DEC Rainbow H7842 Power Supply Failure

The badges are on their side because this was installed in a vertical enclosure.

A couple of days ago, I received a request to format some RX50s in one of my Rainbow machines. So, I got this one out to check that it still works. I was happy that it just powered up fine. I went off to search for some RX50 diskettes to check the floppy disk drive with. When I came back the machine was silent. It had powered itself off. There was a vague burning smell and the circuit breaker on the back had popped.

Not knowing if there was perhaps a short in the machine or a problem in the power supply, I disconnected the fans, the floppy disk drive, the hard disk drive and, probably foolishly, I applied power again to see if the machine would work. At this point there was a bang and a flash in the power supply.

On opening up the H7842 power supply I found that one of the transistors had completely disintegrated. It looks to be the main switching transistor.

Given that before the transistor blew up there had clearly been another failure somewhere else, I tried to find the original failure. There were no obviously damaged parts, so I just probed around near the transistor for any parts that were open circuit or short circuit. I found a diode connected to the base of the transistor that appeared to be short circuit. So, I decided to lift one end to check it. As I de-soldered one of the leads, the diode broke in two. So clearly the diode was either damaged by the failure of the transistor, or it was the cause of the failure. This is the diode:

My problem now is that I can’t make out the markings on the diode, although they might say “D610”. I don’t know whether this diode failed and caused the transistor to blow up, or if instead something else has failed, which has then caused the diode and the transistor to fail.

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2 Responses to DEC Rainbow H7842 Power Supply Failure

  1. Phil Barton says:

    I would look for a short on the low voltage rails on the secondary side of the power supply. DEC frequently used identical diodes in several positions within power supplies. Any Silicon diode rated at 1.0A and say PIV=200 would be more than adequate. On second thoughts you should scrap the Rainbow and give it all to me.

  2. Bernard Klatt says:

    You’ll have to do some circuit tracing, but it has component values (believed to be accurate).

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