17pw20 overvoltage

[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.

[Turnip]

Hi Vic,

Admit - on having a problem, I would try another board - heaps of 'em at council tips, just mend tip chaps' stuff, and then all spares are free - Chris

PS - Some folks say this doesn't work, but chat to 'em a bit first - never fails.

[ProDave]

A cheap and dirty way for a test perhaps is insert a couple of rectifier diodes in series with the inverter power. Will drop about 0.6V per diode.

That might get it running and you can see what the voltage on load settles as.

[Turnip]

Hi Dave,

Could use a variac - plenty still at the local tip - or a couple of 100W tungsten lamps in parellel - (but getting rare now with green concerns) - Chris

[ProDave]

Hi Dave,

Could use a variac - plenty still at the local tip - or a couple of 100W tungsten lamps in parellel - (but getting rare now with green concerns) - Chris

I doubt that wll drop the O/P volts, as most PSU's are now designed to work from 120 to 240V input.

[vic_curtis]

Thanks guys, must admit trying another power board was probably next on my list, but hadn't thought of checking the tip.... I'll have to have a chat with the nice lady in charge down there. In the meantime I must admit it has become a bit of an interlectual challenge to solve the mystery. I like the idea of bunging a couple of
diodes in line with the 24v supply, might give that a shot. Don't have a variac, but an incandescant bulb in series, might just bring the voltage down a bit; especially if the regulation is not quite working correctly.

Will probably return to this when I've finished all the other jobs I'm supposed to be doing, as this just started out as a favour for a friend.

[jimca007]

although i understand what your saying regarding the inverter coming up ok on a bench test the symptom in a real world normal situ of a brief flash from the tubes and a dark picture would almost always be a bad inverter. could be a dodgy mosfet on the inverter on ideal voltage will pass ok but on the slight over voltage locks up with these symptoms i would be looking at the inverter before the psu its the wee flash that gives it away,

[Turnip]

Good thought Jim,

What's the inverter Vic ? Quuq folks might have some - Chris

PS - Thought about tips - Council folks are now trying to save 'their' cash putting recycling to private firms eager to make a survival pound with very few 'Health and Safety' Local Council well pensioned clipboard farts still on the payroll, so pehaps some real recycling can start. Not yet thinking of burning my PCB panels for metal on the local beach - but surely real green recycling is good - just a thought.

[vic_curtis]

Hi Guys, owing to rain stopping my efforts outdoors today, spent a bit more time looking at this and think I've
found the primary problem...R876 (30K 1%) had gone high to 33.5K. Replaced with a 30K and 24v rail is now sitting happily at 24.5v. Now to put the bits back together and see if I have a working set.

Incidentally, R876 could hardly be any more inaccessible! right underneath the edge of the heatsink. Must
admit after wiggling the original out I resorted to fitting the replacement on the reverse side of the board!

Vic.

[jimca007]

Chris with being friends for quite some time with all the guys at my local refuse site ( tip ) I was fairly stunned to hear one of them tell me that all white and brown goods that now come into the yard are no longer sent for scrap but actually sold to a contractor who can either stip and sell parts or refurbish for resale not that it mattered to the guys who are still quite happy to give you what you what for a small gratuity it certainly explains all the clip board bean counters who seem to have sprung up in the last wee while .

Vic... well done indeed if that resistor fixes your prob wouldnt have thought it'd be an easy find !

[vic_curtis]

Result!.... TV now fully working (just need to fit the full complement of fixing screws).

Was initially phased by getting nothing but a "No Signal" indication for both Analog and Digital. Then after
some reflection realised the previously tuned analog channels were showing no signal 'cause the transmitter has now completed its DSO so there is no analog signal. The lack of digital signal I eventually twigged was because the TV had come to me from another area and all the previously tuned channels were different in my area.

Must have taken me about another hour to get to grips with just how you get this beast to re-tune all the
digital channels, including realising I needed to put new batteries in the remote "Doh!". Now all seems to be working a treat.

More than pleased; what was threatening to be a scrap job ended up being fixed by a 12p resistor... just don't ask about the time! Just as well I was doing this mostly as a favour to a friend for his 89 yr old mum.

Cheers all,
Vic