Restoring a DEC PDP-11/05

[wh3016]

Yes. It could be a reason for it not working. You have pullups on the M930. That is why they are high in the backplane. You somehow need to figure out where those signals is cut.

DEC used purple for DC LO and yellow for AC LO. Check the continuity from the PSU for those wires.

It can be that the backplane somehow is modified. It is hard to tell.

I have continuity for both AC and DC LO the connector on the PSU all the way to the backplane.

Looking back, I only had one of the M930’s plugged in to the backplane. Perhaps I’ll take another look and see if having both unibus slots makes a difference.

 

[MattisLind]

A second M930 should not be necessary. A short unibus works well with just one M930.

I can ot understand how there can be continuity while at the same time you meassure different levels at different locations of the same signal.

 

[AK6DN]

A second M930 should not be necessary. A short unibus works well with just one M930.

I can ot understand how there can be continuity while at the same time you meassure different levels at different locations of the same signal.

Could be a connection that is poor and has a high(er) resistance. 'Continuity' at very low current may show ok, but may cause a voltage drop across the junction.

 

[MattisLind]

Yes. That could an explanation. The PU on the M930 is around 500 ohms as far as I remember. So getting a measurment of 3.5 and 4.11V indicate a resistance along the way in kOhm range. Definetely worth investiagting further.

 

[wh3016]

Yes. That could an explanation. The PU on the M930 is around 500 ohms as far as I remember. So getting a measurment of 3.5 and 4.11V indicate a resistance along the way in kOhm range. Definetely worth investiagting further.

Could be a connection that is poor and has a high(er) resistance. 'Continuity' at very low current may show ok, but may cause a voltage drop across the junction.

Honestly given my experience with vintage computers, I wouldn’t have considered oxidation and corrosion on things to be an issue. I suppose when you take into account that there was at one point a mouse living inside, (I found a whole peanut in the power supply) the idea of corrosion seems more plausible. I had vacuumed and wiped down the inside and card slots with denatured alcohol, though I never checked the slots that were populated.

Usually when I clean out things of this nature, I apply the alcohol to a cotton swab, but with a bunch of little tabs inside the card slot, I find they just tend to get ripped up. I’ll pick up a can of contact cleaner and report back.

I’m thinking of taking another look at the AC DC LO’s. It seems odd that I can get a measurement at one place and not the other. Any advice on this?

 

[jackrubin]

Soak a piece of thin cardboard in contact cleaner and move it in and out between the contacts several times to clean inside the slots without damaging the contact springs.

 

[thunter0512]

This thread came to a sudden end. What happened? Did this 11/05 ever came to life?

 

[wh3016]

This thread came to a sudden end. What happened? Did this 11/05 ever came to life?

Not yet. Where I last left off, I had decided that the issue of no DC LOW lies somewhere between the power supply and the M7260/M7261. I tried a second M7260/M7261 and got no signs of life either, which further convinces me that there is an issue with the DC LOW path. I did verify continuity from the power supply to the backplane, though.

My next idea was to find someone or somewhere with a functional 11/05 and see if I could test my boards and the backplane.

A friend of mine recently acquired an 11/05 that is also showing no signs of life, so my interest in getting mine back into operation has returned.

I haven’t had time to work on it since my last post as I’ve had a bunch of other projects come my way…

That being said, I expect to resume working on it in the coming weeks, so stay tuned!

 

[wh3016]

Just an update for those who are following:

First, the bad news: still no luck getting any signs of life from both sets of M7260 and M7261 boards.

The good-ish news: I’d like to thank @cullyrichard for assisting in troubleshooting and providing access to a functional PDP-11/05 to compare mine to. Basically, we were able to confirm that the backplane, power supply, memory drivers, and core array are in fact, functional. This was done by taking the known working 7260 and 7261 from his machine and placing them into mine.
We then took the 7260s and 7261s I had and placed them into his machine.

It appears as though both my 7261s and one of the 7260s are unresponsive. The other 7260 when placed into his machine appeared to respond to the front panel address switches, but we were unable to get it to examine any of these addresses…

While these findings were a bit disheartening, I feel not all hope as lost as we now know the the memory system, front panel, and power supply function properly.

Probably going to be starting a new thread soon, so expect more to come…

 

[m_thompson]

First, the bad news: still no luck getting any signs of life from both sets of M7260 and M7261 boards.

At the same time we found that a spare set of PDP-11/05 boards at the RICM don't work either. We found a M7260 that was really early and covered with ECO wires. We also found a spare M7263 KD11-D PDP-11/04 CPU board that I didn't know we had.

There are several PDP-11/05s in the RICM warehouse along with a GT40 that all need to be tested. We are talking about having another debug day where we could have up to six PDP-11/05 systems in the bench.

