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Old 03-13-2022, 05:11 AM   #3
Dannyx's Avatar
Join Date: Aug 2016
City & State: Constanta
My Country: Romania
Line Voltage: 230VAC 50Hz
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Posts: 3,277
Talking Re: Panasonic PBX shuts off at random

Originally Posted by R_J View Post
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:

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.

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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