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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.....
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File Type: pdf ns500_520_sup2_7115.pdf (1.38 MB, 4 views)
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Old 03-12-2022, 05:46 PM   #2
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Default Re: Panasonic PBX shuts off at random

As usual I would check or better yet replace C32, vcc for U2 and C13 or at least check the vcc voltage of U2 to be sure its not going into burst mode due to low vcc

Check if your +41v at CN100 is there all the time, also if the +15 disappears, the whole secondary circuit will likely not operate so Q15 would likely turn off.

Last edited by R_J; 03-12-2022 at 06:01 PM..
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Old 03-13-2022, 05:11 AM   #3
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Talking Re: Panasonic PBX shuts off at random

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As usual I would check or better yet replace C32, vcc for U2 and C13 or at least check the vcc voltage of U2 to be sure its not going into burst mode due to low vcc
I already replaced those, which leads me to this:

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Originally Posted by R_J View Post
Check if your +41v at CN100 is there all the time
Yes, it is - up to Q15, so point 19 gives me the correct voltage all the time. Point 20 doesn't.

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Originally Posted by R_J View Post
also if the +15 disappears, the whole secondary circuit will likely not operate so Q15 would likely turn off
Yes, that's what I'm expecting, but point 20 still shows 25v somehow. I'm not entirely sure what the power sequence is: does the 15v rail ultimately turn on Q15 ?

I'm trying to work out that mess on the gate of Q15...I see 15v coming in on the gate of Q32 (below the connector), so when that goes high, Q32 pulls the anode of Z14 to GND, completing the circuit, which causes 24v to show up on Z17 which is now forward biased and just passes DC like a regular diode, minus some drop. Z18 is facing the other way, which means it clamps this whole rail to 24v ?.....which seems pretty high for the gate of a FET :| I THINK R106 and R109 act as a divider and drop this down to an acceptable value and the zeners are there for clamping purposes, which STILL seems a bit high too me and it doesn't make sense why there's two of them back to back there - would've probably chose 18v zeners, but heck, they know better...

With Q33 now finally on, I get stuck: anode of D16 gets pulled to GND and then what ???

I also see that 60v rail there which is also generated by an additional winding on L3 driven by U7, so if that dies, Q15 probably turns off. This makes a bit more sense: 60v comes in through R99 and R104 (no idea why they chose two in series), goes through D16 and finally is clamped to 15v by Z11, which is again kinda confusing, since its cathode doesn't go to GND...what a weird circuit. I remember measuring the gate of Q15 and seeing 57v and freaking out, but then I referred to point 21 and that's actually correct ! I'm assuming it's because D15 is referenced to the 41v rail as opposed to GND, so the G-S voltage on Q15 is not ACTUALLY 57v (as "seen" by the gate) - that is what the meter reads referenced to GND, but G to S must somehow still be within the 20v rating...although 41v-15v=26v which is too high still, unless R66 and R74 act as a voltage divider there yet again.....whew...:|
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Old 03-14-2022, 01:44 AM   #4
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Default Re: Panasonic PBX shuts off at random

Here's the situation: I left the PBX running on my desk on Friday over the weekend and when I came back to it this Monday morning it was dead again, with the same symptoms: no 15v output and only a faint 15v on the 41v output. This time however, it wouldn't power back up, no matter what I tried, so something finally died for good, which is good for the sake of the repair, since I don't have to guess anymore. Touching my probe to pin 4 of U7 this time does nothing.

I figure U7 finally packed it for good because it has 41v on pin 1, 0v on pin 5 (SD) yet it just sits there doing nothing. I checked for shorts, didn't find any, and also checked C12 and it measured fine, so I decided to replace it and hope for the best...
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Old 03-14-2022, 07:22 AM   #5
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Default Re: Panasonic PBX shuts off at random

Replaced U7 and it fired up right away and it's been working for around 7 hours now....here's hoping it lasts :|
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Old 03-15-2022, 01:56 AM   #6
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Default Re: Panasonic PBX shuts off at random

Second day of operation and it'd been fine, so I think that was it: U7 sometimes felt like taking a break....
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Old 03-15-2022, 06:03 PM   #7
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Default Re: Panasonic PBX shuts off at random

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Second day of operation and it'd been fine, so I think that was it: U7 sometimes felt like taking a break....
Have you torch tested it yet other than just letting it run idle mode
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Old 03-16-2022, 12:18 AM   #8
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Default Re: Panasonic PBX shuts off at random

Not much I could do with it myself, plus I don't imagine it's doing any major work when in operation anyway, so I just let it run on my desk however it pleased. It doesn't matter in this case, because the guy who brought it in said it'd shut off anyway after around 8 hours or so, no matter what it was doing, so I think we're good.
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Old 03-16-2022, 12:42 AM   #9
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Default Re: Panasonic PBX shuts off at random

Then if this is the case then it sounds like you might have fixed it
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