Rob Jarratt robert.jarratt at
Sun Mar 29 05:39:53 CDT 2020

Thanks, I do have an isolating transformer as well, so I will use that. I have done previous tests with a bench PSU on the UC3842 and found it needs 16V to get going. I just checked the Technical Description and it says the minimum current on the 5V output is 6.4A and on 12V it is 0.17A. I checked my load board and it is only going to sink 3A, so I need more load as I think the IDE disk is not going to be much. Would insufficient load really cause it to shutdown so quickly though?


Not sure I understand your comment about “designators for networks”, is there an example you can point me at? One of the things I have tried to do, but clearly not very successfully, is to minimise the lengths of the wires.






From: Mattis Lind <mattislind at> 
Sent: 29 March 2020 08:51
To: rob at
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk at>
Subject: Re: VAXmate PSU


Hello Rob,

söndag 29 mars 2020 skrev Rob Jarratt <robert.jarratt at <mailto:robert.jarratt at> >:

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.


Do you know the nominal output rating for the supply?

It might be the case that even with load board the load is uneven. But it sounds less likely.  


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


Can be hard. But a good idea is try to minimise the length of wires. Maybe use designators for networks instead of wires to make the schematic more readable. On the other hand I was reading the post on the iPad. 


I have a variac and a bench power supply, so I could do what you suggest. 

To stay safe you need a protection transformer as well. Otherwise the the bench supply will end up at line potential. With the protection transformer in place the circuit will be left floating in relation to earth. With a variac you can then vary the input and keep it within safe limits. If you don’t have a protection transformer and variac then another bench DC supply that can give up to 100 V can be used instead. 


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? 


Absolutely right. I should have been more explicit about where to introduce external supply. But as you say, lift R32 to isolate the switching controller and feed in the bench supply current at VCC. Check the data sheet for what is approriate voltage. 


Good luck!








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