Hi,

I have a Toshiba TV (27" pic) model 27A44 (mfg. 05/04) in my TV service
shop (no SM). Customer told me the TV "sparks" once in awhile, then turns
off. It didn't do it in the shop yet, but checking the HV, it was 30KV
(higher than normal). I checked the regulated B+ and it measured +122V!
Unfortunately, there is no marking on PCB that indicates what this voltage
should be.

I removed and tested the timing capacitors in the horizontal output
circuit. Sometimes an open capacitor will cause the HV to go higher
than normal. They tested just fine.

130V is very common in my experience, but can someone tell me what the
regulated B+ should be in this model TV?

Thanks in advance, Brad

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On Wed, 03 Dec 2008 14:56:54 GMT, (Brad) put
finger to keyboard and composed:

130V is very common in my experience, but can someone tell me what the
regulated B+ should be in this model TV?

I can't help with your particular model, but I recently had to
determine the B+ voltage for a supermarket branded 34cm TV. The B+
capacitor was rated at 250V but the B+ was measuring only 103V. It
turned out that this was correct.

What I did was to trace out the circuit around the optocoupler between
the primary and secondary sides of the switching transformer. The
voltage sensing IC was a TL431.

2.5V
|
B+ o-- R2 --o-- TL431 -- to optocoupler
|
R1
_|_
=

In the circuit above, the value of B+ can be computed from ...

B+ = (1 + R2/R1) x 2.5

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

On Wed, 03 Dec 2008 14:56:54 GMT, (Brad) put
finger to keyboard and composed:

Hi,

I have a Toshiba TV (27" pic) model 27A44 (mfg. 05/04) in my TV service
shop (no SM). Customer told me the TV "sparks" once in awhile, then turns
off. It didn't do it in the shop yet, but checking the HV, it was 30KV
(higher than normal). I checked the regulated B+ and it measured +122V!
Unfortunately, there is no marking on PCB that indicates what this voltage
should be.

I removed and tested the timing capacitors in the horizontal output
circuit. Sometimes an open capacitor will cause the HV to go higher
than normal. They tested just fine.

130V is very common in my experience, but can someone tell me what the
regulated B+ should be in this model TV?

Thanks in advance, Brad

This forum post talks about a B+ of 124V and an EHT of 25kV:
http://www.freelists.org/post/techas...y-in-both-vH,1

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.