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

[vortalexfan]

Originally Posted by Kevin Kuehn

.01 MFD 630V film is fine. We know what the B+ is supposed to be on the detector side, and there should be no B+ on the volume control side.

OK, well I'm going to just take a break from these TVs for a while and work on some of my old radios for the time being. I'm so close with these TVs but like I said there's still a few bugs with the tuner and audio on them yet that need to be worked out and I'm afraid I might screw these up if I don't take a break on them.

[vortalexfan]

Ok so I slept on it a little bit and came back this morning and I went back and checked my voltages again, and the B+ voltage going to C3B and R54 and R56 is around 210V (which I'm going to chalk that up to the fact that the voltages in the Sam's are taken using a VTVM and I was using a DMM which might be why the voltages I'm measuring are a little off) and then the voltage going to pin 7 of the 3BN6 tube is around 145V which is only 10V higher than the voltage given for that pin in the Sam's (again I'm going to attribute that to the fact that I'm not using a VTVM here.)

But I'm still getting some funky readings on the output tube yet which I'm still kind of stumped about, I'm still getting NEGATIVE voltages on pins 5 and 6 of the 6AQ5 tube and also on pin 2 as well (and yes NOTIMETOLOSE I understand that doesn't make sense to you but that is what I am getting.)

I understand that the 6AQ5 tube is supposed to be self-biasing but whatever is in this TV's circuitry to make the tube do that doesn't appear to be working right otherwise I wouldn't be getting those funky voltage measurements and I would be getting a thunderous roar of white noise out of the speaker on the TV but that's not the case, the TV is dead silent, and I can't get the tuner to pull in the signal from my RF Modulator box which is being fed a signal from a DVD Player.

Before you guys ask, yes my RF Modulator works, I had it working on my Meck TV about a month ago so no its not my RF Modulator.

[vortalexfan]

UPDATE: Ok I read through some articles online (outside of here) about how self-biasing tube circuits work, so R57 which is a 680 Ohm 1W 10% tolerance resistor is the bias resistor, and I measured its resistance and it currently measures at around 753 Ohms which I think is a bit high for that application, would an out of tolerance bias resistor screw up the audio circuit, and give me the funky voltage readings I'm getting?

If so it seems I managed to figure it out on my own, which is what you were wanting me to do.

[Kevin Kuehn]

Most DMM have an input resistance of at least 1meg, so using your DMM would not significantly change the voltage reading from what Sams got using a VTVM. There are however notes on the Sams schematic stating the control settings they used when making measurements. That's because some voltages can vary with different control settings.

Your 5AQ5 680 ohm cathode resistor is not out far enough to cause a serious bias issue. But you do need a known good tube in there to move forward. I would think your 6AQ5 would work, if it is in fact a good one.

What perplexes me is how you seem to enjoy back paddling. Last we heard you said the funky negative voltages went away when you temporarily removed C50(.01uf). It was suggested that you remove that cap to correct the negative voltage situation. Have you done so, or replaced it with a know good cap? And if so what was the outcome? Now you're right back to wondering willy-nilly without a clue or sense of direction. I'm trying to provide you a sense of guidance, but like they say you can lead a horse to water, but..

[old_coot88]

Originally Posted by vortalexfan

...I went back and checked my voltages again, and the B+ voltage going to C3B and R54 and R56 is around 210V...

OK. That's close enough for now. At least you've verified B+ on C3b.

The identical voltage should be appearing on pin 6 (screen grid) of the 5AQ5, and on the bottom of the output xfmr primary (red lead).

In other words, there should be solid continuity (zero ohms) twixt all those points. (Forget the negative voltage issue for now) Main thing is to find out what connection's gone open.

With the set OFF, and an ohmmeter set to RX1, connect the black lead to pin 6. With the other lead, check for zero ohms to bottom of xfmr primary (red lead), and to C3b. You'll probably find the connection to C3b is what's open.

BTW, also check the xfmr primary for a few hundred ohms between pins 6 and 5 (plate) of the 5AQ5.

Once this is all sorted out, check for solid continuity (zero ohms) from C3's can to ground. Any resistance here could be contributing to the 'negative voltage' condition.

[vortalexfan]

Originally Posted by Kevin Kuehn

Most DMM have an input resistance of at least 1meg, so using your DMM would not significantly change the voltage reading from what Sams got using a VTVM. There are however notes on the Sams schematic stating the control settings they used when making measurements. That's because some voltages can vary with different control settings.

