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 Post subject: Philco Predicta Chassis 9L37
PostPosted: Feb Mon 12, 2018 12:40 am 
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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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 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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Joined: May Sun 07, 2017 11:35 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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Joined: May Sun 07, 2017 11:35 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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Location: Chicago, IL USA
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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Joined: May Sun 07, 2017 11:35 am
Posts: 149
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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