Toshiba TV model 27A44, Regulated B+ Info

[Brad]

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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[Franc Zabkar]

On Wed, 03 Dec 2008 14:56:54 GMT, bpetria@verizon.net (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.

 

[Franc Zabkar]

On Wed, 03 Dec 2008 14:56:54 GMT, bpetria@verizon.net (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/techassist/TOSHIBA-27A44-raster-pulled-in-horiz-ratty-in-both-vH,1

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

 

[Brad]

Hi Franc,

Thank you for your efforts to help me. I ordered the SM for this TV.

If I fix this problem, I will post what part(s) caused it and regulated
B+ voltage.

If not, I will post the regulated B+ voltage. My customer has set a
limit on how much she wants to spend on this repair.

BTW,

I took a loss on trying to repair a Philips TV 20PT6245/36 (20" CRT)
with a vertical deflection problem. The unfortunate thing about this TV,
it uses all discreet components (no IC amp). See my earlier posts for
details. I got no help from Philips on this problem since I am an
"independent".
Brad

On Fri, 05 Dec 2008 06:32:26 +1100, in sci.electronics.repair you wrote:

On Wed, 03 Dec 2008 14:56:54 GMT, bpetria@verizon.net (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/techassist/TOSHIBA-27A44-raster-pulled-in-horiz-ratty-in-both-vH,1

- Franc Zabkar

 

[Brad]

Hi,

Note: Now I have the SM.

I fixed a lot of TVs over the years that had excessive HV, but this
Toshiba is out of the ordinary. All the parts that would normally cause
excessive HV in the horizontal sweep circuits tested fine. I removed parts
from the circuit to test. The flyback pulse width on the scope was normal,
within 11 to 14 usec at 10% from base. Note: Other FBT voltages were higher
than normal. The SM, "Basic Adjustments", states the regulated B+ should be
"+123 +/- .5V". This voltage, "+123V" or the HV, can not be found in any
schematic in this SM.

The filter capacitor in the regulated B+ was tested. Due to the time
already spent trying to repair this "DOG", I decided upon a couple
modifications to save this TV from going to the dump. I installed a 5 ohm,
5 watt resistor in the B+ line to the FBT. I also added a .0012uf 2KV
sweep cap across C412 .0027 1.25KV. Now the HV is around 27KV, which I
checked several times during a long run, and the TV has a good picture.

Brad

On Wed, 03 Dec 2008 14:56:54 GMT, Brad wrote:

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

Before you type your password, credit card number, etc.,
be sure there is no active keystroke logger (spyware) in your PC.