Recapped My Predicta Tandem, I don't know what to do - Printable Version
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RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-05-2022
Two of the wires are for the AC tube filament supply.
Sounds like your horizontal frequency is way off. The frequency is determined by one of the couplates near the 6CG7 tube. There is also a coil with a mica cap in parallel that should be set to resonate at 15,750 Hz to help the oscillator stay in tune.
When you are feeding a signal the AFC circuit is trying to get the frequency right. Predictas tend to freak out like in you photo when the horizontal get too far off frequency.
Have you tried adjusting the slug in that horizontal coil? The frequency range control plays a part here too.
You should go through the horizontal setup procedure outlined in the service info.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-05-2022
Bob thank you for following up. No I haven't tried to adjust the horizontal slug. I was afraid to change anything like that. My assumption was if it worked before it should again if I don't mess with it. I will look up the adjustment. I have the copulates coming in the mail. You said one of them controls the horizontal frequency so maybe replacing it would fix it?
What do you think caused my blank white screen? I went from the poor screen I had to a non responsive white screen.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-05-2022
Yes, replacing the horizontal couplate could get the oscillator on frequency.
Many possibilities for the white screen. Bad connections between the Tuner, IF, video amp, the long cable etc. Signal injection or a scope could really help narrow it down.
RE: Recapped My Predicta Tandem, I don't know what to do - TV MAN - 01-06-2022
Couple thoughts, if the set has centering rings on the yoke, could be they got bumped and is why the vertical is off center. Not sure on your set. Also you can feed a video signal into the grid of your 1st video amp through a cap to check your amp, output and pic tube. I’ve used a DVD player to check this. It’s crude but works. And as Bob mentioned, if the horizontal is too far off it’ll do all kinds of weird stuff.,
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-06-2022
No centering rings in Predicta. I do believe the vertical output is powered by boost voltage derived from the flyback. So if the horizontal is out of whack it may be dropping the boost voltage and effecting the height. The tandem model is tricky to work on because there's a 25 foot cable connecting the CRT and yoke to the main chassis.
RE: Recapped My Predicta Tandem, I don't know what to do - BrendaAnnD - 01-06-2022
You may have a bad/broken ground shield in the cable going to the CRT. This would cause the smeared video. You could also have a (God forbid) H-K short in the CRT, which would also cause this issue.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-07-2022
Ok the good news is I found the reason for my white screen. The orange wire that goes on to the main board at CircuiTrace point 17 broke off. But stayed close to where it was supposed to be. I don't have a scope so I started checking voltages hoping to find something really off and found it.
The bad news all my voltages are low. I followed the schematic from the switch to C1 the voltage is good leaving C1 145VDC exactly like the drawing. It looks like it goes through one diode and a micah cap then into the choke. The voltage should be 280VDC I have 240VDC. I checked the resistance on the choke it's dead on at 41ohms like the Sam's says. The voltage leaving the choke is 230VDC. At that point I checked the voltages at C2 100MFD it Should be 270 I have 229, 20MFd should be 250V I have 208V, C3 20MFD should be 115 I have 125. Also the 250VDC to the tuner is at 208VDC. Is it possible the (C50) Micah cap is bad and it's bringing the voltage to the choke down?
Also at the Picture tube Pin 7 should be 50VDC I have 30VDC, pin 4 should be 390VDC I have 334VDC. I also checked the junction of resistors 29 and 30 on the Amp board and got 230VDC instead of 270VDC I'm guessing its all caused by the low choke voltage?
What do you guys think?
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-07-2022
Ok the good news is I found the reason for my white screen. The orange wire that goes on to the main board at CircuiTrace point 17 broke off. But stayed close to where it was supposed to be. I don't have a scope so I started checking voltages hoping to find something really off and found it.
The bad news all my voltages are low. I followed the schematic from the switch to C1 the voltage is good leaving C1 145VDC exactly like the drawing. It looks like it goes through one diode and a micah cap then into the choke. The voltage should be 280VDC I have 240VDC. I checked the resistance on the choke it's dead on at 41ohms like the Sam's says. The voltage leaving the choke is 230VDC. At that point I checked the voltages at C2 100MFD it Should be 270 I have 229, 20MFd should be 250V I have 208V, C3 20MFD should be 115 I have 125. Also the 250VDC to the tuner is at 208VDC. Is it possible the (C50) Micah cap is bad and it's bringing the voltage to the choke down?
Also at the Picture tube Pin 7 should be 50VDC I have 30VDC, pin 4 should be 390VDC I have 334VDC. I also checked the junction of resistors 29 and 30 on the Amp board and got 230VDC instead of 270VDC I'm guessing its all caused by the low choke voltage?
What do you guys think?
RE: Recapped My Predicta Tandem, I don't know what to do - TV MAN - 01-07-2022
BSQ, what kind of diodes? Selenium or ? I know a lot of sets used selenium rectifiers in that era and they get bad. I don’t know if your set uses those or not. I can’t tell for sure in the pictures. Might check. That 270v feeds both your vertical and video outputs as well as the horizontal. Your 410v boost is low too. Note that this power supply is a voltage doubler circuit.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-07-2022
This model uses silicon power diodes for the voltage doubler. Did you replace them? I suggest 1N5408 diodes.
Those are ceramic caps, not mica across the diodes. They are there to suppress transients when the diodes switch on and off. Moden diodes are "tougher" than the early ones.
What line voltage do you have coming in? What cap did you use for C1 and C2? Original was 140/150mfd.
All the power for B+ goes through them so it should be a low ESR type with high ripple current rating.
DC voltage on Pin 7 of the picture tube varies with the brightness control.
Side note: Keep in mind that there is no voltage regulation so your readings may vary from what Sams published. Something else to consider is the way Sams created their service info. They would just buy a TV and reverse engineer it. The original manufactures service info will have slightly different voltage values.
For example, Philco service info has +275 for B+ and +265 going to the tuner.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-07-2022
I had my electronics store Cross over the old part number and they said 1N4007 exceeds the original diodes. So I have 1N4007's in there, I did notice that the picture was slightly better with the original diodes, but the voltage was the same.
I have regular electrolytics om C1,2, and 3, I have orange drops everywhere else I can put them.
C1 is 150uf
C2 is 20uf , 22uf, 100uf, 150uf
C3 is 22uf, 10uf, 220uf
TVMAN the original diodes were silicone, but I have modern 1N4007's in there now I left the old ones just laying in there disconnected. The originals had a Philco part # of 34-8048-1 with replacements made by Federal having the part number HF504, international brand is SD-500, and Sarkes Tarzian part# M500. Those are the numbers we used for cross referencing them at the store.
I attached a picture of the setup below. If those ceramic caps shorted could they be dragging my voltage down? I've been checking my choke voltage on that read wire that's leads to the choke.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-07-2022
No. If one of those ceramic caps were shorted, you would have AC on the electrolytics and they would blow. What line voltage are you running the set at ?
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-08-2022
Bob I just metered it, it's 119v. Do you think the store spec'd out the wrong diodes? I saw you use 1N5408.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-08-2022
It makes no difference. Any silicon diode rated at 1A, 400V or better will work the same.
Either some circuit is drawing more current than it should be or the voltage doubler is not operating properly. That's why I was asking what type of capacitors you used.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-08-2022
If I unsolder the Red wire going to the choke, can I isolate the voltage doubler and check to see if it's making 280v?