I will make detailed photos from the RAM boards when I am back in the office during the next days and post them ...
| VCF Latam | Apr 24 - 26, 2026, | Bahía Blanca, Argentina |
| VCF Pac. NW | May 02 - 03, 2026, | Tukwila, WA |
| VCF Southwest | May 29 - 31, 2026, | Westin Dallas Fort Worth Airport |
| VCF Southeast | Aug 01 - 02, 2026, | Atlanta, GA |
| VCF West | Aug 01 - 02, 2026, | Mountain View, CA |
| VCF Midwest | Sep 12 - 13, 2026, | Schaumburg Convention Center, IL |
| VCF Montreal V2.0 | Nov 07 - 08, 2026, | Saint-Lambert, Montreal, Canada |
| VCF SoCal | See you in 2027, | Southern CA |
| VCF East | Apr TBD, 2027, | InfoAge, Wall, NJ |
-
Please review our updated Terms and Rules here
Restoration of a PDP11/60
- Thread starter klapperp
- Start date
pbirkel
Veteran Member
If you'd do the same for the six CPU boards that would be appreciated as well.
I forgot to mention earlier that if you don't already have a copy of the following, then you should get one. It's the FMPS for your memory system. https://bitsavers.org/pdf/dec/pdp11/1160/MP00310_MF11S-K_Feb77.pdf
pbirkel
Veteran Member
Thank you for the photos. So that mostly-Motorola module is an -ED after all. With three dozen Mostek replacements, done quite well as far as I can see from here. Only the 7th row, fourth from the right (as seen from the component side) looks like there is perhaps some flux residue on the solder side. Maybe manufacturing ran short of Motorola components that day and just stuffed randomly with on-hand Mostek?
m_thompson
Veteran Member
The 11/60 Cabinet and Power Supply manual has a lot of detail on how the H7850 works. That should be enough to debug them.
Terri Kennedy
Veteran Member
From the rear side you can see the rework. Since the replaced parts also have DEC P/Ns on them, I'm guessing this was depot-level repair long enough ago that the chips were expensive enough that it made sense to do that level of rework.
BTW, watch out for those electrolytics (particularly the ones in the middle of the board) - they get ripped up by the board in front of them and can short to the adjacent board. DEC often used fish paper on the backs of memory boards, but even that didn't help the first board as it was up against the CPU. I've used this in the past.
pbirkel
Veteran Member
+1 on the fish paper. A thin rugged plastic sheet is another possibility but my money is on the fish paper. Now if we could just keep those rubbed-down ceramic disk capacitors intact. At least they don't go "boom!", or leak.
I can't, but I'll take your word for it :-}. So that's what a depot-level repair looks like. Sure was extensive, but mighty clean. What's your hypothesis for having three dozen chip failures? Perhaps all turned out to be marginal WRT timing requirements? How would they have gone about detecting and localizing each of them, then?
We have all the manuals as well as the schematics from the H785 which is more or less the same except one PCB which makes 12V from 15V.
Other than in most of the other DEC documentation the "theory of operation" is very limited here. So it is a bit a kind of guessing around what is related to what.
We already found out that the regulator has 3 input power sources which may be active at the same time. One AC source from the main transformer, one AC source from the backup battery unit and one DC source from the backup battery unit. Our guess is that at the end at least one of these must be available to turn the regulator on but even with all three of them the regulator does not start.
We are now focusing on this little PCB:
There is a protective circuit which delivers 5V to the regulator in order to start and when we measure this pin standalone on the PCB the 5V are ok. Once we put the PCB back in, the 5V is gone.
@Konstantin focuses now on transistors and the diodes because the chips were tested and are all working as they are supposed to.
I am sure that we will fix the issue soon but a more precise "theory of operation" would have been helpful ...
pbirkel
Veteran Member
Isn't that the H785 Control Board, for which you state that you already have the schematics?
PG31
Veteran Member
I am in the process of restoring an 11/34. I have some bulbs to replace in the regulators. What is the reason for a 3W resistor with the LED? To put some load on the voltage rail or just what you had to hand? I was going to use a 1/4w resistor with the LED.
Yes it is, but I hoped that we find a more detailed "theory of operations" for this complex little part somewhere ...
We decided that a LED is more robust in compare with a bulb and @Konstantin calculated it in order to pull the same current as a 6V 200mA bulb.
pbirkel
Veteran Member
Well the FMPS isn't going to tell you that. I do have a paper copy of the H7850 schematic but that is just the battery backup regulator. It does show the straight-thru connections from J2 & J3 to P4 (which I presume connects to J4 on the H785).
Terri Kennedy
Veteran Member
For example, the center chip in this picture (the -4 board above, to the right of the centerline caps, 7th row down):
It's possible they had some automated test equipment. Or maybe there was a mix of date codes in the memory chips and they just knew to shotgun-replace all memory chips with that date code.
pbirkel
Veteran Member
While editing some new text for gunkies (https://gunkies.org/wiki/MS11-K_MOS_memory) I realized that the MK11-B uses the same battery backup subsystem as the 11/60. Although the applicable FMPS, MP-00272, is named H785 it includes information for the "BATTERY BACKUP REGULATOR". See page 12 (of 24) in the attached FMPS for the schematic. That drawing is identified as 5412343-0-1 Rev. F; the one in the 11/60 FMPS MP-00575 is identified as 5412343-YA-1 (no Rev). Scan follows:
View attachment 1321880
It appears that the only difference between the two versions revolves around the fact that the MK11-B expects +-15v while the 11/60 expects +-12v. The differences appear to be isolated to the hookup of the secondary of T2. I infer that the H7850 was simply a lightly modified H785 incorporating the -YA version of the backup regulator.
See the NOTES on page 18, the schematic for the CONTROL BOARD. The H7850 FMPS uses the exact same same control board drawing, including the NOTES and just above them the same +-15v input labels on J4 pins 19 & 3, although as you can see in the schematic above that pin 3 is actually -12v. The partially clipped comment regarding W3 might be significant in your case? I haven't figured where W1 & W2 are located as yet.
Attachments
@pbirkel Thanks a lot!
BTW. A simple issue related to the flat ribbon cable which is connected via backplane to the console key-switch caused the problem:
@Konstantin fixed it and for now we can confirm that the controler PCBs from both of our H7850 are working
BTW. A simple issue related to the flat ribbon cable which is connected via backplane to the console key-switch caused the problem:
@Konstantin fixed it and for now we can confirm that the controler PCBs from both of our H7850 are working
We re-assembled all the re-worked components from our mains power-distribution and tested them:
We will do the same with the transformers together with the regulators once the H8750 is fully tested!
To be continued ...
We will do the same with the transformers together with the regulators once the H8750 is fully tested!
To be continued ...