While I was using my newly working VAXmate the other day, it suddenly failed. I had left it running for a few minutes and when I came back there was a smell and the machine was not running. I wondered at first if it might be the filter capacitors in the power supply, but if these fail the power supply continues to work.
I took the machine apart and examined the H7270 PSU and other boards for any obviously blown components. However I could not find anything. I have since looked multiple times and still cannot see any obviously blown component.
My first step was to see if the input is being rectified. I found 300V across the two big smoothing capacitors, so, yes, the input is being rectified. The next step was to see if the switching transistor on the primary side was switching, and to see if it is working. I found that it is not switching, but after desoldering it and putting it into my Peak Atlas Component Analyser, it seemed to be working, so the transistor does not seem to be the problem.
My attention then turned to the NE555 timer that drives the switching of the primary side transistor. It is not doing anything and I discovered that it is not getting any voltage to operate.
This means that the current sense circuit on the primary side is detecting an overcurrent condition, which is shutting down the primary side. The causes of this can be multiple, but one is that the crowbar circuit is detecting an overvoltage and forcing a short on the secondary side that triggers the overcurrent condition which then causes the power supply to shutdown.
I have been testing the PSU with no boards attached, and an old IDE disk acting as a load on the 5V and 12V supplies. According to the Technical Description only these supplies are monitored for overvoltage. Any other overcurrent would be a short in one of the other supplies.
So, I have a PSU which won’t start, despite there being no visible damage that can account for the bad smell when it failed, and despite not being connected to any other boards, and only to a known good hard disk drive. The cause seems to be that a problem is being detected that shuts it down. I only have an analogue oscilloscope, and I can’t use it to diagnose a transient condition like this one. I am going to have to get a Digital Storage Oscilloscope that can be used for transient conditions like this. I want a more modern oscilloscope anyway, so I am going to get a Rigol DS1054Z after a friend recommended it to me and after seeing a positive EEVblog review. When I have that I hope to be able to investigate further.
For reference I have reverse engineered the PSU and done a couple of partial schematics. They are probably not drawn logically as I am not an expert.
The component numbers are assigned by me as they are not marked on the board, and I have posted images of the board with the components labelled. A few component numbers are actually named in the Technical Description and I have used those numbers.