Viewsonic PF790 shuts down within seconds

Jun 05, 2008 5 Replies
D

I've started looking into a problem with a Viewsonic PF790 monitor that I've had sitting in the closet for the last year or so. The problem is that about 1 second after turning the switch on, it stops. The green led on the front panel remains on, but the degauss relay abruptly turns off, as does the high voltage. Every once in a while it will turn on without a problem, and work perfectly for hours at a time. There was a time when it would reliably turn on if there was no video signal connected, or if a low resolution signal was connected, but that is no longer the case.



I have read the group's FAQ, and I have access to schematics. My first thought after reading the faq was to check the x-ray protection circuit. According the service manual, it is activated when the voltage at the relevant pin on the controller IC exceeds 8V. With a DMM it was difficult to get an accurate measurement within the second of activity, but it was almost certainly below 8V. I tried halving the voltage divider ratio to see if that would make a difference, but it didn't. I presume that something else is shutting things down.



If anyone has any suggestions as to where I might look next, I'd very much appreciate it.



Dave


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M

I agree. Mine needs some new PS caps. It has been sitting waiting to be repaired when I have time.

Mike

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J

The 4N35 optoisolator in the power supply fails on these, causes the voltage to slowly drift up, takes out capacitors and eventually pops the vertical output IC. I've fixed probably a dozen of these and the similar

17" models with the same problem.
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D

Thanks for the pointers. I took a look at the feedback circuit, which senses off the 180V line. I adjusted the trimmer to lower the voltage a little, and was able to get it to start. The voltage at that point was 175V. After adjusting it back up, it will now start with no or a low freq hsync, but not with a high hsync. Once started, it will remain on when a high hsync is connected. Judging from the trimmer position, the voltage was probably a little high originally, but not more than 185V. Is this the opto-isolator drift you've seen?

Following a hint from the diagnostics in the service manual, I temporarily disconnected the over-voltage circuit (an SCR fed from a zener, from a primary coil on the transformer). The monitor would power up correctly in all situations. The 180V line appears stable. Next chance I get, I'm planning to remove and test these parts. In-circuit testing didn't show an obvious problem.

I don't have a lot of experience with switching supplies, so in the meantime I'll do some more reading on the subject...

Dave

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D

It looks like the problem was the filter capacitor for the winding used to sense over-voltage I measured substantial ripple at the switching rate, and paralleling another cap eliminated both the excessive ripple and the turn-on symptoms. On removal, it measured half its nominal capacitance. I monitored the secondary voltages, and they were stable, so I'm going to assume that this is it for now. I adjusted the focus and G2 while I was there, so now the monitor is working better than ever.

The FAQs for this group were very helpful for someone like myself with an electronics background, but who doesn't usually work with these types of circuits. Thanks!

Dave

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