Memory Voltage on MicroVAX II
robert.jarratt at ntlworld.com
Sat Dec 5 17:01:46 CST 2015
> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Robert
> 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
> > "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
> > should track FAIRLY well. But, maybe during the power-up surge,
> > charging
> > 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
> > went haywire, and I had inadvertently disabled the crowbar circuit.
> > It
> > 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
> > 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
> > 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.
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.
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