Your 5AQ5 680 ohm cathode resistor is not out far enough to cause a serious bias issue. But you do need a known good tube in there to move forward. I would think your 6AQ5 would work, if it is in fact a good one.

What perplexes me is how you seem to enjoy back paddling. Last we heard you said the funky negative voltages went away when you temporarily removed C50(.01uf). It was suggested that you remove that cap to correct the negative voltage situation. Have you done so, or replaced it with a know good cap? And if so what was the outcome? Now you're right back to wondering willy-nilly without a clue or sense of direction. I'm trying to provide you a sense of guidance, but like they say you can lead a horse to water, but..

Well my comments are due to the fact that it seems that my comments have upset certain folks on here and because of that I was trying to take their feelings into account (read notimetolose on post #239) and because of that I didn't want to have to read through anymore rant posts from people who were upset at how I was going about troubleshooting this TV (notimetolose's exact words were that I was making no sense and that I was being a smartass and that I have no idea what I'm doing when it comes to using test equipment which is not true.)

So that's why I'm trying to be more careful with my comments.

Anyways the reason why I went down the cathode bias resistor route was because you kept on referring to the fact that the Audio Output tube in this TV is Cathode-Biased (Self-Biasing) and so I was trying to look into that aspect of the circuit as to why my audio circuit wasn't working right and the voltages were not right.

So yet again it seems that I misread the situation when it came to reading your comments and what you were suggesting that I look into.

I was noticing that you like to throw out subtle clues in your posts when it comes to helping people troubleshoot circuit issues (sort of a way of telling a person what the issue is without actually telling them what it is outright so that they can figure it out on their own.)
I first noticed that when you were helping me with that Meck TV when I was having the issues with the dead short in the B+ circuit in that TV, you told me in a subtle manner to double check the polarity of the filter caps for the B+ circuitry (the big ones that were installed under the chassis originally that had the electrical tape around them) and at first when I looked at how they were installed they seemed correct in how they were installed but then one of the caps vented on me and then I looked at it again and I realized that I had the caps installed positive to positive and negative when it needed to be positive to negative in that situation and when I installed it correctly it was fine, but I wouldn't of figured it out without those subtle hints.

[vortalexfan]

Originally Posted by old_coot88

OK. That's close enough for now. At least you've verified B+ on C3b.

The identical voltage should be appearing on pin 6 (screen grid) of the 5AQ5, and on the bottom of the output xfmr primary (red lead).

In other words, there should be solid continuity (zero ohms) twixt all those points. (Forget the negative voltage issue for now) Main thing is to find out what connection's gone open.

With the set OFF, and an ohmmeter set to RX1, connect the black lead to pin 6. With the other lead, check for zero ohms to bottom of xfmr primary (red lead), and to C3b. You'll probably find the connection to C3b is what's open.

BTW, also check the xfmr primary for a few hundred ohms between pins 6 and 5 (plate) of the 5AQ5.

Once this is all sorted out, check for solid continuity (zero ohms) from C3's can to ground. Any resistance here could be contributing to the 'negative voltage' condition.

OK, I think I may have figured it out, and I should have figured it out sooner, because I had this issue before with the old GE tombstone radio I was working on previously, but I think maybe the negative lead of C3B isn't actually making contact with the chassis (its soldered in place but its not actually contacting the chassis its just "floating" there.

I'll check for sure when I get a chance, right now I'm trying to help my parents clean up water in the basement of their new house because the irrigation system backflow burst on them last night because it froze (because they forgot to turn the water off to the irrigation system before it started getting cold, because this was the first time they've had an in-ground irrigation system in a house so they didn't know you had to shut the water off to it.)

I may be slow on the uptake but I do eventually figure it out.

[old_coot88]

Originally Posted by vortalexfan

...I think maybe the negative lead of C3B isn't actually making contact with the chassis (its soldered in place but its not actually contacting the chassis its just "floating" there.

If the can IS actually floating electrically, it needs to be fixed as the first (not last) order of business. But there's still the separate issue of zero plate and G2 voltage (on pins 5 and 6 of the 5AQ5), as discussed previously. This wouldn't be caused by the floating can.

[vortalexfan]

Originally Posted by old_coot88

If the can IS actually floating electrically, it needs to be fixed as the first (not last) order of business. But there's still the separate issue of zero plate and G2 voltage (on pins 5 and 6 of the 5AQ5), as discussed previously. This wouldn't be caused the floating can.

