BK Precision 1550 Power Supply Repair after Reverse Polarity

[SparkSmokeMan]

Hi All,

I accidentally hooked my trusty old 1550 up backward to a 12V lead acid battery.

At first, it appeared that the supply survived, however I started noticing some symptoms.

-The unit turns on, all buttons and display work.
-Sometimes, the unit works normally, especially at low voltages and currents
-After a few minutes, the output stops creating current, and I cant measure any voltage with my multimeter. The unit needs to be turned off and turned back on and sometimes it begins working again. Sometimes, it will continue to produce current, but only a fraction of what it should be outputting.

Troubleshooting:

-I first checked the main board and front panel board for damaged components and burn marks. None found. There was no smoke or sound when I accidentally reversed the polarity.
-I checked the MOSFETS for shorts, and they all appear to test correctly
-I obtained the schematic for this unit, but am unsure what to check next

Do you have any advice for next troubleshootign steps? I'd hate to thow this out when it appears to be about 75% functional. Any help would be appreciated.

[bostonman]

Can you post the schematics?

[SparkSmokeMan]

Sure thing! Please see attached:

[bostonman]

Sure you have the entire schematic? I don't see where ON/OFF goes (next to PC5 on the right side of the schematic).

Since you claim it tends to work, I'm thinking maybe something happened to U5. It's a D Flop and I'm assuming SW2 is sort of reset because the power switch is SW1 on top.

Some stuff doesn't make sense such as: what is SW2-A (I don't see where it goes), what is M2 and M3, and ZD4 is also labeled 10k. This is why I'm wondering if you're missing a page.

I'd start with the obvious, the 9V and Vcc. From what it looks, the output is isolated from the input through the opto.

Measuring the 9V and Vcc will at least eliminate or isolate (if it's not measuring correctly) the entire upper half of the circuit.