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Old 03-12-2022, 05:03 PM   #1
Dannyx
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Unhappy Panasonic PBX shuts off at random

Good day folks. I wasn't too sure whether I should start this discussion just yet, because I feel I haven't dug deep enough, which I can't do until Monday at work, but I eventually thought I'd just go for it so by the time people start joining in, I'll hopefully be able to complete my measurements.

TL;DR - Panasonic KX-NS500 PBX sometimes dies - PSU issue (schematic attached). 41v rail drops to 25v and 15v rail goes off entirely. Often, even a power cycle doesn't restore it - main cap (C4) needs to be "manually" discharged for it go gain.

Details: to keep it as short as possible, there are a couple of things I noticed so far, since it's all random AF:

Schematic is on page 14 and HOLLY BALL$$ this board is complicated ! SO MUCH stuff going on ! They have the courtesy to explain some of it in the first pages, but it doesn't help all that much. Granted, most of it has to do with protections and signaling and stuff, since at the end of the day, it's just a SMPS producing a 41v output which is then stepped down to 15v by U7. When it works - it works: both rails operate normally and I get all the readings indicated by the manual on the test points - no problems.

First thing I noticed: C4 keeps its charge for what I'd call an insane amount of time ! I swear it was still charged to 300+ V after 2-3 days ! HOW ??? I even pulled it off to check it and it measured fine, so that's a bit weird....

Second: getting it to actually crash is a gamble and I wasn't able to find any connection as to why it happens. Sometimes it works, other times it doesn't start at all and other times (like right now) it just dies after operating for a while. It'd start up fine, give me the correct output, the PBX itself boots, then I'd flip the power switch off and back on a second later and it doesn't start again, though not always and this is not always repeatable ! Once it happens though, you can flick that power switch all you want, but it's not waking it up again ! This is why I mentioned the main cap holding a crazy charge, because normally in such cases, you'd unplug it and plug it back in and it'd get it going a second time, enough to figure out what happened, but because the cap stays charged, you'd have to leave it unplugged for like.....a week for this to happen...

Third: I WAS able to get it to crash a couple of times using the power switch trick and quickly took some measurements while it was dead: the 41v rail before Q15 remains stable, but the output (after Q15) is now only 25v (random AF !). U7 was not operating either, so no 15v on the output. I then wanted to measure the FB pin (pin 4) of U7 and guess what: touching my red probe to pin 4 caused the board to spring back to life right there ! What gives ? I tried it a second time after another crash and the same thing happened ! Once it DOES run, pin 4 reads around 1.6, which is normal according to plot 33. I checked R64 and R65 and they're OK. I also touched up the area with some solder, thinking perhaps me touching the pin is enough to make contact between some weak joints and I was just about ready to pack it up and call it a day, thinking that was it, since it didn't happen again after that, but just as I was relishing in my success, the power LED went off again ! DAMN !

Those are my findings so far...

I'm not entirely sure why Q15 just decides to turn off....it doesn't explain why there's still 25v there. Any doubt that it's residual charge from the caps is debunked by the fact that all these tests were performed with the PSU actually connected to the PBX, so the load should cause that to drain, which is not happening - it's "leaking" from somewhere. Even so, it's not clear what's going on with U7, which is too expensive to replace on its own just to test, since that was my original assumption. Even if it were bad, it shouldn't affect Q15 and the 41v output...unless someone sees some connection.

I'll definitely want to measure pin 5 of U7 during a "blackout" to see if it's still "ON", but not running, or if something in the protection circuit triggered somehow.....
Attached Files
File Type: pdf ns500_520_sup2_7115.pdf (1.38 MB, 4 views)
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