On 12/05/2015 12:29 PM, Robert Jarratt wrote:
-----Original Message-----
From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Jon Elson
Sent: 05 December 2015 18:03
To: gene...@classiccmp.org; discuss...@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
