Get yourself a largeish 24V transformer, I use an old 8A Stancor: http://www.glitchwrks.com/images/dec/pdp1110_psu_repair/5411086_under_test.… The AC input can be between 20-30V so it's not critical. Plenty of other "control transformer" type units would do fine. You shouldn't need a lot of current on the 15V rail. Thanks, Jonathan ------- Original Message ------- On Wednesday, January 26th, 2022 at 19:22, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:

Sorry it has been a while responding. I have been looking further at my H745 regulators this evening. Below is what I have found and my responses to the various questions. In summary, it looks like the rectifier may be faulty. It is marked NSS3514. I believe it is a 35A part. Can anyone suggest a suitable replacement? Maybe something like this: https://uk.farnell.com/taiwan-semiconductor/gbpc35005w-t0/bridge-rectifier-1 ph-35a-50v-thd/dp/2677250?st=rectifier which is rated 50V and 35A? Thanks Rob max. the regardless. That's interesting. On the H744s I have observed that if I have a high load the bench PSU current limiter operates and the regulator cannot output +5V, but if I start with a lower load and then add load, it can continue to operate. Is the H745 different to the point that I shouldn't expect this kind of behaviour? If it is the same, then why do the H744s do this? I have tried waiting a few moments to allow the input capacitor to charge up, but the regulator just does not start. limit? could the > current respond with some linearity to varying the input voltage? > What happens with no load R? Yes, it hits the limit, and it does so even if I have no load at all. > Are you running it for any length of time at 5A? (Sounds like a bad idea at this I daren't run it for long. I just tried running it for 15 seconds and couldn't find anything hot. > Is the 723 socketed? Pull it and run it up while watching what happens > around the drive transistors and elsewhere. > If the 723 is not socketed, consider pulling Q5 or opening it's emitter > connection. With no drive to the drive transistors, input current should be nil. > Are any of the drive transistors socketed, so they could be measured out of > circuit? and other R measurements made without them in circuit? > Pull F1 to isolate circuitry. Still draws current? Well, that is interesting. I pulled the fuse and it does indeed pull in all the current. According to the schematic, the only thing it could be is C1 or R1. I tested C1 out of circuit not too long ago and that seemed fine. I took it out again to be sure and C1 seems fine. R1 also seems to be fine. Which leads me to suspect the rectifier may be bad, although some in circuit tests seem to show it is OK. So, to be clear, I tested with the fuse out, with the rectifier feeding C1, with R1 (3.9K) across C1. The bench PSU hits 5A when trying to supply 20VDC to the rectifier. As the fuse was removed there couldn't be anything else that is shorted. The capacitor does not charge up. This does suggest the rectifier is the problem. > Have you looked for shorts/leaks?, especially leaky junctions in transistors > Q2::Q5. > e.g. R measurements, no F1, no load R, both directions: > Q2.B-C ? > Q2.E-GND ? > Q2.C-GND ? > -15-GND ? > Settling time for cap charge/discharge may be needed. > In answer to your earlier question, no, the +15V is not the reference, it is the IC

I used a Vishay GBPC3506W-E4/51, which is 600V 35A, 400A inrush rated. Looks like they're currently on-order at Mouser, I don't know about other suppliers. The GBPC3506W-E4/51 is a little thinner than the bridge rectifier DEC used originally, which sometimes matters. For the 5411086 board in my PDP-11/10, height does matter, as it shares a heatsink with other parts. I stacked a few washers under the center of the bridge: http://www.glitchwrks.com/images/dec/pdp1110_psu_repair/rectifier_spacer.jpg Thanks, Jonathan ------- Original Message ------- On Thursday, February 17th, 2022 at 17:57, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:

On 2022-Feb-17, at 2:38 PM, Rob Jarratt wrote: Presumably your high test load plus the initial cap-charge current is pushing the bench PS into current limit, that is, with a high load there is less available current to charge the caps before the bench PS starts current limiting. This would slow down the cap charge rate, so it would take longer for the caps to charge. I can't say I see it 'stopping starting', but it would lengthen the time to 'start'. How long depends on the numbers. There may also be some dependance on how your bench PS responds in current limit. On 2022-Feb-17, at 10:19 PM, Rob Jarratt via cctalk wrote: At 30VAC input, peak V is ~ 44V, you're probably better off with rectifiers higher than 50 PIV to provide some safety margin.

On 2022-Feb-17, at 11:40 PM, Rob Jarratt wrote: The 744 derives the +15 supply for the 723 from the same 20-30VAC main input supply, in contrast to the 745 where the +15 is from another source. With the 744, once your bench supply goes into current limit and drops its output voltage, that's also dropping the +15 for the 723. Everything is going to be out of whack at that point, the reference supply, the amp/comparator action, etc. I can't say at this distance and limited info exactly what's going on, there's also the Q7 unijunction shut-down circuit in there. It may be in some sort of oscillation 'trying to start' or it may be latched up into an off state, because the supply voltage is below some threshold. (I've never dealt with these supplies directly, I'm just observing the schematics.) This behaviour may be different for the 745 because the +15 ancillary supply is separate/independant from the main input.

I found a 400V part on Mouser and ordered that.