Memory Voltage on MicroVAX II

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Robert
> Jarratt
> Sent: 05 December 2015 19:49
> To: 'General Discussion: On-Topic and Off-Topic Posts';
> General at classiccmp.org
> Subject: RE: Memory Voltage on MicroVAX II
> 
> 
> > -----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

Checked ripple again and 5V is fine, and 12V is actually not that big.
Putting the memory back in, it started to work. So it just needed
re-seating. Phew! It all works now, I have a working rtVAX 1000. Now it is
time to find out what it can do.

Regards

Rob