My Stereo Theater TV chassis

[electra225]

It's been almost two years since I have worked on this TV chassis. When last I worked on it, I had fixed it enough that it had a good raster with a full screen. I had no audio and no video. R43, the AGC resistor, checked nearly open, so it will have to be replaced before I go much further. Then I will troubleshoot the front end of the set to find out what ails it. I have a B&K TV Analyzer and some extension test sockets so I can inject a signal for troubleshooting. I'll get back on the project if and when they get my shop built. It's been too long.....

[hermitcrab]

The 1968 RCA theatre I got from Bill has the same problem... raster but no sound or picture , I kinda figured it is either in the tuner or the IF strip... because I even tried a convertor box on the antenna and still nothing , never thought to check the AGC circuits... thanks for the tip
Elton

[William]

Elton, do you have a schematic for your RCA?

Bill

[hermitcrab]

Yep first thing I did when I got it home

[William]

I have the schematic for mine as well, and I actually found a brochure for the complete RCA lineup for 1967. Now all I need is the owners manual. My Grandparents always kept it in the record storage compartment but when I got it, it was gone.

Bill

[electra225]

I'm really hoping that I don;t have to pull the tuner. They did not intend for the tuner to be removed. I'll replace that resistor (3.3 meg, I think) and then go thru the IF strip if that doesn't restore video and audio. I don't know if the horizontal and vertical are stable yet, so the diode thingy that usually goes out may be out of this one. My understanding is they can be rebult/reproduced. I have a lot to go. I'm no TV guy by any stretch, but I have learned that taking it a step at a time is the only way to not get lost. Some type of signal generator to inject signals is nice too. I hope you'll share the project. Good luck.

[electra225]

I started making preliminary inspections and developing a plan to finally sort this TV chassis. R43, the AGC resistor, per my notes, is supposed to be 9.1 megohms, but measures 1.46 megohms. I have this item on the order I sent in to Mouser for the Analyzer parts. Of course, last I worked with this chassis in the spring of 2020, I had a good raster, but no audio or video. Our late friend Ed recommended replacing R43 before I went any further, so this will be Job 1. There is a germanium video detector in one of the IF cans. I have located this component and I've read that these are a high failure item, so maybe I should change this before I power up. There is also a dual diode for sync, another high failure item. I also understand that shotgunning parts without testing in between replacements may ultimately prove to be shooting myself in the foot if I add faults I don't have now. I don't want to fool around in high voltage any more than absolutely necessary. I'm terrified of high voltage. So, I want to replace any parts that involve digging very deeply into the chassis before I power up. I have test sockets and have a couple avenues of investigation I want to pursue. I also don't want to have to pull the tuner until it is a last resort. With the cabinet sitting on the floor instead of on a table, that will be a back-breaking project. The safety glass and mask around the CRT will have to come out so I can remove the remote control channel selector along with the tuner. I think we decided when I worked with this last that my problem was in the tuner or in the IF strip. I hope the Analyzer will help with some of this. TV guys, if you have any suggestions that might help, I'd sure appreciate your guidance. It will be a few weeks before I actually get on this project, but I can do preliminary work while I work on the Analyzer and get it going.

This chassis is the C36-20-11 version. The schematic I have didn't survive storage. I could sure use a good, readable schematic for this chassis.. IIRC, Steve Johnson didn't have the right schematic, I had to improvise. The one I had was a copy and is now unreadable. I don't remember where I got it. I got a lot of the service information I have from a forum member, but I don't remember where I got the schematic.

[electra225]

I now have a good, readable schematic. And I have the AGC resistor I needed. I got to reading the Sams for this chassis and learned a couple things that may stand me in good stead to take care of while I have the chassis out of the cabinet replacing the AGC resistor. The video detector diode, buried in the discriminator (?) transformer can may cause a no sound/no video condition. It will be easier to change with the chassis on the bench rather than me trying to change it sitting on the floor behind the cabinet. Then the horizontal phase detector diodes on the video board. Those are a high-failure item and may cause the horizontal sweep to be unstable. Again, easier to change with the chassis on the bench. The video detector diode is given in Sams as a 1N60. The service literature does not give the value for the horizontal diode. Hopefully, one of the members with more TV experience than I have will chime in with a suggestion. I need to get back on this project and get the TV sorted. I have been told that a "Maggotbox" like the one I have tends to have a "soft jug", so I really hope mine is the exception. I'll test it again.

