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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 2:19 pm 
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Joined: Dec Fri 13, 2019 9:33 pm
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Thanks for explaining that to me Tom. I've never seen them before but that explains it. The TV was overscanning but I've made adjustments and can get an okay picture but it is jittery. I am still not certain I have the tuner wires connected correctly. My tuner seems to be a bit different than the one shown in SAMs.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 4:46 pm 
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Location: Pewaukee, WI
The connection point you used is probably wrong. The tuner was originally supposed to have a shield covering the bottom and sides (as viewed with tubes pointing up) completely hiding the wafer switch... Sloppy techs would pull the shield and discard it (bad practice that looks like your set was victim of).... usually the connection points are brought through the top of the tuner to solder lugs or feed through capacitor terminals exposed top side....look for those first. I think some early sets (but not predictas) had connections inside the shield and ran out via wires.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 5:32 pm 
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Hi. Thanks for your information. I have the shield off temporarily because of the broken wires. In the pic, I had these connections not through the top, just for testing. I have since repaired the plug by adding the wire that was missing, cleaned up the connecting points in the tuner and neatly re-connected them through the top. I can get a pretty good picture now but the vertical sync is not good, there is instability in the upper 25% of the picture and the sound is still low. I'm going to re-examine the tuner connection points. Also, I want to make the tuner is not the source of some of my problems. I should see if I can bypass the tuner. I don't think it demodulates the signal so maybe I can disconnect the rf cable and directly feed into the next stage. With this tuner, I can get channel 3 on channel 3 and also pick up a channel 3 with the tuner selected to 11 or around there. I am not sure if that is normal.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 7:03 pm 
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Location: Pewaukee, WI
Are you using videotape for your signal source? If so that will explain the likely horizontal instability at the top quarter. VCRs put out non-standard sync timing that is beholdent to the accuracy of the speed of the motors in the VCR (in consumer grade VCRs the accuracy is pretty poor). Sets made after 1980 tend to have faster horizontal time constants and advanced counter based sync separators and will minimize flag waving at the top of the picture, but early sets will exaggerate the problem. The solution is to get a time base corrector and place it in the signal chain between the VCR and the TV or to switch to DVD or other NON-tape source.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 8:24 pm 
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Hi,

Yes I am using a VCR. I have a DVD player and TV test unit as well so I'll try that out. Thanks again.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 13, 2020 11:15 pm 
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Signals from the test equipment or a DVD player definitely work better as there is more stability at the top. I think the tuner is a major source of the problem however. I can pick up channels on channel selections opposite of the correct ones. Also, when I feed a channel 3 test signal and include a 1khz tone, I can barely hear the tone when the tuner is on channel 3 but I can hear the tone much more loudly a few channels over.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Tue 14, 2020 1:46 am 
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Location: Ann Arbor, MI
As for picking up a signal on the other side of the dial: I think it is likely that you are picking up a harmonic of the channel 3 signal. Every signal generator, such as a modulator, will produce some output at twice and 3 times the main frequency. I think that is what you are getting.

As for the sound being better when tuned to channel 4 instead of channel 3: It does sound like the tuner is a little off frequency. First, put the shield back on. The shield being off could de-tune things some. Then turn the fine tuning back and forth. See if that helps. If the sound gets better all the way to one end of the fine tuning, you may have to adjust the oscillator adjustment. The oscillator adjustment is like a coarse fine tuning. If you look at the front of the tuner you will see a series of holes in a circle with screws behind them. Those are the oscillator adjustments and there is one for each channel. If you have the Sams for the set, it should tell you which is which. You need a non-metallic screwdriver to adjust them. You would adjust the one for channel 3.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Tue 14, 2020 2:20 am 
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Joined: Dec Fri 13, 2019 9:33 pm
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Hi. Thanks for the suggestions. I am familiar with harmonics but I expected the signal to be significantly weaker on the other channel but it is not or at least the TV image looks the same. The tuner has the channels tuning holes marked with the channel number on the metal. I'll put the shield back on and play with the fine tuning adjustment screws with a non-conductive device as you suggest. Thanks again.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Tue 14, 2020 3:31 am 
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With the shield on, the picture is stable. Just as bit of vertical elongation but overall it has an excellent picture now. So the tuner may be okay. I can't improve the sound yet. It is still very low and peeks when the channel is tuned a bit off frequency but even then, is still noisy and low. The audio worked perfectly before I replaced an intermittent tube socket in the audio section and at the same time replaced a ton of components. So I likely have an incorrect value of one or more capacitors. It is not in the in the audio amplifier section because I injected a 1khz tone into that section and it is good. The problem is somewhere else.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Thu 16, 2020 10:59 pm 
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I have a question about something that may be a clue to the low volume audio problem. The 3BN6 tube is used as the audio detector. I am not sure if this is a clue or not but on pin 2 (which is a grid pin measuring 0VDC) when I attach a scope lead to it or touch it with even a small screwdriver, the sound completely disappears.

