Anyone [H]ere work for Viewsonic?

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
My father gave me a Viewsonic TV -- it's an N2630w, aka VS11749-1M; it's from 2007 -- to try and fix a few months ago. Some capacitors in the PSU went splat. Replaced them and got a blinking red light, so I just ignored it for a while, figuring it was over my head.

Pulled it back out earlier today to have another go. Replaced an obviously faulty 7805 (!) on one of the two logic boards (according to the Service Manual I have, it's the "DTV board"). Now I get two red blinks of the power light and that's it -- and I have to wait a long time for it to give me those blinks again... like a half hour...

Before that regulator was replaced, I was getting at one point six or seven blinks, and at one point three blinks.

Of course the Service Manual doesn't include a list of POST codes for the damn thing, and the "troubleshooting" flowchart is basically ornamental.

Anyone know how to decode those blinks, so I know what to replace next?
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
UPDATE --

I know what the problem is, now. A transformer on the power board has gone and died. Four leads on the primary, four leads on the secondary, and an extra wire coming out the top that I think is a tap.

The entire secondary is dead out shorted. Most of the primary is, too -- I get some resistance between the first and last pins but that's it.

This transformer is one of the secondary transformers, off of the input transformer.

How do I measure the voltage going into it, easily? I tried with my DMM -- got 000 for DC (it was only there real quick like, I don't *think* I popped anything) and a vast array of fluctuations on 600v AC mode.

For the record, this is my DMM, only in Fluke-Got-Mad discontinued yellow and dark grey instead of the new grey and dark grey. (I bought it before the kerfluffle happened.)
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
The transformer in a switching power supply runs at a far higher frequency (tens or hundreds of kilohertz) than your multimeter is likely designed to measure (AC in your house is 60Hz), and doesn't have a nice sine wave, either.

When you say the entire secondary is shorted, how did you come to that conclusion? Generally, transformers should have next-to-zero resistance when measured with a DMM--after all, they're just a long piece of copper wire.

The fact that you're getting a blinking red light means that the power supply is supplying at least *some* power correctly, and there's some diagnostics being run. You should be able to measure the DC values coming out of the power supply (be careful to avoid the AC side!) with your DMM. I'd expect to see a 12V or 24V output, and a 5V or 3.3V output.

My guess at this point is one of a few things: 1) backlight driver has gone bad, 2) popping voltage regulator took some of the other hardware with it, or 3) there are still some bad capacitors.
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
I had replaced three or four caps on the board, after this specific transformer, and it's still blinking at me. I've also replaced a 7805 on one of the two logic boards.

The transformer in question supplies 5v, both during standby and normally. Standby current is rated at 1a, and on-current is rated at 6a. No-load, IIRC, the connector for the /standby/ current (+5v, GND, and a standby on/off trigger) is around 5v. I recall it being higher but I don't recall it being /much/ higher. I don't think it was a full volt.

Also, the transformer in question ticks when power is applied to the supply board.
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
Does it tick repeatedly, at regular intervals? If so, that most likely means that its overcurrent (short-circuit) protection is kicking in. It still could be a shorted winding in the transformer, but my money's on something downstream being shorted to ground. What do you see when you measure the full-on 5V line?
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
Repeatedly, at about 2-4Hz I'd say. Haven't measured the standard 5v line, but probably not very much until I figure out how to take it into / out of standby mode...

EDIT: FWIW, I'm taking the board to a friend of mine in about an hour, he'll know a lot more than I do ;)
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
UPDATE: a rollaway (MELF) diode had shorted near the transformer. It was replaced with a similar, but still-functioning diode from a nearby circuit board. Should work well enough at least to test.

It was color-coded. Three bands, black black and black ;) (meaning, heaven only knows what it really had been...)
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
...well, somebody screwed up somewhere, the thing won't turn on at all now :(
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
Sounds like I was right. The problem with a situation like this is that you don't really know what else might have been damaged.

If you really want to get it fixed properly, you'll probably have to reverse-engineer the board, draw a schematic, and start testing components one at a time. My guess is that the diode you replaced was part of a snubber circuit (if it was a secondary-side rectifier, it would probably have been on a heatsink), but I'm just spitballing here.
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
The diode is part of a circuit that includes two 330R chip resistors as a voltage divider, and a TL431. I suspect that the TL431 has gone to visit The Great TV Repairman In The Sky. I've scavenged three replacements out of an old PC PSU (it was working, but permanently out of use from complications of ATX connector replacement surgery). I'm going to put one in and test the TV. If it's broken I'll reverse the diode. If it still doesn't work I'll try another TL431.

This should be fun :D
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
To be of any help, you'll need to actually trace out a schematic.
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
I'm fairly certain at this point that the problem is elsewhere in the TV. That 7805 I replaced never turns on... may mean something, may not. I'm gonna swap it out a second time, prolly Wednesday night (I doubt I'll be out again before Wed).

Besides, there's some pretty hefty blocks of metal covering parts of that PSU, and I really don't feel like mucking with them.
 

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
Update on this...

After some work of my own and additional help from one of my IRL friends, I've determined that the "DTV" board has a bad chip somewhere. It's one of two Marvell chips, either the BGA CPU or a secondary chip that's SMT but not BGA. Looks QFNish to my eyes.

Doesn't matter, I can't buy either chip by itself. (If it's the BGA chip, I can't even put that in...) The whole board will have to be replaced -- by someone else since I don't have $60...
 
Top