[electra225]

I use a B&K 445 CRT tester. I haven't used it much, so I don't "know" it well. The CRT in this set checks right at the edge of the green, at "300" which the instructions for the tester says is minimum for a good tube. There are no shorts and the cutoff test is inconclusive. I may not be doing it right. It takes several minutes to get any reading on output of the CRT, but it has been three years since it has been used. If I increase the voltage from 6 volts to 7.3 volts, the tube reads about 600 on the green scale. This is about how it tested last time, according to my notes. I should probably see if my tester is working right. I have a little Zenith 19" B&W TV that the "jug" tests "soft" in, yet it produces a nice sharp picture. It seems to me that if the tester is working right, there shouldn't be as large of a jump in output between 6 and 7.3 volts as there is. I need to see if the 6 volt setting is actually giving me 6 volts at the CRT. This CRT totally flunks the life test, so it may be a tad on the soft side for sure. Turning the voltage on the tester down to five volts and the emissions go to almost zero. The best test may be to power up and see if I get a good raster.

Next, I need memory refreshed on how to test a diode. IF I put the positive lead of the ohmmeter on the anode and the negative on the cathode, I should get continuity, right? Reverse the leads and it should show open, right?

I believe what I'll do now is to power up slowly without the horizontal output tube installed. Reform the capacitors, then connect the HOT and see what happens. If I get a raster, I'll proceed further. I suppose it's possible that in moving and standing unused for three years, the picture tube might have gone soft, too soft to use. I need to verify a raster before I go much further. The CRT is coded 274, which is RCA. The date code is smeared, but I believe it reads either the 32nd week of 1962 or the 12th week of 1962.

[electra225]

Now I don't have a raster.

I wanted to get a baseline of performance on this before I pulled the chassis. I powered up slowly to let the filters reform. I'm not sure I buy the reforming part, but Shango and Bob Andersen seem to think it is good to do. I started out at 40 volts, then gradually, over about a two-hour period, got it up to full line voltage. I had pulled the horizontal output tube. I put the HOT back in, then powered up at full voltage. The CRT remained dark. With the HOT out, it pulled 250 watts. The wattage didn't increase with the HOT back in. The filter caps remained cool the entire time. I'm beginning now to wonder if I have lost B+. Next step is to measure B+ to see where I lost it. This TV chassis really does nothing except the filaments all light. Sams says the TV chassis pulls 160 watts. The stereo pulls about that, so the wattage draw is about 75 or 80 volts low. I thought I heard a pop when I was ramping up voltage slowly. I didn't see anything and can't swear I really heard anything.

As an aside, I wanted to see if my CRT tester was putting out the correct filament voltages. Right on the money, so we can safely say the CRT in this stereo is soft. I still think it will produce a picture. I had a raster before, so something has happened while it was in storage, some component decided not to work.

[electra225]

I have B+. I have several problems with this TV chassis. I'm in the market for a high-voltage probe now. It seems to have lost high voltage. I don't see anything red-plating and it is not drawing too much juice. Back to the drawing board.....

[William]

Sorry to hear this, Greg. I guess your TV is like my VM speaker/amp in that it should work correctly but does not.

Bill

[electra225]

Not to worry, Bill. If it was easy, everybody would do it. We'll keep at it. I was looking at the drawing which is a Godsend for this project. You remember I got this thing going three years ago. I had a raster, but no sound or video. Then I repopulated it with all new RCA tubes. I hadn't used it with the new tubes. I wonder if one of the tubes like the HOT, damper or high voltage rectifier is no good. I may have a handle on the sound issue as well. I'm going to check and change the video detector diode while I have the chassis out. I'm glad I found this before I worked on the chassis then might have to take it out again. Having a good schematic, then doing a thorough baseline check has given me clues.

[electra225]

Like Shango likes to say "the fun never stops on this project".....

I am starting with the "no audio" condition. First thing I did was to determine if the connections within the tuner would accept audio from the TV chassis. I rigged up a cable from my little Sony transistor "test" radio to the TV audio input on the tuner. Western Red came thru just fine, thank you. Next, I checked continuity of the audio cable from the TV chassis to the tuner. I used three meters and got three different readings, but they all indicated that the audio cable from the TV chassis is shorted to ground. The plug that goes into the tuner on that cable looks wonky, with bare wires. I will remove the plug, clean up the wires, then see if I have continuity back to the TV chassis. If yes, I'll replace the RCA plug on the audio cable. IF no joy, I need to rig up another audio cable. Wonder if that cable absolutely has to be shielded?

