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Digital Equipment Corporation PDP-11/04 & PDP-11/34 Minicomputers - Teardown

I

intabits

Experienced Member
Joined
Jan 22, 2019
Messages
148
Location
Melbourne, Australia
I've had these two machines for many years, and I've been very naughty, because I've done nothing with them. They've just been sitting in my "museum room", collecting dust.

So it's time to do something about that, clean them up and get them operating, at least as standalone units that can be operated via the front panel.
First up, have a look inside, note what's there and make a video.

Image links show up too big in this forum, I've added them at the end, but here's a link to 3 more viewable images:-

https://imgur.com/gallery/1czglgJ

I'll try to add images of each board over the next few days.


Teardown video:-

https://www.youtube.com/watch?v=3YK2nhc_kUw


Links to some of the many documents available on BitSavers:-

PDP11/34:-
http://bitsavers.trailing-edge.com/pdf/dec/pdp11/1134/

PD11/04:-
http://bitsavers.trailing-edge.com/pdf/dec/pdp11/1104/

PDP11 general:-
http://bitsavers.trailing-edge.com/pdf/dec/pdp11/

Power supply:-
http://bitsavers.trailing-edge.com/pdf/dec/unibus/EK-BA11L-TM-001_Oct77.pdf


The next step will be to check/fix the power supplies. Coming soon...


Images in link above. (They don't show up so big in another forum that I posted this message to)
bcTvkzy.jpg



The card cages. 11/34 at top, 11/04 at bottom:-

qR2oKtg.jpg



A selection of the boards. Power supply covers removed:-

PeUMfR5.jpg
 
Nice machines! The BA11-L chassis are a bit of a pain to work with though in my opinion. But far lighter than than the heavy BA11-K.

Both your machines have RX11. Do you have RX01s for them as well?

You know that there is a DEC specific subforum / group in this forum, right? Maybe your post will get more attention there.

This should be quite easy to have them working. Now it was quite some time ago since I worked with my 11/04 with KY11-LB. But I think I remember that you need to have bus grants in all empty slots to get it to work. It is a DMA device in a way and has its own 8008 CPU. In maintenance mode you can probe IO ports without the main CPU in the system. Even a non functioning CPU I guess as long as it doesn't hang the bus completely.

The H777 PSUs should be quite easy to get going. Secondary side switchers as usual in DEC equipment from this period. Mine had a bad output capacitor on the 5V.

The PDP11GUI is a quite useful tool that I use a lot for loading diagnostics.

Good luck!
 
MattisLind said:
Nice machines! The BA11-L chassis are a bit of a pain to work with though in my opinion. But far lighter than than the heavy BA11-K.

Both your machines have RX11. Do you have RX01s for them as well?

You know that there is a DEC specific subforum / group in this forum, right? Maybe your post will get more attention there.

This should be quite easy to have them working. Now it was quite some time ago since I worked with my 11/04 with KY11-LB. But I think I remember that you need to have bus grants in all empty slots to get it to work. It is a DMA device in a way and has its own 8008 CPU. In maintenance mode you can probe IO ports without the main CPU in the system. Even a non functioning CPU I guess as long as it doesn't hang the bus completely.

The H777 PSUs should be quite easy to get going. Secondary side switchers as usual in DEC equipment from this period. Mine had a bad output capacitor on the 5V.

The PDP11GUI is a quite useful tool that I use a lot for loading diagnostics.

Good luck!
Click to expand...

Thanks for the kind words and advice.

I do have 2 RX01's, and some discs. I think they came with the 11/34 (or 11/05). The 11/04 came with nothing else.

Not sure how I missed the DEC subforum. I posted my only other thread, (IBM7094 mainframe emulator) here, so with no further thought posted this here as well, since "minis and mainframes" also covered the PDP-11s.

There's only one bus grant card between them, so I suppose it's done with backplane wiring. Something else to check.

I only recently found out about the maintenance mode, and I'm looking forward to seeing what can be done without even a CPU installed.

Yes, the PSUs are the next step, which I will be taking very cautiously.

In my student days, we had a PDP11 running a small turnkey Fortran cardinput-compile-run-print system. Apart from that I've only ever used a PDP to toggle in short loops to test the basic operation.

I'm not sure if I want to go down the rabbit hole of doing much more than that again. But if I do, I expect that my limited experience will result in a multitude of dumb questions here!
 
I've just done a teardown, clean, checkout, repair, modify and test of the H777 power supply from the PDP-11/04.

The H777 PSU

7Pa6TTT.jpg



The main +5v @25A regulator PCB

K4NhbQY.jpg



The "MOS" regulator PCB, providing +/-15V @1A (and a smaller +5V @4A supply for use by a battery backup system)

NIv2UWT.jpg



At over an hour, the video is quite long, (and that's been distilled down from over 3 hours of recording).
But I think that anyone who is very interested in this topic, and especially anyone planning a similar exercise, would want to see as much detail as possible. Hence the long video.

