplifier: 5BZ7

2. Osc.-Mixer: 6u8

3. 1st Video IF Amp.: 6CB6

4. 2nd. Video IF Amp: 6CB6

5. 3rd. Video IF Amp: 6CB6

6. 1st. Video Amp-
2nd. Video Amp: 6AN8

7. Video Output: 12BY7

8. AGC Cathode Follower -
AGC Keying: 12AU7

9. 1st. Sound IF Amp.-
2nd. Sound IF Amp: 6AN8

10. Ratio Detector: 6AL5

11. AF Amp. -
AGC Clamper: 6AV6

12. Audio Output: 6V6GT

13. Sync Separator: 6CS6

14. Sync Phase Inv. -
Horz. AFC: 6SN7GTA

15. Vert. Osc. -
Vert. Output: 6BL7GT

16. Horiz. Mult. 6SN7GTA

17. Horiz. Output: 6CD6G

18. Damper: 6AU4GT

19. HV Rectifier: 3A2

20. Diode Coupler: 3A2

21. HV Doubler: 3A3

22. HV Regulator: 6505

23. Chroma Bandpass
Amplifier: 12BY7

24. Chroma Bandpass
Cathode Follower -
Quandrature Amp.: 12BH7

25. Burst Amplifier -
"G-Y" Amplifier: 6AN8

26. Chroma Reference
Osc. Control -
Chroma Reference
Osc.: 12AT7

27. Chroma Sync
Phase Det.: 6AL5

28. "B-Y" Demodulator: 6AL5

29. "R-Y" Demodulator: 6AL5

30. "B-Y" Amplifier -
"R-Y" Amplifier: 12BH7



One thing I also should ask. I could identify and purchase all of these tubes except for #6505. Nobody seems to know what this tube is, and I can't find any literature on it. Is it something propriatory to Motorola?

Hope this helps. From reading the list, it appears tubes 3A2, 3A3, and 6505 are involved in rectification or voltage regulation.

The 3A3 and 3A2s are the high voltage rectifiers. They wouldn't have any effect on the distorted sound.

The 6505 is a gas high voltage regulator. It also wouldn't have anything to to with the sound. You won't find a replacement for the 6505 - it was made just for that set and I've never seen one outside of a chassis.

Your problem is almost certainly not a tube. It may be one of the selenium rectifiers, or some other part in the power supply.

Thanks. It seems most shops in my area treat this set like it was made of Kryptonite. I'm lucky that I found a TV shop about 15 minutes away that likes to work on the old tube type TV sets. Since I'm not quite up to speed yet on going beyond swapping tubes, it looks like I may be giving them some business.




Joe

Joe,

Was it a typo when you listed the RF amplifier as a 5BZ7? That tube should be a 6BZ7.

Maybe, I caught myself hitting the "5" instead of "6" a couple times. I'll have to double check the documentation this evening to be sure.

I take it that it's unlikely to be a coincidental tube failure in the video and audio tracks? With 30 tubes, I wouldn't be surprised to have multiple tubes fail once in a while, even two at the same time.

I have a Hickok 533A tube tester on its way (bless the man who founded eBay). If it works as good as advertised against known good tubes, I don't see the harm checking the tubes (my time is cheaper than a shop's). My guess is checking the tubes associated with the audio branch, then the ones listed under "No picture, sound normal" in my DIY TV trouble shooter guides. As long as I don't accidentally break anything removing & replacing the tubes, it can't hurt to try.

It's a good thing I enjoy tinkering--I can see why most people wouldn't have the patience for tube driven stuff. It's been fun learning more about the old stuff. I just got to remember to unplug the set first and not touch anything but the tubes when I'm in there.



Thanks again,
Joe

You're right, it was a typo. That RF Amplifier is a 6BZ7, not 5BZ7 as originally entered.

Joe,

There is a 5BZ7, which is identical to the 6BZ7 except for the filament voltage. It is used in sets where the filaments are wired in series, which your set clearly is not due to the other tube types used. I mentioned it because if someone stuck a 5BZ7 in there, it would work but would have a short life.

I opened up the set and turned it on just to see if all the tubes lit up. I found there's an extra light show going on in there. I didn't have the set on long enough to pinpoint it (for fear of doing more damage), but I could clearly see (and faintly hear) the flashes of something arcing below the picture tube.



Joe

Joe,

You will need to determine what is arcing. Did it look like it could be coming from the high voltage cage (the box with the 3A2s and 3A3 in it)? If it is, see if you can remove the cover before powering it up just for a few seconds to see what's going on. Pay close attention to the flyback transformer (inside the cage). Also watch the tubes in the cage, plus the damper tube & horizontal output tube.

ATTN Steve McVoy: you are familiar with this chassis. Any suggestions?

I would suggest you let your local tube TV repair shop deal with it. Even if you figure out what is making the noise, the chances are you don't have the knowlege, parts, etc. to fix it.

I'm with Steve on this. I don't have the expertise yet to get too deep into repair. I can tell it's not coming from the high voltage cage. It's clearly coming from something electrical below the picture tube. Unfortunately the picture tube itself blocked my direct view so I couldn't get a good look. Since I didn't want to do further damage to parts made of "unobtainium," I quickly shut it off without looking around to see exactly what was arcing.

Thanks to everyone for their helpful suggestions. Looks like the suggestion it not being a tube was right. I'm going to have a shop to look at it and tell me what's arcing and if it's something they can fix.



Joe