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 - TV MAN - 01-08-2022
Opening the circuit would give you false readings, no load= higher voltages. Could check current. Look for anything getting too hot, resistors, coils, output tubes, etc.
RE: Recapped My Predicta Tandem, I don't know what to do - OZ4 - 01-08-2022
I feel compelled to say something here. apERNA1885 while the people here are knowledgeable and helpful I think you should look at some youtube videos on tv repair. I have been watching way too much youtube lately and found this guy (shango066) really knows his stuff on most any tube TV! He doesn't do full restorations but he shows what is necessary to get them working. He has a real feel for the circuits and what is the most important to look at first And he does work on a couple of Predictas. I find his videos easy to watch with all the cutesy ceremony removed. He can be pretty blunt and straight to the point. He states that the parts in the vertical section are the most critical for proper display. On your set I would start there first. If you don't find his videos to your liking there are many other videos on TVs. Sounds like I am promoting youtube NOT. Good luck.
P.S. I would not try to watch any videos on a cell phone. You need at least a large monitor to see the detail.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-08-2022
OZ4 I've watched both Bob and Shango a lot, but they're seasoned vets. They never have the everything new but this chassis is haunted episode. They typically rebuild it and it's D**n near flawless or they find a problem in 2 seconds. Honestly their videos are what inspired me to take this on. I regret it a little, because I'm lost, spent a ton of money, heard a bunch of I told you so's, and why dont you take it to someone who knows, and still have a dead tv. I'm hoping the K networks help. Because really I'm out of ideas. Watching them on You Tube is like watching HGTv and calling your self a contractor. Without doing it your self and learning you'll never get anywhere. I'm hoping to learn from asking questions, and trying to trouble shoot it with the diagrams I have but who knows.
I don't have values for current or voltage in a lot of places, I'm tempted to try messing with C1 or C2 section A and see if raising it's value will bring up the voltage. This chasis looks like it saw a lot of moisture in it's lifetime, and I wonder if something has just had it and I may never find it. I have a really nice chassis I bought after the fact, Im tempted to try recapping that one and see if it works.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-08-2022
I recall that in one of my Tandems all my voltages were also low but the set worked fine so I just let it go. As you mentioned, I typically pull the boards out and do a total rebuild since these are difficult sets to troubleshoot.
The voltage doubler uses the two capacitors to build up charge that gets added together to create B+. If the capacitance is too low to meet the load, B+ will drop. Likewise, if the internal resistance (ESR) of the cap is high, B+ will drop.
A simple thing to try is tack a capacitor in parallel with C1 and C2 and see if it brings B+ up.
Also, can measure the total current draw of the set? It should be 170 watts or 1.65 Amps according to Sams.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-08-2022
I should also add that I really don't think low B+ is the cause of your issues. I think it may be a symptom. There is something else very wrong. The video is smeared, bad horizontal lock, and lack of vertical. I don't recall if you have sound?
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-08-2022
Bob, first thank you for the explanation, It makes a lot more sense to me now. What kills me is I'm used to being able to separate things when I'm troubleshooting, and I feel like I can't here. I'd love to know that everything is good up to the choke or just past the choke and know where to look. It seems to me and I could be wrong, but the issue could be anywhere.
I'll try putting a cap in parallel with C1 and C2 that's a cool idea, should it be the same value? I will also check the current draw, I'm not sure if I can tonight but definitely by tomorrow.
I'm not sure if I'm putting too much weight on swapping the K Networks, but I'm really hoping having them replaced will at least help eliminate some of the variables. Once the picture isn't folded over on itself and fills the screen maybe we can get some direction?
Also my sound works really well it's actually loud and clear.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-08-2022
I would add around 47uF to each cap. Going too large will stress the diodes on startup from the surge current.
Good sound is a good thing. It means that the tuner, IF and video amp are all working. Yes, replacing the two couplates associated with the sync and horizontal circuits could very much improve the situation.
RE: Recapped My Predicta Tandem, I don't know what to do - OZ4 - 01-08-2022
Your B+ is good enough to have a raster. Both the horizontal and vertical oscillators are running. I haven't been reading too close. Do you have a Sams Photofact? Or something with waveform drawings? It should have voltages too. Also you will need a scope. It doesn't need to be anything special.
I see from your pics that the horiz osc looks to be running too fast. and the vertical waveform driving the yoke is distorted and or way too fast that's why the foldover and saggy picture. Most likely caps causing the distortion. Look for tired resistors too. Oh and apparently the tube sockets are fragile. They break it where they clamp the tube pins just from pulling the tubes out.
Again once you know the voltage here is good. I would start on the vertical deflection it doesn't depend on the tuner, IF, or the horiz section. It's pretty straight forward. It runs at only 30hz. It should deflect full screen all by itself. Again double check the caps found there and make sure It is wired correctly. Look at the waveforms here and look to the vertical output tube for goodness. The sams will have test points to check with an oscilloscope. You will also need a tube tester or a known good output tube. It takes some watts to drive the yoke.
Adjusting the vert and horiz controls just speeds up or slows down the oscillators so they will lock to the incoming signal IF. Look to see if the pots are dirty or dead.
Set the vert adjustment at half way and find the vertical oscillator coil and try to lock the picture in. Don't be afraid here. The oscillators only need to be close enough to lock in. I would use a large mirror so you can see while you are behind the set twisting on things. You should be able to see the rolling speeding up on not. If you can get the picture to stand sill but won't lock the sync-intergrader in not working. That's the next chapter.
Watch a bunch of tv repair videos and it will start to stick in your brain. There a ton of videos.
RE: Recapped My Predicta Tandem, I don't know what to do - OZ4 - 01-08-2022
After looking at your pictures again. I appears the horiz osc is way off frenquency. Again set the horiz control at half way and adjust L22 the horiz osc frequency. The number of lines should change. Once you get close it may lock.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-09-2022
That "freewheel" coil helps keep the oscillator on target, but it isn't the only factor. If the RC components of the multivibrator are way off, I'm not sure the coil can compensate.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-09-2022
I don't want you guys to think I'm not trying, I went down today to measure the amp draw but it turns out both the testers I have here only test DC Amperage. I have to wait until tomorrow and grab my good meter from work.
RE: Recapped My Predicta Tandem, I don't know what to do - OZ4 - 01-09-2022
I would try and see if the coil can bring it in. Then you would know if it's time to look at the circuit components. It may bring it in just the way it is. Wiggle and move stuff around to see if the osc is stable or jumping around or totally out of wack. And what condition the Horiz pot is in.
I remember trying to get the vert to lock when I was a kid. Stupid TV just keep rolling. Now the stupid TV is so bright it hurts my eyes.
RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-10-2022
I just checked it out. The set climbed up from .7 as it warmed up to 1.016 amps
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-11-2022
About 0.6A is just from the series string tube filaments. That's means B+ is pulling 0.4A. I clearly remember going through all this when I restored my first Tandem. I'll have to go back and watch the videos to see what values I got. I'm pretty sure I got closer to what you are measuring than what Sams stated.
RE: Recapped My Predicta Tandem, I don't know what to do - Bob Andersen - 01-11-2022
Mine draws about 1.1A so similar to yours. I think Sams was wrong with the 1.65A claim.