Rob Jarratt robert.jarratt at
Sun Mar 29 01:55:24 CDT 2020

Thanks Mattis. I do test it with a load, although to be honest I forgot to do this when I took the measurements yesterday. I use a modern-ish IDE disk and a load board from a MicroVAX 2000 as the dummy load. I don’t know if that is sufficient.


I don’t know enough about PSUs to make the secondary side drawing more logical unfortunately.


I have a variac and a bench power supply, so I could do what you suggest. Could you be a bit more specific about where to apply what, so I don’t do it wrong or damage something? Would you put the bench PSU across the UC3842 Vcc and Gnd pins? I am not sure what would happen if the normal supply to the UC3842 was still in place with the bench power supply also trying to supply power. Would it be wise to lift R32 so nothing conflicts with the bench power supply? 






From: Mattis Lind <mattislind at> 
Sent: 29 March 2020 06:39
To: rob at; Rob Jarratt <robert.jarratt at>; General Discussion: On-Topic and Off-Topic Posts <cctalk at>
Subject: Re: VAXmate PSU


Hello Rob,

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




Any thoughts gratefully received.



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