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Attn: ZZac - Pioneer TV update

I haven't had time to check the raster as of yet since I have been working 7/12. I have adjusted the B+ voltage back down to 135V, which you were dead on about, since it says "135V adj". This has brought the voltage down on the Hot collector to 101V, from 119V. The darlington reads 135V on it's collector, and 101V on both emitter and base (crossed by a small diode). Looks like this darlington is supplied by

135V from another winding on the flyback. The darlington emitter ends up at the Hot collector via low ohm resistors and a diode.

There are two test points on the board. Both say "PD 135V". I traced this all the way to an 8 pin socket that says PD at this terminal. Other terminals are labeled +25, -25, gnd, etc. I'm not sure what "PD" stands for, but both test points read 0V. Both points are electrically connected and tie to either the base or emitter of a small transistor. The other two legs read right at 135V.

This may be normal since I have the power board on my dining room table out of circuit. The rest of the set is about 15 miles away. I'm not sure I mentioned this in the original post. You are exactly right in this being a real mother to figure out. I have only been working on sets for a couple years (self taught) and with this groups help, and have never seen a power supply like this. I try to stay away from big screens because of the hassle of going back and forth to the set. I already have a 46" Mits in my kitchen that is trash since I can't get a replacement tube.

Again, I will post follow ups as I get info. This set belongs to friends that have seen better financial times, and I would love to get this thing going for them since they can no way replace it. I'm willing to bite the bullet on the majority of parts costs. My time will be free as usual. Thank you for your input and interest.

Reply to
Golf
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Hmmmm, what model of Mits and which tube ? We've had a couple with really bad coolant leaks and the tubes are just fine.

Thing is, on the Pioneer, 101V might be right, you need other observations to determine that. Can you determine if it has seperate HOT and HVOT ? Look at the yoke plugs. Where the red wires went they are all hooked together. Do they go to the flyback or not ? Sometimes they go to a somewhat smaller transformer.

Now lemme tellya something about electronic design. They do not use a darlington because they want to feed it with a high current source.

When you start finding your way around that board, in the PS there is a darlington, and it breaks the rules. It is fed with a low impedance source. But that is the exception. Normally darlingtons are used because of lower drive current requirements. That means usually, if your DVM reads 0.468 E to B forward, something is wrong. Pioneer has a habit of using these 2SD12 somethings, and in some cases you can just throw a 2SD401 in there, and in others you can't. And it is very difficult to tell without a print.

Also, inspect that board very carefully for bad solder joints. More than one have been fixed by simply resoldering them. Of course it is still $300 because it is such a bitch to do, but actually those sets are relatively reliable.

JURB

Reply to
ZZactly

Thankyou again for your help. As far as the Mits, model #WT-46807. Kept going into shutdown at power up. Can hear and see sparks flying inside the neck of the red tube. Disconnected the anode and the set stays powered up fine. No evidence of coolant leak that I can see. I'm assuming a crack or something in the tube, but if a coolant leak could still be the culprit, I would be really interested in how to fix this. So would my buddy who is without it now! Either way, no big loss since he got it free from a friend who was going to trash it (set was dead, replaced pico fuse in vertical? supply), and worked perfect for about a year until now.

Now the MOFO, the three tube yokes go to another component (trippler?) and from this to the flyback. I checked the darlington that I replaced, got 0.90 drop E to B. I'm again assuming this was probably good, but not knowing I replaced with a sub - NTE2547. Of course got the exact same readings and results.

I have seen numerous suspect solder joints under the magnifying glass and hit them all. Haven't finished the entire board yet, but did flex it in circuit with no effect to the tubes (raster) brightness. I figure I am getting a clue to the problem when I touch a particular diode off the flyback (with positive lead of my fluke) and can make the raster brightness flicker brighter on all 3 tubes. I resoldered and checked this diode good, but get the same results.

If you are looking for more clues or have other questions, please feel free. If I don't hear from you, I will do as said and post updates as I get them. Just don't know how long I will be stuck working these damn 7/12's. Thanks a ton.

Reply to
Golf

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