1950s or '60s vintage Zenith TVs, Need some info

[vortalexfan]

OK so I powered up the TV with R54 and R56 tied together and C3B out of the picture and the voltage went up to 145V (still down a bit from where it should be but not as much, the voltage I think is still getting lost after C50 somehow because the voltage before C50 is 145V and after C50 its becomes a negative voltage that is in the 10v range, and then from there the voltages at pins 5 and 6 of the 5BQ5 output tube become basically non-existant.

[vortalexfan]

OK so I put C3B back into the circuit and the voltage is still around 145V upto C50 then after C50 the voltage drops off to basically nothing, so I'm suspecting C50 and C51 are both defective which is why the voltage is non-existant at p5 and p6 of the 5BQ5 tube socket.

[Kevin Kuehn]

Originally Posted by vortalexfan

OK so I powered up the TV with R54 and R56 tied together and C3B out of the picture and the voltage went up to 145V (still down a bit from where it should be but not as much, the voltage I think is still getting lost after C50 somehow because the voltage before C50 is 145V and after C50 its becomes a negative voltage that is in the 10v range, and then from there the voltages at pins 5 and 6 of the 5BQ5 output tube become basically non-existant.

Unfortunately I can't follow what you're doing there. R54 being 390k cannot pass the kind current that you have being drawn through the 475 ohm( R56). The fact that B+ came up that much when you disconnected C3B is a strong clue you'll find more of your issue very close to that same area. Is R56 still in fact measuring 470 ohms, or has it been overheated and drifted much higher? You may get another clue if you temporarily disconnect either the blue or red leads of the output transformer, one lead at a time, while monitoring the voltage at the left end of R56. Once all loads are disconnected B+ should measure the same between both ends of R56 to ground. No load equals no voltage drop, so your goal here is disconnecting current paths until you find where the higher than normal draw is coming from. When you say you have no voltage on pins 5 or 6 of the 5BQ5 that strongly indicates a short some place before those points.

[Kevin Kuehn]

Originally Posted by vortalexfan

OK so I put C3B back into the circuit and the voltage is still around 145V upto C50 then after C50 the voltage drops off to basically nothing, so I'm suspecting C50 and C51 are both defective which is why the voltage is non-existant at p5 and p6 of the 5BQ5 tube socket.

If C50 is shorted you could have positive bias on the grid(pins 1-7) of the your 5AQ5 and that tube would be very hot. Did you measure voltage on pin 1 and 7? I thought not.

[Kevin Kuehn]

While you're at it go ahead and lift one end of C50 and 51. They don't need to be in circuit to get correct DC voltages on pins 2,5, 6, 1-7 of the 5AQ5. All awhile keep in mind that defective(open) caps will not have any affect on DC current draw, they would need to be leaky or shorted.

[vortalexfan]

Originally Posted by Kevin Kuehn

If C50 is shorted you could have positive bias on the grid(pins 1-7) of the your 5AQ5 and that tube would be very hot. Did you measure voltage on pin 1 and 7? I thought not.

I checked the 5AQ5 tube in the TV and it turns out it has a H-K Short, which would explain why there was there was not much voltage on pin 2 which is the cathode of the 5AQ5 tube, as for pins 1 and 7, pin 7 is empy, it has nothing tied to it, pin 1 has one of the leads from the volume control pot tied to it, if that makes any sense.

[Kevin Kuehn]

Originally Posted by vortalexfan

I checked the 5AQ5 tube in the TV and it turns out it has a H-K Short, which would explain why there was there was not much voltage on pin 2 which is the cathode of the 5AQ5 tube, as for pins 1 and 7, pin 7 is empy, it has nothing tied to it, pin 1 has one of the leads from the volume control pot tied to it, if that makes any sense.

If I'm not mistaken pins 1-7 are internally connecting inside the tube. So now we have a heater to cathode short. I'm feeling dizzy from being dragged around in circles.

[vortalexfan]

OK so I installed a NOS 6AQ5A tube into the audio output tube spot and I measured pins 1 and 7 (I looked at the RCA Receiving tube manual and saw that pins 1 and 7 are connected internally which is why nothing is connected to pin 7 of the tube socket) and it says on the Schematic that there should be 0V there and I'm measuring -5V there or so.

As for pins 5 and 6 I'm getting -22V there which doesn't quite make any sense unless like I said C50 and C51 were bad because before C50 I've got ~145-150V there and but then on the other side of C50 I have a negative voltage of well under 100V.

[vortalexfan]

Originally Posted by Kevin Kuehn

If I'm not mistaken pins 1-7 are internally connecting inside the tube. So now we have a heater to cathode short. I'm feeling dizzy from being dragged around in circles.

I apologize for that, I've not worked on a TV that's this complicated before (the Meck TV I worked on was a lot Simpler than this and it just worked once I recapped it, it didn't have all of these weird complications that this Zenith has been having, and the funny thing is that the Meck TV was stored in harsher conditions than this Zenith was and it still worked without much issue after a full recap.)

[Kevin Kuehn]

Originally Posted by vortalexfan

OK so I installed a NOS 6AQ5A tube into the audio output tube spot and I measured pins 1 and 7 (I looked at the RCA Receiving tube manual and saw that pins 1 and 7 are connected internally which is why nothing is connected to pin 7 of the tube socket) and it says on the Schematic that there should be 0V there and I'm measuring -5V there or so.

As for pins 5 and 6 I'm getting -22V there which doesn't quite make any sense unless like I said C50 and C51 were bad because before C50 I've got ~145-150V there and but then on the other side of C50 I have a negative voltage of well under 100V.

