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 Post subject: Re: My Predicta went dark
PostPosted: Sep Thu 09, 2021 10:31 pm 
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Location: Albion, CA, USA, 95410
OK, I am here with the set and trying to test a few things.

I made a little jumper so I could use my test socket on V11, the HOT, 12DQ6A. Pin 4 has 105V instead of 175. Pin 5 is just barely negative at -.15 instead of -45. So I think we can say the horizontal multi-vibrator isn't running. I did get a small Sains-Smart oscilloscope to look at the horizontal multi-vibrator wave form but without it running at all it seemed pointless to try it.

Looking at the power supply side of things, the junction of the rectifier diodes (M1 and M2) starts at 150V and drops to 126 as the set warms up. The output of M1 is 245V instead of 290. So I decided to see what the current passing into the set is. The schematic shows it as 270mA. Once the set is warmed up the output current reaches 350mA. So I guess the power supply is OK and something is dragging the voltages down.

The couplates, including K6 have been replaced. I have subbed another 6CG7. I routed the sound through the set. I can get some sound with the volume cranked and the local/distance switch set to distance. However, it is weak and distorted.


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 Post subject: Re: My Predicta went dark
PostPosted: Sep Fri 10, 2021 12:02 am 
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Location: Albion, CA, USA, 95410
Update: At the suggestion of somebody on Videokarma, I went and checked and checked out L22, the horizontal frequency coil. It appears mine is open. On a complete chassis I have it tests as it should. Somewhere I have a board I already cannibalized for parts that should have one but I can't seem to find it.

Anyway, are there issues that can cause L22 to fail that I should now look for? Or is it just bad luck?

Thanks!


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 Post subject: Re: My Predicta went dark
PostPosted: Sep Fri 10, 2021 12:46 am 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
It is probably a random failure. Even if the oscillator tube shorted there is a 15K resistor in series which would limit current to something it could handle.
Many times a coil like that will go open at the location where the wire leaves the winding and goes to the terminal lug. Sometime you can actually repair the coil by finding the wire break and unwinding a turn or two to reconnect it to the lug.

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It's not the Destination, It's the Journey.


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 Post subject: Re: My Predicta went dark
PostPosted: Sep Fri 10, 2021 1:37 am 
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Location: Ann Arbor, MI
madlabs wrote:
Looking at the power supply side of things, the junction of the rectifier diodes (M1 and M2) starts at 150V and drops to 126 as the set warms up. The output of M1 is 245V instead of 290. So I decided to see what the current passing into the set is. The schematic shows it as 270mA. Once the set is warmed up the output current reaches 350mA. So I guess the power supply is OK and something is dragging the voltages down.

The horizontal output tube is drawing too much current and dragging the voltages down. All will be well when you fix the multi-vibrator.

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 Post subject: Re: My Predicta went dark
PostPosted: Sep Fri 10, 2021 3:58 am 
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Location: Albion, CA, USA, 95410
So, at the recommendation of banderson, I shorted out L22. And what do you know, it restored HV! So now I know for sure that L22 is the problem. I dug up my parts chassis and 2 of the 3 I have are bad. I will see if it is possible to repair the coil. If not, I do have one good one but will start to look for another.


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 Post subject: Re: My Predicta went dark
PostPosted: Sep Fri 10, 2021 5:06 am 
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Do look closely at the bad coils and see if you can see where the breaks are. Maybe one of the coils can be repaired.

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 Post subject: Re: My Predicta went dark
PostPosted: Sep Sat 11, 2021 3:55 pm 
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Location: Albion, CA, USA, 95410
Well, I ran the set for about 4 hours last night. It plays perfectly with the coil shorted out. So I think I will pay homage to Madman Muntz and leave it shorted. With the help of some folks on Videokarma I found a cross referenced part and ordered it, just to have on hand.

I did pull a dead coil off of one of my parts sets. The open is in the interior on that one.


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