> -----Original Message----- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Jon Elson > Sent: 05 December 2015 19:19 > To: gene...@classiccmp.org; discuss...@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-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
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
