My Stereo Theater TV chassis

[electra225]

It does not give wire color on the picture tube connections. It gives 25 volts on pin 6. The problem with that is that pin 6 has the yellow wire, usually the cathode wire. I need a high voltage probe in the worst way. I'm not familiar enough with how this thing should act to use the "jump spark" test and be able to know what it's telling me. Something happened suddenly, a component failed or is failing. I think I'll pull the horizontal output tube, then measure element voltages and see where I lose voltage. I'm about ready to believe the picture tube has gone out, but it still tests good. I'm chasing my tail at the moment. I need to develop a plan....

[electra225]

I am having to make some "guesstimations" between the set I'm working on and the Sams I'm using. So far, that has not been an issue other than some chasing my tail. Better than flying totally blind, though. The 23MP4 on the diagram and the 27ZP4 in this set are basically wired alike. I had high voltage before. Since I fired it up after setting for over 3 years, I don't. It has to be something simple. The CRT has come awake with fairly decent emissions and cutoff, so I don't think it is an issue. I repopulated the chassis with all NOS RCA tubes after I got it running initially, so I may put in all the old tubes in the deflection circuits and see if it makes a difference. I am concentrating on getting the HV back. I can't do anything without a raster. I'm going to have to break down and buy a HV probe. I'm not going to be able to get along without one it seems.

[William]

I just checked eBay and there are all kinds of them listed, with varying prices and styles. I even see one that looks like the kind we had at the Gamble Store way back when I worked there.

Bill

[electra225]

I think I want a Pamona. I just need to decide for sure if I need one. I have fumbled this project so badly.......

The only saving grace to this little faux pas is that even Shango makes one occasionally. I really messed this project up, got to running down a rabbit hole. Besides not knowing what I'm doing, I don't have the proper equipment to do it. Some of the more experienced TV guys can tell how much high voltage they have when they short out the HV section with a screwdriver. I have no idea. What I thought was high voltage was something, but it wasn't high voltage.

I was stuck, had no idea where to go next, and was embarassed to ask for help. The guys on VideoKarma are willing to help, but I don't even know what to ask. Help! is hardly giving them much to go on.....

I decided to remove the horizontal output tube and the damper, then treat the chassis like a radio chassis. Then to take voltage measurements at key locations in the high voltage/deflection section. First up was the 6CG7/6FQ7 horizontal oscillator. You RCA stereo owners may recognize this tube as the first audio tube in RCA stereo amp chassis. This is a dual triode tube. Pin 1 which is a plate has 280 VDC on it with spec 210 VDC. Pin 3 which is a cathode has 9 VDC where 5 VDC is spec. The other plate has 0 VDC where spec is 210 VDC. Pin 8, the cathode for that section has 300 VDC, where spec is 5 VDC.
The 6DA4 damper has zero voltage on the plate, pin 5. Should have 240 VDC or so. Of course, there would be no negative voltage on Pin 5 of the horizontal output tube, therefore starting the high voltage. The only reason I didn't fry the HOT was that there was no B+ on the damper, thus no B+ on the high voltage rectifier, so the HV could not have worked even if there had been negative voltage on the grid of the HOT.

An interesting fact about this whole mess is that the damper is fed from the same 245 volt leg of B+ that the working half of the horizontal oscillator is on. The non-working half of the horizontal oscillator is on a 235 volt leg of B+ that the vertical oscillator is on and its voltages are normal. Therefore, I have a break somewhere in B+, probably on the terminal strip on the backside of the chassis. I have no boost voltage, but I THINK that is because there is no high voltage. Boost voltage is developed in the flyback transformer (horizontal output transformer). This is also why there is no voltage on Pin 6 on the CRT, because this 245 volt leg feeds the brightness control. I have had the belief for some time that I didn't have any B+ voltage. Nothing worked except the filaments all lit. I am going to revisit the audio and video boards and measure voltages there again. They were fine the other day. I need to make sure voltages at the 6GC5 cathode follower are correct. And to check the various branches of the three B+ voltages, 120 VDC, 235 VDC and 245 VDC.

[electra225]

I have found that the voltages on the 6GC5 cathode follower are not right. The 245 VDC leg is good, the 120 VDC leg is good, but the 235 VDC leg only has about 150 volts on it. I'm told there is a B+ branch connection that may corrode or there may be a cap that has opened or a resistor that has drifted, killing or lowering B+. I figure I got lucky by not having plate voltage on the damper tube. Had it been working, with the horizontal oscillator not working, I would have likely killed the horizontal output tube or maybe even damaged the flyback transformer. In the future, when I work with a TV with a dark CRT, I will pull the horizontal output tube and the damper to give me time to troubleshoot without further damage. I still want to verify voltages in the IF strip before I pull the chassis for repairs. I'd like to figure out how to power the chassis while on the bench so I can test voltages without having to remove it from the stereo cabinet again.

[William]

It sounds like you have made some progress, Greg, and any progress is always good. I enjoy reading your posts on your adventure with TV, I just wish I understood just a little of what you are talking about.

Bill

[electra225]

So do I....

And I learned something that everybody else probably knew. The guys who are knowledgeable about TV in videos that take shortcuts are not doing those trying to learn any favors by featuring shortcuts in their videos. The old "jump spark" method of checking high voltage is dependant upon knowledge of how much spark you need for correct voltage. I heard something under the suction cup of the anode wire that I THOUGHT sounded like a spark, but obviously wasn't. When you are starting out especially, you need the right equipment. If I had had a high voltage probe, I wouldn't have chased my tail so much. Since I have discovered that this TV has been tinkered with before I got it, I need a high voltage probe to make sure I have the CORRECT high voltage, if I ever get it back.

