Mattis Lind mattislind at
Sun Mar 29 00:39:17 CDT 2020

Hello Rob,

lördag 28 mars 2020 skrev Rob Jarratt via cctalk <cctalk at>:

> I have posted here a couple of times because I have a failed VAXmate PSU. I
> have just posted a little bit more information here:
> analysis-of-the-vaxmate-h7
> 270-psu-failure/ with some scope traces and a greatly improved schematic.
> Although the schematic is likely to have errors still. Unfortunately, a
> stray scope probe ground lead blew the fuse so now I have to wait for a new
> fuse to arrive before I can continue work.
> I would really like to know if all the spiking I am seeing is to be
> expected, and any suggestions why it appears to be detecting an
> overcurrent?
> There do not appear to be any shorts on the secondary side, but that could
> be wrong of course. I don't know if a genuine short anywhere would cause it
> to trip the SCR quite so quickly (within 20ms of the switching transistor
> starting to switch).
This is fly back design and I would expect some spiking when the transistor
shuts off.

Then for over-current. It might be so that there are over-voltage
protection on the outputs that kicks in. A crowbar that short circuits the
output. It looks like there is such a circuit on 5 and 12 V. But to be
honest the output circuit schematic is hard to read.

If you have no load or little load or un-even load the PSU might hae
problems to regulate. I know for fact that the PSU in the MicroVAX 2000
need to have a dummy load when no hard drive is installed otherwise there
will be uneven load which it has hard time to handle sonce the output
regulation is based on the sum of the outputs somehow. It will trip the
crowbar on over voltage on one of the outputs otherwise.

What if you supply the control circuitry on the primary side using a bench
lab supply and then connect a protection transformer and a variac in series
to the normal AC inlet.

Slowly increase input AC voltage while monitoring source voltage and output
voltages.  At what AC input voltages does it trip? What is the output
voltages at this point?

If both voltages exceed normal and the crowbar trips I would think that the
feedback network somehow reports to low output voltage to the control
circuitry. Maybe the opto coupler is bad?

Sorry. A lot of guessing here. But it is hard to tell withour more


> Any thoughts gratefully received.
> Thanks
> Rob

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