SPS-9400/MP3090/BK-1692 15V 40A Power Supply Repair

[ZigmundRat]

Having acquired a good BK-1692 supply, I then picked up one that was nonfunctional, figuring I could repair it using the available service manual and schematic, and using the good one as a guide. Unfortunately those docs are for the Ver 1 of the supply and it’s changed a bit since then. If anyone has any newer version schematics they would be appreciated. I asked BK Precision but have no answer yet.

Anyway, I was able to effect the repair without much difficulty. It turns out the soft start resistors were both open. These white ceramic resistors can be seen at the upper left side, next to the bridge rectifier (metal square thingy) in the attached photo.  They are in parallel in series with the one side of the AC line before going to the rectifier. They get shorted out by the relay just below when it pulls in. I’m trying to get my head around the failure mode here. These are switched out of circuit almost immediately after turn on. So why would they BOTH be open? There’s no sign of heating on them or elsewhere for that matter. Any ideas?

Thanks!

[apefoot]

I have the same BK-1692 variant with the same problem (open soft-start resistors).  Was replacing the resistors a permanent solution?  Did you ever determine the failure mechanism?   Did BK Precision provide a schematic?

[rfengg]

Hi there,
Try emailing "support@bkprecision.desk-mail.com".......I had contacted them once and they digged out a SM for a power supply that was discontinued since 1992.
Pretty helpful folks imo.

[apefoot]

I replaced the resistors, turned the unit on, and got fireworks in the high-voltage section.  Off to Silicon Heaven for that one!

[aeg]

Maybe it's a shorted switching transistor? It could still be possible to repair.

[apefoot]

Too many blackened parts for my skill level!   

[Harry_22]

Hi!
Make a photo of fire area from top and bottom.