IFR A8000 tech question power supply

[HarryHydro]

Hi Folks:
I'm working on an A8000 that seems to have a flaky power suppy.
After changing the electrolytic caps, the stink is gone but after
sitting, it still blows fuses. On 12V input, it still blows the 7.5A
fuse. If I change it, turn it back on, it works. I've ran it all
night like this, with no overheating. Let it sit overnight, off, just
plug it in in the morning, and it tries to come on, blows the fuse. I
have a limited 12V supply on it now to see what's happening before it
blows the fuse. It seems to be making 5 and 12V +-, but the snubber
looks like it's getting too hot. The resistor is black now.. The
snubber-cap looks good. Maybe this diode is shorting? Maybe this
thing shouldn't be ran without the internal battery connected (it's
dead)? Why would this be coming on by just plugging it in? Sometimes
when I press the 'battery' side of the switch, the supply puts DC
voltage on the gates of the FETs and that's pulling my limited supply
down. The FET's then go linear and quickly get hot! Why would it try
to run at low voltage? I see no switcher-chips, just op-amps. A clue
maybe, there is some substance under one of the caps I changed. Maybe
electrolyte? Freon TF doesn't seem to take it off. I don't know... I
have 2 1200'service monitors that have had no problems.
Thanks for any leads
Harry

 

[HarryHydro]

Hi Folks:
I'm working on an A8000 that seems to have a flaky power suppy.
After changing the electrolytic caps, the stink is gone but after
sitting, it still blows fuses. On 12V input, it still blows the 7.5A
fuse. If I change it, turn it back on, it works. I've ran it all
night like this, with no overheating. Let it sit overnight, off, just
plug it in in the morning, and it tries to come on, blows the fuse. I
have a limited 12V supply on it now to see what's happening before it
blows the fuse. It seems to be making 5 and 12V +-, but the snubber
looks like it's getting too hot. The resistor is black now.. The
snubber-cap looks good. Maybe this diode is shorting? Maybe this
thing shouldn't be ran without the internal battery connected (it's
dead)? Why would this be coming on by just plugging it in? Sometimes
when I press the 'battery' side of the switch, the supply puts DC
voltage on the gates of the FETs and that's pulling my limited supply
down. The FET's then go linear and quickly get hot! Why would it try
to run at low voltage? I see no switcher-chips, just op-amps. A clue
maybe, there is some substance under one of the caps I changed. Maybe
electrolyte? Freon TF doesn't seem to take it off. I don't know... I
have 2 1200'service monitors that have had no problems.
Thanks for any leads
Harry

Hi Folks:
I fixed this supply. I used a can of contact/control cleaner &
lubricant. The problem was electrolyte from the old caps. The plated
traces had a dull grey appearance under the caps and all around the
caps where the opamp oscillator was. The oscilator was starting too
late, and after spraying things around, didn't start at all. With
this spray and some paper towels, I swabbed it clean and it's
working!
Harry