Rob Jarratt robert.jarratt at
Sun Apr 5 08:05:39 CDT 2020

I found time to follow Mattis’s suggestion today and I got some interesting results.


I powered the UC3842 with about 16V from a bench power supply. I lifted R32 so that the transformer would not supply it. I then used an isolating transformer to power a variac and applied the variac to the AC inlet. I also used a load board from a MicroVAX 2000 and an old RD53 disk as the load, so there should be enough load.


I found that I can vary the AC input up to a maximum of about 40VAC before the SCR triggers, the  5V output reaches about 400mV. If I raise the AC input more slowly, it will usually cut out before that, around 30VAC. I noticed that the inrush thermistors also get quite hot at these low AC voltages, I don’t know if this is because of the relatively low AC supply voltage, or if this indicates a problem of some kind.

The voltage coming out of L3 into the T1 “bounces” somewhat. I guess this is because the AC input is only 20V or so, or it may be expected ripple from the smoothing capacitors? In the description below, the peaks of the bounces are used. Throughout the variation from 0VAC to 40VAC the duty cycle of the oscillation of the UC3842 output does not change, I guess because the output voltage has not reached its target value.


With the AC input at about 25VAC the circuit seems to be stable (apart from the bounces mentioned above). At this supply voltage, the voltage at the source of Q1 reaches 2V. The current sense resistor is 1 Ohm, which means 2A must be flowing through it at that time.


When the Q1 source is at 2V, the other end of R14 is at about 0.5V, which is just below the trigger voltage for the SCR. This makes sense because R14 and R15 form a voltage divider that looks to be nominally 25% of the Q1 source. Given the SCR nominally triggers at about 0.8V, this means that the current sense resistor is set to trigger the SCR at about 2.5A, I think. This would suggest that the duty cycle on Q1 is too high and causing too much current to be drawn. So presumably the feedback to the UC3842 is not working correctly.


I tried setting the AC input at 120V and using a one-shot sample. Q1 is switched for about 30ms and then there is a spike on the SCR gate to 2V and it triggers. The gate voltage then remains at 1V. However, there is no spike across the current sense resistor (R13), so I don’t know if the spike is because the SCR is being turned for some other reason. There is nothing unusual on the anode of D19 to cause it to trigger due to avalanche breakdown. I got the same result when the AC input was 220V. I wonder if the SCR is behaving slightly differently because I have lifted R32?


Since there might be a feedback problem, I looked at the VFB input to the UC3842 when doing a one-shot test at 240VAC. I can see VFB steadily rise over the period when Q1 switched, up to a maximum of 4V. I don’t really know if this is how it should behave though, but it seems to make logical sense. During all that time the duty cycle of Q1 does not change.


I am not too sure where to go from here. I hope the above makes sense. I would appreciate any further thoughts.







From: Rob Jarratt <robert.jarratt at> 
Sent: 29 March 2020 11:40
To: 'Mattis Lind' <mattislind at>; rob at
Cc: 'General Discussion: On-Topic and Off-Topic Posts' <cctalk at>
Subject: RE: VAXmate PSU


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 <mailto:mattislind at> > 
Sent: 29 March 2020 08:51
To: rob at <mailto:rob at> 
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk at <mailto: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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