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17pw20 overvoltage

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vic_curtis:
Hi, new here, but been reading lots of useful info recently while trying to fix a Sanyo CE32LD81 with 17MB22
mainboard and 17PW20 PSU.

Symptoms were that when connecting to mains, set seems to come into standby (blue led lit). Remote
doesn't seem to work but ch+ or ch- on set seems to get set into run, but no picture.

Initial check on voltages based upon schematics I tracked down via this site, seemed to indicate that all
was actually OK, both standby and run. But no backlight. Now suspected inverter so pulled that out and had
a closer look - nothing obvious, tried a quick bench test with bench PSU and it seems to run OK.
Puzzled, I hooked back up the set PSU but the inverter still refused to run. While pondering this I had an idea,
re-measured the psu outputs and noticed that actually the 33v, 24v, and 12v were all a bit high (before I had
just been checking for lack of volts not too many!). 12v was reading just over 13v and 24v was reading about
26.7v. Hooked the bench PSU back up to the inverter and raised the voltage to 26.7 and lo! the inverter stops.

So it seems that the problem is down to the run psu not regulating its output down correctly.
Not exactly sure how the PSU regulation works on the 17pw20 but looks like everything hangs off the MOSFET Q837 connected across the SoftStart caps on the SG2525A.

Need more time to investigate further, but thought I'd post my experience so far and see if anyone else
has seen a similar problem with this power board. The schematic I have is not an exact match for the V3 I have in the set as I have found a couple of component differences already.

Vic.

Turnip:
Hi Vic,
Would suggest - be brief - leave it to Quuq folks to chew the probs with pertinent questions - a novel ain't the biz.

Short attention span, sadly  - Chris

vic_curtis:
Hi Chris, good point,

Brief summary, 17PW20 power board - 12v, 24v and 33v rails all reading too high.
24v reading 26.7v which seems to be too high for the inverter to operate without tripping
out. Any ideas as to possible cause?

Vic.

jimca007:
uunfortunately on these your in a wee bit of a chicken and egg situation, you'll prob find once the inverter powers up the load will drop the voltage back to 24 could be the main board not switching the inv on

vic_curtis:
Thanks Jim, I had wondered about that too, but I have tried powering up with a voltmeter on the inverter
inputs; 3 voltages required: Main supply 24v, Inverter enable 3v and analog dimming control about 2v.

When all these are present and correct (from bench supply) the inverter fires up fine, but if I connect to the
TV supply the 3v and 2v are present OK but the main supply is over 26v and the inverter fires up momentarily
then shuts off. I get the same behaviour from the bench supply if I increas the 24v to 26.7v.

I'm pretty convinced the problem is in the regulation circuit of the run supply, but haven't got my head around
the whoce circuit yet. I've suggested to the owner (not my TV) that we try a replacement supply board, but
he is yet to decide whether to spend more on it or not. In the meantime, when I have spare time (nothing worth watching on TV!) I might spend a bit longer checking out the circuit.

Vic.

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