For those not so inclined, the executive summary is:-

* Teardown and clean of the PSU.
* Reform the 3 large electrolytic capacitors.
* The 3900uF/6V cap on the 5V supply output is bad (which might be a chronic failure point for these units).
* A salvaged replacement cap is bodged into service.
* Testing reveals that the +/- 15V, and +5VB supplies (from the MOS regulator board) are good,
* But the +5V supply from the main regulator is producing no output.
* The +5V supply is found to be in current limit mode.
* When investigated out of the case, the +5V supply comes good.
* No obvious cause is found, but it's suspected that the replacement capacitor, and/or regulator module were reinstalled incorrectly.
Further suspicion falls on the interference fit of the PCB modules inside the unit.
Theory of failure is that this tightness is causing mechanical stress that may be affecting some components or solder joints.
* The two tall 560uF/20V capacitors on the MOS regulator PCB were replaced with modern 1000uF/25V caps that are much shorter, and so eliminate the interference between the PCBs.
* Unit is reassembled and all voltages are good.
* A connector and cable harness are made to attach the PSU to a set of dummy loads.
* All supplies operate satisfactorly at near rated load (but only at about 19A on the +5v supply).
The need for a better dummy load for the +5V supply is clear.


The video:-
https://youtu.be/MpV6pyaxZ6M

Links:-

Mattis Lind's (DatorMuseum in Sweden) PDP-11/04 restoration, with similar H777 PSU issues:-
http://www.datormuseum.se/computers/digital-equipment-corporation/pdp-11-04


Documentation resources at BitSavers:-

861B Power controller:-

http://www.bitsavers.org/pdf/dec/po...D_E_F_Power_Controller_Users_Manual_Nov76.pdf


BA11-L mounting box (used for both 11/04 & 11/34, contains extensive H777 theory of operation)

http://bitsavers.trailing-edge.com/pdf/dec/unibus/EK-BA11L-TM-001_Oct77.pdf
http://www.bitsavers.org/www.comput...C-11-HBKEF-A-D BA11-K Mounting Box Manual.pdf


PDP-11/04 Engineering drawings (H777 PSU starts at P104):-

http://bitsavers.org/pdf/dec/pdp11/1104/MP00019_1104_EngrDrws_Feb78.pdf


PDP-11/34 Engineering drawings (H777 PSU starts at P183. Component overlay is more legible):-

http://bitsavers.trailing-edge.com/pdf/dec/pdp11/1134/MP00082_1134_Vol2_Sep76.pdf
 
Good work!

Interesting that you also had a problem with the output capacitor on the 5V.

Don’t forget to check the AC LO L an DC LO L signals!

/Mattis
 
MattisLind said:
Interesting that you also had a problem with the output capacitor on the 5V.
Click to expand...

Yes, it is.
Although the 6v rating is 20% above the 5v operating voltage, it is *only* 1 volt. Maybe transients (and possibly negative ones) from the inductor and the switching action, (or short surges that somehow got past the crowbar?) did some damage over time?
I think 20% is sailing a bit close to the wind anyway...

Don’t forget to check the AC LO L an DC LO L signals!
Click to expand...

Yes, I'd forgot about those. Though if I ran into problems caused by them, I'm sure I'd be quickly reminded of them by your blog post on the 11/04!

Your blog also steered me straight to looking for a short on the output.
So thanks for making it, I'm sure it will be of further help as I progress...
 
Following on from the checkout, fix, modify and test of the H777 power supply from my PDP11/04, the exercise is repeated for the PDP11/34 PSU.

For my future reference and hopefully that of others, I note the differences found between the two PSUs, plus steps to take (or avoid) that I found helps streamline disassembly and reassembly.

The same problems were found as for the PDP11/04 PSU:-
- 3900uF/6V cap on +5V output was bad
- Replaced "too long" 560uF/20V capacitors on the +/-15V supplies
Plus, the fan worked, but not very well, and needs to be replaced.

A quick test under load confirmed the output voltages were OK.

Finally, both PSUs were reinstalled into their BA11-L mounting boxes, and checked that the console power switch operated the supplies. (No boards inserted yet)

jVSM6nc.jpg



Video:-
https://www.youtube.com/watch?v=ZkTM2mcPnRk
 
I just wanted you to know how helpful this is, and really is going to help when I find some time to get my 11/34 running.

I'm hoping to find a BA11-L (smaller) as I really don't have space for my BA11-K (larger).

Right now I'm working on getting a QBUS system going as my PDP11-beginner project, hope to get to my 11/34 after.

Jonathan.
 
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