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 Post subject: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Fri 13, 2019 9:49 pm 
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Hi. I've been electrically restoring a Philco Predicta Pedestal TV. I've replaced all electrolytic and paper capacitors and all out of tolerance resistors. All the k modules have been rebuilt (replaced) on the main board. I have a problem in the final audio section. The 12CA5 tube (V7) has about 139 volts on pins 2 and 5. It should have about -1.5VDC on those pins. All other voltages are good. I am waiting for a couple of tubes for my Stark Tube Tester so I cannot confidently test the tube but I had bought an NOS 12CA5 and the results are the same.If I disconnect the 260 volt source from R45 (820 kohm) the voltage goes to zero. Is it possible that I have 2 bad audio output tubes?

Thanks for any tips. PS -The audio works but could be louder and clearer.

NP


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 3:16 am 
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Check the schematic for notes on measuring the voltage on those pins. You may find that the voltages on those pins should be measured with the negative lead of the voltmeter somewhere other than circuit ground.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 4:49 am 
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What Tom said. That set uses the "stacked B+" configuration so the grid voltage is measured from the 140V source which feeds the cathode, not chassis ground. That is noted on the schematic.

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"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 3:34 pm 
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Thank you! That solves that issue. I have a few more questions;

Does anyone know where to get replacement tuning cores for the audio section? Mine are seized and I've cracked one trying to adjust it. Removing them might be challenging.

A couple of issues;

The audio seems to build up and get louder when background music is heard whereas when voice audio is heard, it is much lower volume. Also, the contrast control introduces noise when the contrast is increased.

The picture is quite good now however I can pick up the same channel on 2 different channel selections on opposite sides of the channel dial. Something must be wrong with the tuner. Anyone seen that before?

Thanks again.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 4:33 pm 
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Location: Albion, CA, USA, 95410
I have had the same problem with the audio tuner coils. The coil was open and the slug stuck. I ended up picking up a parts chassis to get one. This was on a Holiday.

I have experienced the same audio/music/speech issue. I have no idea why that is.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 6:04 pm 
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Thanks for the reply. I'm going to try heating the slugs to see if I can move them. They are likely set right already but I am trying anything to improve the audio. But I'll need a replacement slug or an entire adjustable inductor.

One thing that does not look right is that at trace point 26, I measure about 190VDC which doesn't make sense based on the schematic. Maybe I have a path from a high voltage point under the PC board. I replaced that socket as it was intermittent. I'll have to remove that board for the 10th or so time and have a look. That may explain why the contrast control effects the audio, adding noise when the contrast is increased.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 7:15 pm 
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Sets such as this one with intercarrier sound do sometimes get increased buzz or background noise with increasing contrast. Typical sets of that vintage are not engineering masterpieces, they found the simplest way to get what they saw as "acceptable" video and sound and went with it. As a result, everything has to be exactly right in order to get the best possible sound.

You will also probably notice an increase in buzz or background noise when there is white writing on the screen.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sat 14, 2019 7:22 pm 
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I think to some degree the contrast does affect the audio, especially when there are big differences in contrast on the screen, like credits or titles.

Another issue I had was the buzz control. Not sure if yours has one. Mine was a wire wound pot and was really dirty and needed a serious cleaning. I also found that I hadn't got the fine tuning set perfectly when I adjusted the audio and buzz pot and the best picture didn't end up where the best audio was.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sun 15, 2019 12:10 am 
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Thanks for all of the feedback. It appears some of those things are normal for the design and vintage. I do have a buzz control but the audio is quite suppressed if I adjust it to eliminate the buzz. The loudest volume also has the worst buzz. And as stated, via the channel fine tuning control, the best picture and best audio are not at the same spot for this TV as well. However I do think the audio worked better before and the high voltage on trace point 26 (and both sides of the volume pot, other than the 140 volts), doesn't look right. Also, now that I know to measure the grid voltages referenced from the 140 supply for the 12CA5, I am measuring just under +1vdc. It should be about -1.5 so I still may have an issue.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sun 15, 2019 2:20 pm 
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The quadrature coil and buzz control do interact. On your audio output problem, you may have a leaky coupling capacitor. Also, check the grid bias resistors. If the output voltage on the cathode is not correct, the stages supplied from there will not work properly.

Many manufacturers used the "stacked B+" output stage circuit; Admiral, Dumont, GE, and RCA, to name a few. All have similar problems.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Sun 15, 2019 11:21 pm 
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Stacked B+ audio output stage= a really bad idea that saved them the cost of a 50 cent power resistor to drop the B+ to the other stages it feeds.

