5 Dec
2015
5 Dec
'15
3:01 p.m.
-----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
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
-----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:
How BIG a spike? In a lot of these power supplies, they don't
regulate
the
-----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?
"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