Polaroid FLM-3234B Power Supply

[kevinm34232]

I'm working on this TV which wasd dead, and found a burnt transistor and a couple bad zener diodes on the primary side. After looking it up I found some other posts about it. They listed the part #'s and those are what I ordered.
2 transistors have the same part #, but Q2, I just noticed is a K3667 (http://www.digikey.com/product-searc...376381?k=k3667)
and what I had ordered without looking at it was a FDP18N50-ND (MOSFET N-CH 500V 18A TO-220).

Are these compatible? The K3667 is all black where it screws to heat sink, whereas the 18N50 is metal..

Thanks!

[tom66]

Re: Polaroid FLM-3234B Power Supply

You will need an insulation pad, mica or similar. If you have a dead computer PSU, you can harvest one out of it.

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Well I thought this would fix it but still a short somewhere.. Im trying to use the lightbulb trick here for the first time since it blew the main fuse 2x. I only have a 60W bulb and that glows brightly when plugging in TV.

So far I replaced Q8, Q9, Z2, and Z4. I did not replaced Q2 as it wasn't showing a short between pins. After replacing those components the first time fuse blew and Z4 was shorted again. The main filter cap also was testing bad in circuit, although it looked OK. Out of circuit it wont even give a reading on the ESR meter.

With the known bad and questionable components removed from circuit (Q2, Z4, main cap) the light bulb still glows brightly. Not sure if this is a correct technique though. Could it be IC2??

PS board part #: http://www.shopjimmy.com/polaroid-86...wer-supply.htm

By the way originally Q8 had right leg burned off and Z4 was shorted.

[tom66]

Re: Polaroid FLM-3234B Power Supply

Sometimes the bulb will glow brightly if the board can't start up.

The way you determine if it is a short or not is to measure the voltage being applied to the board, across the live/neutral wires, AFTER the bulb. If it's less than a few volts AC there's probably a short. But if not, it could just be a power supply struggling to start.

I also suggest you isolate parts to see if you can at least get 5V standby.

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Will power supply struggling to start blow the fuse? Because initially the first time the bulb didn't light up when I plugged it in. Then I put the fuse in and it blew. And I had 120vac at the ps board after the fuse.

As I said it also blew the diode again. I ordered new parts again including the main cap which was apparently bad too but I failed to test it first because it looked OK.

[tom66]

Re: Polaroid FLM-3234B Power Supply

No, it won't blow the fuse, so clearly you have another fault.

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Replaced main cap, 3 transistors and diodes, still seems to have a short. The lamp I have wired across the fuse glows bright, and I have only about 2 VAC at PS board. What can I remove to start t/s'ing?

[tom66]

Re: Polaroid FLM-3234B Power Supply

Tested bridge rectifier?

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Well I tested ohms between legs there was nothing low what should I look for? Could anything on the secondary side cause this or must it be on the primary side?

[tom66]

Re: Polaroid FLM-3234B Power Supply

Could be a damaged controller IC with output stuck high, though that would normally only cause problems at a higher voltage. You need to remove all power components and do the lightbulb test when they are out of circuit then put them back one by one until problem comes back.

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

I removed the bridge rectifier and the short went away. I put it back in and removed the power IC and short was back. I pretty much checked or replaced everything from the brdige rectifier to the transformers but it's still there..

Unless the bridge rectifier is bad but not showing a short?

[tom66]

Re: Polaroid FLM-3234B Power Supply

Have you tested all combinations? There are four diodes in a bridge, and any one can fail short without it being obvious.

[selldoor]

Re: Polaroid FLM-3234B Power Supply

I removed the bridge rectifier and the short went away. I put it back in and removed the power IC and short was back.

Think you found the problem then passed it by?

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Well what's the proper way of testing the bridge? Do I do it in diode mode each pin combo, then switch the leads around? I figured if the lightbulb went out when the bridge was removed that everything before it must be good, so it was either that part or something beyond it.

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Here's the readings when I test in diode mode (in circuit) first with black lead on left, then with red lead on left:

Black lead on left:
1-2: .525
1-3: .542
1-4: -0.00
2-3: 1.056
2-4: .535
3-4: .524

Red lead on left:
1-2: .535
1-3: .524
1-4: -0.00
2-3: 1.027
2-4: .524
3-4: .541

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Here are readings out of circuit:
Black lead on left:
1-2: .513
1-3: .531
1-4: .963
2-3: .0L
2-4: .528
3-4: .518

Red lead on left:
All combos read .0L

[tom66]

Re: Polaroid FLM-3234B Power Supply

1-4: -0.00

Shorted.

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

OK, that's why I tested it out of circuit too, 1-4=.963, and .0L other way? Still a short?

[tom66]

Re: Polaroid FLM-3234B Power Supply

Hmm, may be OK, is there a short between 1-4 on the board with the bridge removed?

[kevinm34232]

Re: Polaroid FLM-3234B Power Supply

Yes -0.00 both ways..

A lot of the diodes show shorted in circuit aswell but fine out of circuit.
I've used the lightbulb since I replaced the components that were bad, can I be confident that they didn't blow again Im not really 100% on the concept..??

Basically I cut off the plug to a table lamp, soldered each wire to each end of a blown fuse, and then stuck the fuse in the holder. I have a 60W bulb in the lamp.. correct?