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?

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.

Tech Data wrote:
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.

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.

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

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 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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