I got this 7854 to repair.
Main symptom:it blows the 4A main fuse. After several tests and measurements, the culprit was found in one of the two leveling capacitors (C16), 950uF 200V with a serious leakage.
I replaced both capacitors C16 and C17 for safety. The SMPS chassis sits on my workbench, fully extracted from the mainframe.
I put a dummy load across +15 and -15 V (one resistor 33 ohm 10W), insert a jumper on P72 on LV board (thermal switch) to avoid a false FAULT sense condition and power up the SMPS.
The fuse does not blows. Good. The security indicator neon lamp is steady on.
I got the somewhat expected tick-tick from the SMPS. Damn...
I already read the various topics on this forum regarding the infamous 7xxx SMPS and all the possible documentation and/or links about that.
So I proceed with measurements and investigation.
With my surprise, the tick-tick is not as usual (one tick/burst every 500 ms or so) but is very irregular
With my oscilloscope I see short random pulses followed by one, two sometimes even three or more bursts (pin 9 of U75, base drive control).
On pin 1 of U75 I see a signal that grows towards + 0.6V but in a very irregular manner (in correspondence with the very short pulses) and then reach 0.6 / 0.7V when (in a random manner) the normal burst begins.
At this point I am totally puzzled because due to this random behavior I cannot understand how the correct signals should be.
To be sure, I've disconnected completely the LV regulator board and I've put the dummy load directly across the +17/-17V.
I checked all the voltage levels on the low-voltage rectifier section: during the burst they reach more or less the nominal value, so there are no short circuits or lossy capacitors. The +108V are present.
The fault sense pin shows an irregular signal of 50mV or less so I don't think that's the cause.
The over-voltage stop seems O.K. in the sense that the emitter of Q45 is LESS positive than its base. The zener diode VR45 is correctly regulating to 120V.
I've checked with my analog VOM (ICE 680R) all the bjts. All of them seem OK. The programmable UJT Q30 is OK also.
Checked the power transistors Q34 and Q40: they are good.
Checked for leaky capacitors OK
Checked all the diodes OK
I've been working and studying on this beast for three days but with no results, except a big headache.
Any ideas and/or suggestions?
Max
