Memory Voltage on MicroVAX II

Jon Elson elson at
Sat Dec 5 13:18:56 CST 2015

On 12/05/2015 12:29 PM, Robert Jarratt wrote:
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Jon Elson
>> Sent: 05 December 2015 18:03
>> To: General at; Discussion at and Off-
>> Topic Posts
>> Subject: Re: Memory Voltage on MicroVAX II
>> On 12/05/2015 09:51 AM, Robert Jarratt wrote:
>>> I finally fixed my H7864 PSU so I can now run my rtVAX 1000. However,
>>> I think the machine is damaging memory boards. I checked the ripple
>>> and 5V looks OK, but 12V looks suspicious. Is the 12V supply used by the
>> memory?
>> I don't have my print set anymore, but I highly doubt it.  I think the +/-
> 12 V is
>> only used by serial comm boards, and possibly by some graphics boards.  I
>> would not be surprised if the regulation of the 12 V supplies is not very
>> precise.
>> Jon
> There seems to be a spike on the 12V supply, surely that isn't good, even if
> it doesn't affect the memory?
How BIG a spike?  In a lot of these power supplies, they 
don't regulate the "auxiliary" voltages.  So, whatever power 
is needed to be sent to the main voltage, the aux. windings 
just tap off some of that energy.  If the flux produces the 
right voltage on the +5V (with so many turns on that 
winding) then they put a few more turns on the 12 V 
windings, and assume it should track FAIRLY well.  But, 
maybe during the power-up surge, charging all the caps on 
the +5 network, the +12 might surge a bit.

I built my own power system on my uVAX-II system, and had 
separate supplies for +5 and +12 (mostly for the disk 
drives).  One day the 12 V supply went haywire, and I had 
inadvertently disabled the crowbar circuit.  It went up to 
22 V and damaged the disk drive and blew caps and the RS-232 
driver on the CPU board.  The disk drive worked for another 
week, and then totally croaked.  I ordered the parts for the 
serial transmitter and got it fixed fairly easily. (He he, 
glad I had that print set!)

So, I can say that if the spike gets close to 22 V, that 
would be real bad.


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