An easy way to check the caps is to see what the voltage across them is like when you put the system under load and the 'regulated' voltage drops. If the cap voltage drops substantially, or if you get a high ripple under load, that's a good sign the caps are bad.
You mention that the +5V holds up with just the CPU and 3 memory boards and then drops as you add more. The CPU and memory boards should pull a fair amount of power on their own so it seems strange the 5V isn't off to some degree with just those boards. Have you tried just those additional boards without the CPU and memory boards to see if they may have bad components on them that are loading the 5V rail? Bad decoupling capacitors on boards can short out/load up the supply rails and cause the problem you are seeing. David Collins -----Original Message----- From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Dominique Carlier via cctalk Sent: Saturday, 19 August 2017 6:17 PM To: Curious Marc <curiousma...@gmail.com>; General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> Subject: Re: DCC-116 E / DATA GENERAL NOVA 2/10 / Nixdorf 620 - Restoring and restarting Hello Marc, In fact no, i am not really sure, I checked twice all the big filtering caps but as you with a low voltage tester. These caps comes apparently from a new old stock and some doubts persists now. I admit, I don't know how to test these caps at rated voltage with my tools or in situ with the caps inside the working (and thus closed/packed/connected) PSUs Dominique On 19/08/2017 07:11, Curious Marc wrote: > Great work, you have isolated the supply fault. Looks like it's not regulating, but the fact that it starts at 5V tells me it actually is, at low current drain. Are you *really really* sure the filtering caps are good *at rated voltage* (we had a recent case of caps that tested perfect with my low voltage tester but were duds at their rated voltage)? Bad caps would cause something like this. If not I'd usually start to check the regulating power transistors. Could be anything else of course, having the schematics would allow for a much more intelligent conversation instead of blind speculation. > Marc > > On Aug 18, 2017, at 10:25 AM, Dominique Carlier via cctalk <cctalk@classiccmp.org> wrote: > > Some news ! > > Following a risky way (but I did not see how to do otherwise), I deactivated the Power Fail by hiding the contact number 23 of the two power supplies. > The idea was to avoid automatic protection by lowering the regulated voltages (+5V and 15V) and see first which unit was involved (G1 or G2), and also which voltages became weak, at what level it is lowered, and according to which board (model or number of connected boards). > > Results of the observations: > - This is definitely the regulated +5V of the G2 power supply. More I add boards more the + 5v level goes down. +5v, +4.8v, +3.6v, +2.9v. It remains stable however with just the CPU and the three core memory boards, it becomes difficult for the power supply when I add boards in addition to these. > - This is definitely not a problem at the level of the Power Fail circuit. > - The big capacitors are not in fault (I rechecked twice). > - So this maybe a problem at the level of the regulation itself, the +5V balancing system ? > > Question: a faulty voltage regulator can behave in this way? I always thought it worked or it did not work, but not between the two states depending on the charge. > > Anyway, suggestions are always welcome ;-) > > PS : I'm starting to want to put another power supply for that regulated +5V, and bypass the +5V regulated of G2, but it would be a shame and not in the spirit of a restoration in my opinion. > > > > > -------- Forwarded Message -------- > Subject: Re: DCC-116 E / DATA GENERAL NOVA 2/10 / Nixdorf 620 - Restoring and restarting > Date: Wed, 16 Aug 2017 23:33:31 +0200 > From: Dominique Carlier <d...@skynet.be> > To: Christian Kennedy via cctalk <cctalk@classiccmp.org> > > > > Hi ! > > I finally find some time to work again on my D-116, try to find the > problem(s), thus principally at the level of PSUs. > As you suggested, I inspected particularly the large capacitors of > both power supplies. I replaced those that appeared suspicious > according to the results via my ESR meter, but note that this one is > not supposed to be able to verify the capacitors of more than 22000μF. > I have also some doubt about the results (capacitor working with a real charge or not). > > Anyway, unfortunately the problem is still there. I don't know where > to search now. If I understand well, the two power supplies can cause > a Power Fail if one of the regulated voltages were out of range. At > this point I do not know which of the two is in fault, because when > the Power Fail is active the + 5V is automatically dropped around 1.5V. > Following the schematics I have focused my attention on the value of > some resistors with an important role in triggering this state (eg R18). > I found nothing abnormal, I checked all the capacitors, a large > package of resistances. > > At this point what I know is that I can simultaneously connect the CPU > board and the three core memory boards without problem. If I add the > controller board for the removable hard disk drive or for the tape -> > Power Fail. > Interesting thing: if I connect only the CPU board and the disk > controller: Power Fail too. Maybe the PSU in default is the one that > supply the + 5V for the boards in the upper part of the rack? (slot 1 > for the CPU, solt 4, 5 and 7 for the mem, slot 10 and 12 for tape and > hdd) > > I can provide pictures, schematics, ... > > Regardless of this failure, I try to find information about what I > could install as an operating system on that big beast. If you have > too any ideas about that? > > I would like to be able to do simple tasks such as managing files > (copying files from disk to tape and vice versa), being able to create > directories and sub directories, writing text, print on my drum > printer, programming in a simple language such as BASIC, and also, if > I find a communication board (on the CPU-board I don't found any trace > of components that evoke me an RS-232 interface), communicate with > another machine, print on a teletype ... Does this seem possible for > you with this type of machine? If yes, with which OS? > > I took tons of pictures of the machine from all angles, I will post > them soon ;-) > > Dominique > >>> If I removes all the boards (printer, core memory, scanner, disk >>> controller, etc.), the Power Fail light eventually goes out, I get >>> again the 5VDC, so the power has become "too weak" to power the >>> computer when it is fully populated. >> It's a switcher; look at the caps in the LC filter (downstream of the >> series pass transistor) that, together with the inductor, form the >> energy storage mechanism of the power supply; check the source supply >> as well. The fact that it eventually comes back suggests that the >> reference, comparator and pass device are probably functioning. >