If the grid is 1 volt + in relation to the cathode, then there is a problem, although that isn't going to quickly kill the tube nor will it drastically affect operation. Could be grid emission in the 12CA5, or a resistor off value, or a leaky coupling cap.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 1:44 am 
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Hi. I replaced C33 again but no change. The grid voltage on the 12CA5 is now about 0.18 volts. I'm trying to find the source of the high voltage on the opposite side of the volume pot, opposite of the 140 volt source. I disconnected R43 and R44 which come from the volume potentiometer and determined the high voltage source is coming from the PC board. The 3BN6 audio detector has 220 volts on pin 7 whereas it should be 115 volts. I tried replacing the tube but the results were the same. I also replaced C25 in case there was leakage but that did not help either.

Somehow the contrast pot is affecting the voltage on the volume pot leads that go to R43 and R44. When I adjust the contrast, a huge amount of noise is introduced into the audio. I've tried adjusting the fine tuning control and the buzz pot a million times but no luck so far. I wouldn't be surprised if I made and error or two in building the K2 module.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 5:49 am 
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While it is not usually a problem, rosin from rosin core solder can sometimes become conductive. That is more likely if the rosin is has been burned. So conductive rosin bridging two foils on a circuit board could introduce a voltage where there should not be a voltage. You could use 91% alcohol to remove the rosin. If it is badly burnt you may have to scrape it off.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 4:27 pm 
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Location: Albion, CA, USA, 95410
Have you replaced all of the resistors and caps on the PC board? I replaced all the tube sockets too, I had a couple of bad ones and considering the difficulty in removing the board I decided to do it all. With the bad audio coil I ended up taking it off again anyway.

Have you had someone check your work? Before replacing the board on mine I had my wife look over the board with the Sam's. Sure enough, she found an incorrect value cap.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 5:49 pm 
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Hi. Yes the pc board has been cleaned with flux remover and alcohol, every time. No I don't have anyone to review my work. I replaced one bad socket, tightened and cleaned the others, replaced all paper and ceramic capacitors and all out of tolerance resistors which was around 80% of them. I also built new couplates (k modules). Lastly, I cleaned and bent tube pins to ensure conduction. With all those changes in 2 batches, there is a lot of potential for errors. I did all of the resistance measurements and many additional resistance measurements as well. I'll remove the board again, recheck the couplates and look for the mysterious high voltage path.

Thanks for all the tips.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 7:14 pm 
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Bending tube pins won't help with the bad sockets in predictas.... their primary failure mode is not their contact with the pins of the tube. How predicta sockets typically fail is that the the metal tab of the socket that electrically connects the tube pin to the PCB will crack into 2 pieces inside the plastic of the socket...It will still grip the pin solidly, and from all appearances look fine, but if you touch a DMM to the tube pin hole and the solder contact on the board you will find there is no continuity due to the crack in the metal...the only good cure for a bad predicta socket is to change it.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 7:49 pm 
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Okay, thanks for that information. I'll try to check all socket pins.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Dec Mon 16, 2019 9:31 pm 
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What is the Philco part number for the coil with the broken slug? A part number is needed to locate a replacement unless one is scavenged from another set.

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"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Sun 12, 2020 11:50 pm 
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Hi! I'm working on the Predicta again. A family death kept me away.

Is there anyone that can verify the connection of the plug wires to the VHF tuner? The black wire is good but the orange wire disconnected or broke off and the white wire broke off from the tuner and the plug. According to the tuner schematic, pin one of the plug is the white wire that should go to point "T", so I temporarily attached an alligator lead from that point on the tuner to pin one of the plug. But I am not sure if I have the right point on the tuner. The SAMs picture points to what I thought was a solder stud. Is this actually point "T" and also one side of C206?

The orange wire should go to C220 but again, this looks like a solder stud. After hooking these up, I get mostly raster. I Can get a picture but it is very weak and unstable with bad or no vertical sync.

Just a note that the white wire must have been disconnected from the tuner and the plug for weeks. The picture what quite good before but the sound was very low.

I've attached a picture of the current tuner hookup. Thanks for any help!


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Philco-Tuner-1.png
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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 6:08 am 
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What you are calling a solder stud may well be a type of capacitor. There is a type of capacitor that is a ceramic tube with metal on the inside and on the outside. There is a wire passed through the tube and soldered to the metal on the inside. Often the wire comes out both ends of the tube. These capacitors are usually placed in a hole in one of the metal shields and the outside metal is soldered to the metal shield. These are often used as feed through capacitors. Power may be applied to one end of the center wire and taken off of the other end. That way power can be gotten into a compartment but any possible signal or interference is bypassed to ground by the capacitor.

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