RCA 42LA45RQ no backlights, bodged cap replacement on PSU

[johnboy1313]

I have an ad on Craigslist looking for TVs in need of repair. I got a call the other day regarding this set. The price was right so I bought it. Based on the described symptoms, brief backlight that shuts off, this video...

http://www.youtube.com/watch?v=no_e1kq6KnY

and this link...

http://www.tv-forums.com/forum/TV_Eq...cture_P103843/

I thought it would be a fairly easy fix. I was apparently wrong.

I got the TV opened and could immediately tell that it had been worked on before I got it. All four 15pf caps had been swapped out. Whoever removed the original caps did a number on the board. I can't find a schematic so, unfortunately, I'm not entirely sure if I can even repair this. I removed the caps that had been put in, cleaned the board a bit and tried to determine how to fix the damaged traces. I put the caps back in and did my best to fix the traces with jumpers. At this point, I checked for continuity where I believe there should be continuity and put the board back together. I still get a brief flash of the backlights. I'm not sure if the board was repaired correctly and I have a separate fault or if the board was not repaired correctly. Here are some pictures. Stay tuned for voltage readings when powered on.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

more pics

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

More pics. Other side

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Do these caps, 15j 6kv, have a polarity? Do they need to be installed a certain way?

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

No polarity.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

So in my search across the web, I found that Coppell offers this chip to fix a similar issue. I'd like to try to trouble shoot the chip before I blindy replace it and find myself still needing to buy a new PSU. Any help trying to determine which pins to probe for voltages would be appreciated.



http://www.ebay.com/itm/OB3316QP-BAC...-/110890297821

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Check pin 1: high signal (>2V) should be observed
Check pin 7: VDD should be present (4V to 7V)
Check pin 16: should measure >1.25V.

All ground referenced, if on primary, you need to check to the -ve of the big cap.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Are you saying if the chip is on the primary side it should be referenced to the negative leg of the big cap?

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Yes. Is it?

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

The chip is on the cold side of the PSU.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Measured against chassis ground, I'm getting 4V at pin 1 and 5V at pins 7 and 16.

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

The IC is being told to turn on. So, it should be working, suggesting it may be bad. Check any and all fuses in the inverter section.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

I was looking at the inverter or maybe actually the balancer board last night and noticed a discolored and possibly slightly puffy 15pf 6kv cap. Most of the single caps on the board are 27pf 6kv caps. In this spot, there were two 15pf caps in series. I didn't have any 15pf caps so I pulled both caps in that spot and replaced with three 10pf 6kv caps in series. You can see them in the pic if you look close. At any rate,it looks like there may be another discolored cap or two in the board. Can I follow budm's inverter test procedure and connect a single fluorescent bulb to the power connector on the PSU to see if the inverter section of that board is functioning properly which would then point me to the balancer board?

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Two 15pF capacitors in series makes a 7.5pF; three 10pF in series makes a 3.3pF. They need to be in parallel. (Capacitors in series always have a total value less than any one part.)

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Yeah, I'm a dumbass. I meant parallel.

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

You could use my favourite trick which is to use a neon bulb near transformer outputs. High voltage makes them glow. I need to upload a video I did about that.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

I don't think I have a neon bulb, but I'm interested in the video nonetheless.

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Used PSU installed and the set is working great.

[tom66]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

Nice fix, I guess you decided repair was too expensive?

[johnboy1313]

Re: RCA 42LA45RQ no backlights, bodged cap replacement on PSU

I wasn't certain that the IC was the problem. If I knew for sure, I would have given it a try. As it is, the price for the PSU was good enough. I didn't want to risk it.