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Hitachi 27AX2BC

Hi, I'm working on a Hitachi 27AX2BC 27" television. I have the full schematic at my disposal, which is nice. But the problem I'm having is very intermittent. The TV will constantly turn on and go into protection due to excessive B+. If I continually cycle the power with the power button, the TV will come on and work flawlessly continously. When the TV is on, all B+ voltages check out, and the TV works very well.

After I shutdown the TV again, and attempt to restart, the P/S goes into shutdown, squealing like crazy. Unplugging the cord and plugging back in again will get me back to the point where I can cycle the power again until the TV eventually comes to life.

I'm thinking that the problem is somewhere in the Horizontal drive circuit, perhaps during startup the horizontal pulses don't exist, or they are intermittent, and the protection circuits detect a high B+ due to insufficient loading, and the HV shutdown circuits kick in. I have replaced C723, which is apparantly a common failure for this TV because it is known to cause Horizontal drive problems. Does anyone else have any clue as to why this TV would do this?

Reply to
iblackford
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I'd have to do some digging for the location number but, a common fail for that series was a 12volt zener near the horiz. drive transformer, ZD7xx? It's part of the 12 volt run circuit. It gets leaky and causes slow start.

Reply to
Tech Data

Hi, If you have any more info on that, it would be great. I checked out the schematic, and there is a zener ZD710 that is connected to the Horizontal Drive Transformer. However, this is an HZS33-2, a 32V zener.

I have found similar startup problems when searching the net, and they referr to ZD744, a 12V zener. Could this be the one your are referring to?

Any help would be appreciated on this model, I'm a little stumped.

Reply to
iblackford

On 22 Apr 2006 02:07:19 -0700, "iblackford" put finger to keyboard and composed:

I'm not familiar with your set, but often this kind of fault is due to a small electro on the primary side of the SMPS. When it dries out it sometimes causes overshoot on the B+ rail.

- Franc Zabkar

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Reply to
Franc Zabkar

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