The A12 transformer output (if that is what it is called) is connected to grid pin 2, and measures 2.7 ohms to gnd which is normal. How could I be killing the sound with a high impedance scope probe or with the end of a screwdriver? Is it because the frequency is high at that point? Thanks again.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Thu 16, 2020 11:32 pm 
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penelopepug wrote:
I have a question about something that may be a clue to the low volume audio problem. The 3BN6 tube is used as the audio detector. I am not sure if this is a clue or not but on pin 2 (which is a grid pin measuring 0VDC) when I attach a scope lead to it or touch it with even a small screwdriver, the sound completely disappears.

The A12 transformer output (if that is what it is called) is connected to grid pin 2, and measures 2.7 ohms to gnd which is normal. How could I be killing the sound with a high impedance scope probe or with the end of a screwdriver? Is it because the frequency is high at that point? Thanks again.

With either the scope probe or the screw driver you are adding some capacitance to ground which will change the tuning of the transformer. My 10x scope probe is specified to have 13 pf to ground.

Basically any two conductors separated by any insulator is a capacitor. So you and the earth form a capacitor and the screwdriver and you and also the earth form a capacitor.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 20, 2020 11:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1687
Location: Phoenix, Arizona
I used to repair Predicta's in my misspent youth and I hated it when one cam in for repairs.

I always replaced all the tube sockets on the sweep board. The metal that grips the tube pin wraps over the top of the Bakelite socket and then to the part that pushes through the board. Where it wraps over the top is where it cracks and breaks. The metal can't be solderer'd and was plated where it solders to the PC board. And of course the solder is always broken loose on some pins, often the filament.

The same problem exists with the wirewrap pins and I don't believe you can get solder to flow.

The Gated Beam audio discriminator was a good design but the parts need to be on value and adjusted properly.

I have never done repair work on an IT PC board.

I have a pedestal Predicta and the sweep board is still original and actually still works. I replaced one capacitor by soldering on top and several thermistors in the warmup circuit. I also have a rebuilt CRT from Hawkeye.

Jim

Flyback transformers are touchy and I hate opening the can.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Mon 20, 2020 11:29 pm 
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Hi Jim,

Thanks for your information. Can you please tell me what the sweep board is? I've only replaced one socket that was intermittent in the audio section. I checked all others and they are good ( I think....). I have a pretty good picture now but the audio is still very low. I can pick up the audio a bit better on adjacent channels or when I adjust the fine tuning off frequency it improves but is still poor. I cannot adjust the iron cores in the audio section because they are seized. Do you have any tips on sorting out the low audio problem? I should mention too that audio amplifier section is good. I injected a 1KHZ sign wave into it and it was clear and reasonable volume.

Also, do you know if there is a way to bypass the tuner to determine if the tuner is the problem?

Thanks.

NP


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Tue 21, 2020 2:25 am 
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Some test equipment manufactures made tuners in a box to be used as a substitute tuner for test purposes. They may show up on ebay from time to time.
But I really don't think that there is anything wrong with your tuner.

Perhaps a little more on how things work. The main IF strip is designed to amplify some frequencies more than others. This is required because of the way the signal is transmitted. You could look up 'vestigial sideband' if you want the gory details. In addition to amplifying some frequencies more than others, it also purposely amplifies the sound carrier much less than everything else. This is done to prevent the sound carrier from showing up in the picture as a fine grain. This should be more than made up for by the gain of the sound IF strip if the sound IF strip is working properly. Now when you adjust the fine tuning or switch to a different channel you are moving all of the received signal over into a different part of the main IF strip pass band (the band of frequencies it amplifies). One effect of this is that you can move the sound carrier into a section of the IF strip pass band where it amplifies things more.

The bottom line is that the increase in sound when you adjust the fine tuning or switch channels is expected behavior. If you look in the alignment section of the Sams, there is a graph of the frequency response of the main IF strip.

