Memory Voltage on MicroVAX II

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Jon Elson
> Sent: 05 December 2015 19:19
> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic and Off-
> Topic Posts
> Subject: Re: Memory Voltage on MicroVAX II
> 
> On 12/05/2015 12:29 PM, Robert Jarratt wrote:
> >> -----Original Message-----
> >> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Jon
> >> Elson
> >> Sent: 05 December 2015 18:03
> >> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic 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.
> 
> Jon

I would have to measure again, but I don't think it was anywhere near that
big a spike. I did measure the ESR on the capacitor on that output and it
seems OK, although marginally higher than the same cap on some of the other
outputs, so I am thinking of replacing it anyway.

As the 5V seems fine, the ripple seemed to be about 20mV (although I am
going to check again), I do wonder what could be causing the memory modules
to appear to be failing. I am hoping that re-seating will cure it.

Regards

Rob