[electra225]

I have the chassis disconnected, ready to pluck from the cabinet. It wasn't as bad as I remembered. Removing the tuner will be as bad as I remembered. I need to get it out to check voltages. I believe I can lay this chassis on its front, with the tubes facing down, and power it up to check voltages. If that is the case, what a blessing that will be. I'll leave the HOT and damper out so I won't have any high voltage. I can check B+ and the oscillator should still be able to run.

[William]

Good luck, Greg, and be careful around all that high voltage.

Bill

[electra225]

I will have the high voltage section disabled with the tubes out. I can power up the rest of the chassis like I would a radio chassis. Keep one hand in my pocket and follow the meter when I work with a component. All I want to test is B+.

[electra225]

Today is Western Red day, so while I listen to arguably the best country music program on radio anywhere, I'll dig into this TV chassis and its ailing B+. Rube Goldberg designed the line voltage distribution system, I'm sure. Line voltage enters via the interlock on the rear of the cabinet, then goes to the TV chassis, then back to the remote control on/off switch, then to the remote, or to the tuner, depending on the position of this switch. There is a RCA plug on the TV chassis that goes to the motor on the TV tuner. There is a line plug that goes to the amplifier that eventually powers up the tuner and MPX chassis. The remote control operates three levels of volume up and down, changes TV channels, turns the entire instrument on or off. It has no control whatsoever on the record player or radio tuner. It may control radio volume, but I don't know that yet. The TV chassis is powered from the tuner. I have pictures of this mess and will share them in due course.

[electra225]

I did find the reason I had no B+ on the damper tube. There is a choke in the B+ rail ahead of the damper tube. One wire on the choke had broken. I will accept half the blame on this one. The break was about 1/4 of an inch or so below where I had soldered in the new damper tube socket. The original socket was plastic and was partially melted, so I had replaced the damper tube socket with a nice ceramic one. I don't see any reason for no B+ on half of the horizontal oscillator tube. All the components in the B+ circuit on both plates in that tube check good. A 6200 ohm resistor, a 120K ohm resistor and a choke all check good. They checked good on my original inspection. This may be stupid idea, but I'd still like to power up this chassis while on the bench to check B+.

[TC Chris]

There is always a reason for the mysteries, isn't there?

Chris Campbell

[electra225]

I have a question that will require someone with more experience to answer. The wire from the TV tuner to the IF strip is shielded. The shield wire has been cut at some point. Will the shield wire being cut kill the output from the TV tuner?

Everything else checks out on the chassis. I found a .047 cap in the high voltage section that is a 630 volt cap and should be a 1000 volt cap. That I'll have to change. That is my fault, I put it there. Nothing looks burnt or that it has been too hot. I have found and have identified the three B+ rails, along with the boost line. I haven't answered the question as to why I have hum in the dead audio. Could it be because there is no boost voltage (B+) to the audio detector? I still have a couple things I need to complete, then check the video detector diode. Small progress is still progress.

[TC Chris]

Sometimes shields are grounded at only one end, to prevent ground loops. That can cause hum in audio. Not sure if it creates problems in RF.

Chris Campbell

[electra225]

I may not have explained it right. There is a four contact flat plug in the wiring between the TV tuner and the TV chassis so you can remove either component without cutting any wiring. The ground for the shield runs externally from this plug, and it has been cut at some point. I am trying to come up with a reason that I don't have any audio nor any video out of this TV set. I am checking everything, trying to cover my bases.

I changed the AGC resistor that had drifted. I checked the detector diode and it checks good. About 100 ohms resistance one way, open the other way. I need to resolder the shield over the detector. AES has the 1000 volt cap I need. They are pretty spendy, but I have to get supplies there anyway, so that will work.

If that open shield won't affect the tuner in any way, I may be down to having tuner trouble. If I get a raster on the screen, I can check that issue further.

[electra225]

I promised some pictures, so here you go.....

Here is one reason I had no high voltage. This choke goes to the plate on the damper tube.

Some general pictures of the miracle that is a Magnavox Stereo Theater. It is a miracle that it actually works. Miles and miles of unruly wiring.

[electra225]

This TV chassis appears to have low hours. It probably broke early in its life and sat the rest of the time. There are a lot of little things amiss with this thing, so I could see that happening. There are four three-way power switches in this stereo. Actually five, if you consider the function switch. The main power switch, the remote control switch, the remote control itself, and the switch in the changer. The series 77 tuner in this stereo doesn't exist in any service documents I have come across. I wonder if it was built specifically for the remote control-equipped 400 series Magnavox Stereo Theater models. If I remove the damper, horizontal output tube and the vertical output tubes, why couldn't I run this chassis on the bench long enough to test B+? There is no audio output trans, and the horizontal (flyback) and the vertical output transformers would both be without power. Why wouldn't that be okay? I could handle it like I was working on a radio at that point.

I'm told that the severed shield may be used as a return for the filaments and B+ in many cases. The tuner is mounted on wood, so it would need a "ground" or a neutral of some kind. There are four wires in the cable from the tuner to chassis. The drawing does not show this shield or what it does. I'll fix the breach to be on the safe side.

[TC Chris]

so with all those power switches, do any of them kill the video B+ to preserve tubes when not watching TV?

Chris Campbell

[electra225]

The function switch turns the TV off so you can use the stereo features by themselves. But you have all 36 tubes in the stereo hot when you have the TV on. Even the Multiplex adapter. The Energy Star people would have a field day with this thing..... I'm guessing it will pull around 300 watts with the TV on with everything working right.