Another thing. With the audio cable from the TV chassis connected to the tuner, I have a substantial hum in the speakers. When I rigged up my test cable, and unplugged the original cable, the hum went away. This further strengthens the case of a short in that audio cable from the TV chassis.

[TC Chris]

IF no joy, I need to rig up another audio cable. Wonder if that cable absolutely has to be shielded?

Twisted pair? My 1951 Magnavox has a "TV" input (i.e., line level) input and it works fine with a twisted-pair input. And later if it's too noisy, you can do a fancier cable.

Chris Campbell

[electra225]

I fixed the issue with the audio cable. The RCA plug where it connects to the tuner had bare wiring that was shorting. I removed that plug, tossed it, skinned back the wire and shield, resoldered that to a new male RCA plug. I can now get audio cleanly thru that cable, but I still have the hum. I need to dig further.....

I have tested the tubes, all good. I have no audio, no video, no high voltage. The no high voltage issue is the most important to tackle. I had high voltage, then I didn't. No smoke, no red plating, no blown fuses or circuit breakers. It's like I lost B+ to the high voltage section. I'm not sure what to do next. I can inject an audio IF signal with the Analyzer and see if I can get some audio thru the set somehow. Then I reckon I pull the chassis and see if I can find something open. I mentioned that I thought I heard a pop, but no smoke. I wasn't dead sure, but that may be significant. I'll bone up on test procedures using the TV Analyzer then maybe somebody will be good enough to nudge me in the right direction. I'm wondering if all the issues are related.

I learned something I was not aware of on VideoKarma. I used the little yellow Chinese caps like we put in radios in the boost and high-voltage sections of this TV chassis. I read over and over how those cheap caps, that work perfectly well in radio and audio, won't last an hour in the high voltage circuits in a TV set. That is about as much raster as I've seen in this TV. Remember when I was powering up slowly, I reported hearing a pop? Apparently, according to what I have read, those yellow caps will go "pop" when they blow up. They won't handle the high pulse voltage in a TV set. The recommended way to make voltage measurements without risking further damage is to remove the horizontal output tube, then make measurements with the tube out. I need to check the horizontal oscillator, then see if I have lost boost voltage.

[electra225]

I've been looking at the drawing for this chassis. There is some nomenclature I'm not familiar with. Some of the circuits have lines with an "X" in them. I don't know what the "X" signifies. The brightness control does not work in the cathode circuit on the CRT. It is on a grid, but it could cut off the beam if the control was open or misadjusted. I don't really understand how the contrast control works, either. The drawing also gives 12 MA for the cathode current on the horizontal output tube, which, obviously, is incorrect. Methinks they left off a digit, probably 120 or 125 MA is more likely the correct value. I have an old milliamp meter that I may rig up like Shango does to measure cathode current on the HOT. There is a boost control and a B+ control, but those are allegedly for the Magnalux automatic brightness/fine tuning control. I won't putz with that until I get a raster and an image, then I'll do more research into that. I need to learn to manipulate all the controls before I suspect chassis issues. I try not to mess with things that may get me into trouble. I'll take some voltage measurements, for learning value, if for no other reason.

[Hi-Fi-Mogul]

I've been looking at the drawing for this chassis. There is some nomenclature I'm not familiar with. Some of the circuits have lines with an "X" in them. I don't know what the "X" signifies.

From some reading and asking other folks before,
"X" and lines through it indicate a part of the circuit that
was not instituted in all chassis' designs.

In the simple hi-fi schematics I've gone through, it may
have indicated a capacitor or resistor that was not
used in all models.

A few other schematics had a more complicated piece
of the circuit left out as designated by the X's.

[electra225]

I have no voltage on pin 6 of the CRT. An orange wire. I think I remember reading about a color code for CRT connections. I have heard that the yellow wire is for the cathode. I cleaned and checked the controls. I have high voltage. I had a raster, turned the set off, turned it back on and had a black screen. I don't know what my next step is. I can't do much else until I get a raster.

[TC Chris]

Doesn't the schematic call out the CRT pins by number & wire color?

Chris Campbell