What is your reasoning about C50 and C51 that makes you think they could make pins 5 and 6 go negative? Can capacitors reverse the polarity of DC voltages?

[vortalexfan]

Originally Posted by Kevin Kuehn

What is your reasoning about C50 and C51 that makes you think they could make pins 5 and 6 go negative? Can capacitors reverse the polarity of DC voltages?

Those are the only components in the path of the area where I'm getting those funky readings besides R56 and R54 but when I asked about those resistors you said that they weren't going to cause those kinds of issues.

Also C3C and C3B are around there as well but those like I said they are hooked up like they're supposed to be.

The only other thing I can think of is C3C which is a 20MFD 25V electrolytic (which I replaced with a 25MFD 100V electrolytic) that the schematic shows is in parallel with R57 which is a 680 Ohm 1W resistor going to ground from pin 2 of the 5AQ5 tube.
The way I wired up that capacitor is that I wired up the positive side of the capacitor to the same terminal on pin 2 of the 5AQ5 tube socket as R57 was wired to and then the negative side of the cap I wired to the same "chassis ground" terminal that R57 was wired to rather than just tying the positive lead of the capacitor onto the side of the resistor facing pin 2 of the 5AQ5 tube socket and tying the negative side of the capacitor to the side of R57 that was facing the "chassis ground" terminal.

[Kevin Kuehn]

Originally Posted by vortalexfan

I apologize for that, I've not worked on a TV that's this complicated before (the Meck TV I worked on was a lot Simpler than this and it just worked once I recapped it, it didn't have all of these weird complications that this Zenith has been having, and the funny thing is that the Meck TV was stored in harsher conditions than this Zenith was and it still worked without much issue after a full recap.)

I've got a couple dumb questions.

What DC voltage do measure from end to end of the 470 ohm R56?

And from what point have you been getting your B- or ground reference when taking DC voltage measurements? The reason I ask is that you're 245 B+ measured about 25 volts low, and also you mentioned measuring -22 volts at some point.

[vortalexfan]

Originally Posted by Kevin Kuehn

I've got a couple dumb questions.

What DC voltage do measure from end to end of the 470 ohm R56?

And from what point have you been getting your B- or ground reference when taking DC voltage measurements? The reason I ask is that you're 245 B+ measured about 25 volts low, and also you mentioned measuring -22 volts at some point.

This may sound weird but now suddenly I'm getting 210V before and after R56, and R56 is not getting hot to the touch, but I'm still getting the weird voltage readings after C50 though, and still getting low voltage readings at pins 5 and 6 of the 5AQ5 tube socket.

I removed the output transformer completely from the circuit and took voltage readings and I'm getting 0V now at pins 1 and 7 of the 5AQ5 tube socket and pin 2 around 5V on the cathode and and still getting 22V on pins 5 and 6 of the 5AQ5 tube socket.

The -22V figure came in because of the fact that for some reason or another when I take voltage measurements on anything after C50 all the voltages are negative unless I flip my probes so that I use my positive probe as the negative probe and the negative probe as my positve probe and then I get positive voltage readings, and my reference to ground is the chassis because that's what its supposed to be according to the schematic.

[Kevin Kuehn]

Originally Posted by vortalexfan

This may sound weird but now suddenly I'm getting 210V before and after R56, and R56 is not getting hot to the touch, but I'm still getting the weird voltage readings after C50 though, and still getting low voltage readings at pins 5 and 6 of the 5AQ5 tube socket.

I removed the output transformer completely from the circuit and took voltage readings and I'm getting 0V now at pins 1 and 7 of the 5AQ5 tube socket and pin 2 around 5V on the cathode and and still getting 22V on pins 5 and 6 of the 5AQ5 tube socket.

The -22V figure came in because of the fact that for some reason or another when I take voltage measurements on anything after C50 all the voltages are negative unless I flip my probes so that I use my positive probe as the negative probe and the negative probe as my positive probe and then I get positive voltage readings, and my reference to ground is the chassis because that's what its supposed to be according to the schematic.

You really don't want to run the set with the output transformer removed other than to take quick measurements because the 5AQ5 screen grid(pin 6) is still getting B+ and it's not healthy for the tube without the plate also getting B+. I really can't follow what you mean when you say everything after C50 is negative? Exactly where are you measuring negative voltage in respect to B- ?

I'd like to suggest we focus only on this little section instead of jumping all over the map. C50 is only a audio coupling cap that passes audio from the detector stage to the volume control, so we're really not worried about anything other than this little portion of the schematic. Once we figure out what's ailing the audio output then we can move onto the bigger picture.

[Kevin Kuehn]

For our OP I sketched over the schematic using conventional current the direction DC flows from B+ through the circuit and returns to ground. The 5AQ5 being self (cathode) biased sets the tubes quiescent current. Under normal operation there should be no DC flowing in the volume control or grid circuit. C50 is the grid coupling cap that's purpose it to isolate the 5AQ5 grid from the preceding plate circuit's DC. C51 basically damps out very high frequencies in the output trans primary. It's this flow of quiescent DC current that produces the appropriate voltage drops across the resistance or impedance of the various components. Healthy capacitors should not pass DC therefore would not change this quiescent or idling current in the circuit. You could remove C50 and 51 and the circuit should still work completely isolated from the rest the TV. Normally if you touch a screwdriver or finger to the grid or volume control it will produce a buzz in the speaker. Anyhow I though it might help remove some of the mystery surrounding how this little section of the TV works. Once you understand what's suppose to happen it sometimes helps to find the source of trouble.