I suspect that in replacing parts in the sound IF strip you slightly changed the various stray capacitances and therefore changed the tuning of the tuned circuits. Replacing a physically large part with a physically small part can do that. Perhaps someone can chime in here with a method to un-seize those cores. I suspect that adjusting them will fix things.

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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Jan Tue 21, 2020 4:06 am 
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Location: Belrose, NSW, Australia
I agree, the sound IF should be aligned.

I've done a lot of these rescues (I used to work in the service industry) but I need to know a few things first:

Paper-phenolic or plastic formers?
If plastic, is it a thermo-softening plastic or thermo-setting (i.e. does it melt?)
Hex hole or screwdriver slot? If screwdriver, does the slot go all the way through the cores? If it does, are the cores cracked?
Has the screwdriver slot been butchered?

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Wax, paper, bitumen, cotton, high voltages - what could possibly go wrong?


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Feb Mon 10, 2020 1:12 am 
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Joined: Dec Fri 13, 2019 9:33 pm
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Hi. Sorry for my late response. All the adjustable inductors in the audio paths have hexagonal metal cores. None were cracked but they are all seized. The good news is that I was able to purchase a parts pc board and transplanted the 3 components over. Now that I could adjust the audio inductors, I was able to get excellent audio, loud and clear. There is a slight buz but it is quiet and I could not get rid of it, even via the buzz pot. Unfortunately I had a double image after i re-installed the pc board and I determined the horizontal multiplier was operating at a frequency out of range. I stuck a small screwdriver in the core of L22 because it was seized and this changed the frequency range. I was then able to adjust the vertical hold and suddenly the double image was gone. To solve this problem permanently, I installed a larger capacitor (c42) to bring the frequency into range.

The TV operates great now with a clear picture, very good and loud audio and the it syncs easily. The only thing that is a bit strange is that adjusting L17 (part of the audio detector) has no effect on the audio.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Feb Sun 16, 2020 11:29 am 
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Joined: Jan Thu 16, 2020 12:29 am
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I'm not sure if anyone on this forum is familiar with this trick, I have used it many times to remove cracked & broken slugs. It works if the slug has a screwdriver slot and even better with a hexagonal hole.

The problem is that the if the slug is cracked in two, when you attempt to rotate it with a tool, the two halves are pushed apart and this expands the average outer diameter of the slug, but worse than that, its sharp cracked edges dig into the former threads and prevent it from rotating.

So to get around this I use a very small amount of 24hr epoxy resin on the tool, (small amount, if too much it will spread across the slug surface to the threads and if that happens it game over), position the tool correctly while it dries. 24hrs later the tool will rotate the slug without it jamming in the threads and usually its possible to get it out and replace it, saving the coil.

This method also can work if one side of a slotted slug is broken away.

If the slugs are not cracked, but tight, try putting WD40 on them and leaving them for a day or two, it generally works better than heat and is less risky, you can clean it off later with CO contact cleaner.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Feb Sun 16, 2020 2:18 pm 
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Joined: Dec Fri 13, 2019 9:33 pm
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Excellent information! Maybe using vice grips to slightly compress the plastic tube would ensure the broken slug is repaired tightly. (for that type of inductor) I have two other seized inductors (not broken) that I'll try to free up using your tips. Thanks again.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Mar Fri 06, 2020 3:28 pm 
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I've nearly completed this Predicta restoration however I am trying to do final adjustments to the screen centering and for linearity. Can someone please tell me how the adjustments on the yoke work? They do not have any impact. I assume to adjust them, you spin the center tab CW or CCW or is it the outside insulated material adjusted?

I took the black plastic encapsulated part off the back of the yoke and was able to remove a circular (donut shaped disk) insulator but that's seems to be as far as I can go to access the mechanics of the yoke adjustments. I'm guessing the yoke adjustments move tiny magnets or metal to tweak the geometry and centering but I don't think mine are moving.

Any thoughts on this? I want to adjust the center and top linearity.

Thanks for any tips.


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 Post subject: Re: Philco Predicta Pedestal Audio Problem
PostPosted: Mar Fri 06, 2020 4:01 pm 
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Don't take the back off the yoke. There's nothing in there you need to adjust.

You bend and twist the bar magnet on the left and turn the two circular magnets on the yoke cover.

This photo is from a Predicta Tandem set so ignore the circuit board on the right.

Image


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