1.) Yes, exactly, I do need to fix the floating negative on my replacement cap that I used to replace C3B that was in the can, and the floating negative lead would definitely cause my negative voltage readings in the audio circuitry.

2.) Yes I do still need to figure out the issue with the low voltage readings on pins 5 and 6 of the 6AQ5 tube socket and the none existing cathode voltage (which should be 17 volts but measures a measly 5 mV.)

3.) As for the floating ground issue, it is easy enough to do especially when you're trying to tie a new individual electrolytic cap negative lead onto a previously used chassis ground tie point that has tons of old solder already blobbed onto it.

[vortalexfan]

OK, I checked all of my negative lead connections on my electrolytics and they aren't floating, so that's not the problem.

But I think I definitely figured out this time for sure where my voltage is getting lost at on my Zenith Bugeye TV.

C49, a 470pF ceramic disc capacitor that goes from TS8-4 to chassis ground measures open (measuring the resistance across it, it quickly goes to O. L. from 25 MOhm and measuring it on my cap checker it measures 25pF, and when I measured the voltages across that cap my voltages were right there going to ground.

So am I onto something here?

[Tom9589]

There's something confusing about the SAMS schematic that Kevin Keuhn attached in post 192. We all know that dashed lines on a SAMS means an alternative circuit that may have come about on a later chassis.

Look at C3D. The alternate circuit has the positive side of C3D connected to R54. However, it also shows that the negative side of C3D is connected to +245V. That is definitely an error as C3 is a four section electrolytic with a common ground. Since C3A-C are all in the power supply, their negative sides all go to ground.

What gives? SAMS error?

[vortalexfan]

Originally Posted by Tom9589

There's something confusing about the SAMS schematic that Kevin Keuhn attached in post 192. We all know that dashed lines on a SAMS means an alternative circuit that may have come about on a later chassis.

Look at C3D. The alternate circuit has the positive side of C3D connected to R54. However, it also shows that the negative side of C3D is connected to +245V. That is definitely an error as C3 is a four section electrolytic with a common ground. Since C3A-C are all in the power supply, their negative sides all go to ground.

What gives? SAMS error?

What my hard copy of the Sam's for this TV shows is that C3A-C3C are in one can and C3D is a separate non-polarized cap in some models in a small note on the bottom of the schematic which is what that is all about.

I hope that helps you out.

[Kevin Kuehn]

Originally Posted by vortalexfan

What my hard copy of the Sam's for this TV shows is that C3A-C3C are in one can and C3D is a separate non-polarized cap in some models in a small note on the bottom of the schematic which is what that is all about.

I hope that helps you out.

Yes and the alternate circuit has C3D negative lead going to B+ and the positive to B Boost, so it's still polarized correctly for that scenario. I suspect they did it that way so that full B boost voltage wasn't across C3D, because B Boost likely swings above 500v during warmup. Off hand I have no idea why they went to a non polarized cap unless it's exposed to negative going spikes on the B Boost. If by chance you used a 450v cap for C3D it could become unhappy with full B Boost across it.

[vortalexfan]

Originally Posted by Kevin Kuehn

Yes and the alternate circuit has C3D negative lead going to B+ and the positive to B Boost, so it's still polarized correctly for that scenario. I suspect they did it that way so that full B boost voltage wasn't across C3D, because B Boost likely swings above 500v during warmup. Off hand I have no idea why they went to a non polarized cap unless it's exposed to negative going spikes on the B Boost. If by chance you used a 450v cap for C3D it could become unhappy with full B Boost across it.

Yes I did use a 450V cap there, but it is rated for 550V surge so it should be fine.

[Kevin Kuehn]

Originally Posted by vortalexfan

OK, I checked all of my negative lead connections on my electrolytics and they aren't floating, so that's not the problem.

But I think I definitely figured out this time for sure where my voltage is getting lost at on my Zenith Bugeye TV.

C49, a 470pF ceramic disc capacitor that goes from TS8-4 to chassis ground measures open (measuring the resistance across it, it quickly goes to O. L. from 25 MOhm and measuring it on my cap checker it measures 25pF, and when I measured the voltages across that cap my voltages were right there going to ground.

So am I onto something here?

Are you measuring C49 out of circuit? And even if C49 were open it would have no influence on the weird B+ voltages you're reporting. I'd say it's nothing more than a RF filter to the audio signal leaving the detector plate.

Next definitely for sure possibility please.