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 Post subject: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 12:40 am 
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Location: Coopersburg, PA USA
I'm back to work on this TV again after many months off of it. Sometimes, life just gets in the way.

To review, I'm a radio guy (an average one at best) who chose this difficult set mostly because of the unique styling.

Here is where I stand.

1. I'm feeding a DVD into the set.
2. I get clear and strong audio.
3. The set is pulling 1.5 amps at 115 volts - very close to spec of 1.56 amps.
4. I have absolutely nothing on the CRT.
5. I jiggled all the tubes to no avail.

Help this TV newbie out - what should I do next or look for? TIA

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Stan Saeger
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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 1:13 am 
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Location: Phoenix, Arizona
Nothing on the CRT could be bad.

I would look in the back of the CRT enclosure and see if you can see the filament lit up.

Missing high voltage would cause this problem.

CRT's in Predicta's were trouble from the beginning and used, I believe a 2.3 volt filament. Flybacks are fragile.

If you are missing high voltage you could have a problem in the B+ supply.

There is no rebuild service for CRT's and Flyback's are hard to find. The wax breaks off leaving the HV wire unsupported.

Jim


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 2:29 am 
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Can see glow in the back of the CRT

Started taking resistance and voltage measurements. This first thing I found out of whack was pin 5 of 12DQ6 horiz output should be -45VDC, it measures -1 VDC. Plus, the 12DQ6 is HOT.

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 4:14 am 
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OK, the horizontal oscillator is not running. Don't run it too long like that, you will damage the 12DQ6 and possibly kill the flyback as well.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 4:17 am 
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Measure plate voltages around the horizontal oscillator. You might have an open resistor or a bad tube. Resistor most likely.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 6:38 am 
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By the way, does anyone know if you can fit a 21CEP4 into a 21" Predicta? Data sheets show original CRT has overall length of 13", 21CEP4 is 14.5", i.e. it has a longer neck. How much clearance is there in the hump?
Heater current is the same at 600mA, although original tube runs at a lower heater voltage.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 5:25 pm 
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There isn't very much clearance. Not enough for an extra 1.5" Perhaps you could add spacers and use longer screws for the back cover.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 5:27 pm 
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Have you recapped the set ? That main circuit board is a little tricky to remove but not that bad once you get the hang of unwrapping the wire from the posts.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Tue 13, 2018 2:20 am 
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irob2345 - I will measure voltages. When you said horiz osc, did you mean the 6CG7, horiz mult?

Here are voltages and resistance at the 6CG7 pins (spec/measured)
Voltages
1 175VDC/138VDC
2 -.5VDC/-.5VDC
3 6.5 VDC/0VDC
4 12VAC/9.8VAC
5 19VAC/15.4VAC
6 175VDC/205VDC
7 -15VDC/0 VDC
8 6.5VDC/21VDC

Resistance
1 10K/open from 280V source
2 1.8M/1.8M
3 1K/1.1K
4 4/3.2
5 5/4.6
6 53K/54K from 280V source
7 100K/205K
8 1K/open

Looks like I got some issues at that tube. I guess I'll have to pull the board again.

banderson - I should have mentioned I replaced all electrolytic and paper caps, I also rebuilt the couplates plus one I even bought from you, Checked and replaced resistors out of spec, also.

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Last edited by Stan Saeger on Feb Tue 13, 2018 3:05 am, edited 1 time in total.

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Tue 13, 2018 2:50 am 
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Yep, the 6CG7 would be the one.

I'd look first at the plate load resistors, I don't have a circuit so I can't give you a reference.

You might have made a wiring mistake when replacing something around the horizontal multi. That is the danger of using the shotgun approach - you can accidentally insert faults that weren't there before...

There should be a capacitor that connects from one of the 6CG7 plates to G1 of the 12DQ6. My money's on that being in the wrong place....


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Tue 13, 2018 3:19 am 
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Looking at the schematic, it looks like the capacitor that connects from then 6CG7 plate pin 6 has a 3300mmf cap which then connects tto 12DQ6 pin 5. This cap is inside couplate K6. (I think couplate is the term).

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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Tue 13, 2018 4:08 am 
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OK, good, now we know where it is!
A look at what you measured shows the 1k cathode resistor on Pin 8 is open circuit. That checks out with the other voltages and explains perfectly why the oscillator is not running.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Sun 25, 2018 10:03 pm 
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OK, back to this Predicta. I now have a pretty clear picture, but the one issue is vertical hold. No matter how often I get the picture correct by adjusting vertical hold, , I begins to roll after a few seconds.

Any ideas what to check?

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Stan Saeger
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http://www.saegerradio.com/


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 26, 2018 6:45 pm 
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Stan Saeger wrote:
OK, back to this Predicta. I now have a pretty clear picture, but the one issue is vertical hold. No matter how often I get the picture correct by adjusting vertical hold, , I begins to roll after a few seconds.

Any ideas what to check?

That is commonly caused by leaky capacitors in the vertical circuits. More than any other circuit in the set, the vertical section needs all of the capacitors and resistors to be their correct value and to not vary with temperature. A leaky cap is very temperature sensitive.

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Tom


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 26, 2018 11:37 pm 
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There are many possible causes of loss of vertical sync. Maybe a mistake made when re-capping?

If horizontal sync is OK, look around the vertical integrator - that's the R - C network between the sync separator and the vertical oscillator. If there's one of those couplates there, that will be it.

An unexpected cause of no vertical sync is an open circuit, missing or miss-wired AGC bypass capacitor, either on the tuner or the IF. There should be about 0.1uF or so there.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Tue 27, 2018 9:24 pm 
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I replaced all the paper and electrolytic caps and replaced all resistors out of spec.

Having said that, I will double-check my work around the vertical circuit.

Thanks

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Stan Saeger
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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Wed 28, 2018 12:52 am 
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Don't forget to check the caps on the AGC line to the tuner and IF....


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Wed 28, 2018 3:21 am 
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You mentioned couplates earlier. Did you replace both couplates K3 and K4 (Sams notation)?

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Wed 28, 2018 3:46 am 
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Closly inspect all the tube sockets. The type Philco used were prone to the develop cracks where the metal pieces are raised. I had to replace all of them on my set.


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 Post subject: Re: Philco Predicta Chassis 9L37
PostPosted: Feb Wed 28, 2018 5:00 pm 
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and make sure you used a cap rated for at least 1,000 for the 0.0015 uF vertical feedback cap (C38 on the Sams)


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