 

[wh3016]

At the same time we found that a spare set of PDP-11/05 boards at the RICM don't work either. We found a M7260 that was really early and covered with ECO wires. We also found a spare M7263 KD11-D PDP-11/04 CPU board that I didn't know we had.

There are several PDP-11/05s in the RICM warehouse along with a GT40 that all need to be tested. We are talking about having another debug day where we could have up to six PDP-11/05 systems in the bench.

I look forward to that day! Six 11/05s in one place would be quite a rare sight…

Even though we weren’t able to get them to work, thank you for digging out those boards!

 

[m_thompson]

Even though we weren’t able to get them to work, thank you for digging out those boards!

We need to get the 'scope and logic analyzer out and find out what has failed on these board sets. Hopefully we can fix a few.

 

[cullyrichard]

We need to get the 'scope and logic analyzer out and find out what has failed on these board sets. Hopefully we can fix a few.

I think some of these boards are exhibiting some of the same or similar faults. I am hopeful that the faults are not on the various bipolar ROM chips that are used for glue logic. The good news there is that we have known good ROMs on my board we will likely be able to make copies should they be an issue.

Next time we set up for a repair session I'll throw my big HP analyzer and scope(s) in the car and hopefully we can find the chips at fault. I've already ordered another M9312 and some replacement chips for my fried one so we can run the diagnostics rom on both machines and get to the bottom of these faults!

 

[Dare]

Next time we set up for a repair session I'll throw my big HP analyzer and scope(s) in the car and hopefully we can find the chips at fault. I've already ordered another M9312 and some replacement chips for my fried one so we can run the diagnostics rom on both machines and get to the bottom of these faults!

I'm actually kinda jealous of a marathon PDP-11/05 "repair session". These systems can be quite fun to debug!

 

[thunter0512]

I think some of these boards are exhibiting some of the same or similar faults. I am hopeful that the faults are not on the various bipolar ROM chips that are used for glue logic. The good news there is that we have known good ROMs on my board we will likely be able to make copies should they be an issue.

Next time we set up for a repair session I'll throw my big HP analyzer and scope(s) in the car and hopefully we can find the chips at fault. I've already ordered another M9312 and some replacement chips for my fried one so we can run the diagnostics rom on both machines and get to the bottom of these faults!

I am worried about both bipolar ROMs implementing logic and the microcode ROMs. I believe these vary with board revisions, so are not interchangeable.

On the other hand unsoldering all ROMs to extract their contents for backup is a high risk exercise unless you have a set of spare boards at the same revision level.

I wouldn't want to destroy a working CPU board to make backups of the ROM contents. If the ROMs would be socketed, I wouldn't hesitate. If the ROMs would be soldered without bent pins I would seriously consider it. With DEC manufacturing's bent pin method of holding the ICs in place during wave soldering I am always very worried when unsoldering any ICs intact as there is a real risk of damaging the PCB.

If I want to replace a faulty IC I always just cut it out next to the body of the IC and then carefully unsolder each individual pin. This is the only 100% safe method if you want to preserve the PCB.

I know of no 100% safe way of preserving both PCB and IC when unsoldering typical DEC boards with all pins bent.

 

[William Maddox]

I know of no 100% safe way of preserving both PCB and IC when unsoldering typical DEC boards with all pins bent.

A proper desoldering gun perhaps, such as a Hakko FR-301?

 

[1944GPW]

I was watching a desoldering channel on Youtube (might have been Mr Desolderer or something like that I think) and to remove a device with a series of pins, a piece of thickish copper wire from mains flex was soldered along the pins then two soldering irons were applied at the same time which I thought was rather ingenious.

 

[cullyrichard]

A proper desoldering gun perhaps, such as a Hakko FR-301?

It’s very fragile and it can take some pretty careful planning to replace components on the boards. The best thing for us to do would to be to rebuild the proms based on the drawings.
At the very least, we can compare the working ones in situ

 

[thunter0512]

A proper desoldering gun perhaps, such as a Hakko FR-301?

I suggest re-reading my post #135. Your Hakko FR-301 won't make a difference when IC pins are bent flat onto the pad. When trying to unbend the pins you risk scraping off the pad. You might have more luck with modern PCBs where the copper-to-expoxy adhesives used are stronger than what DEC used in the 70s.

BTW not all DEC boards are manufactured with bent pins and then any decent vacuum desoldering station will make it easy and safe (for both IC and PCB) to remove the ICs if used properly and with care.

 

[djg]

I wouldn't want to destroy a working CPU board to make backups of the ROM contents.

Random thought. Do you know if it does a checksum of the ROM as self test? If so you might be able to wire up a DIP clip with electronics to capture the address and data in circuit. With slow EPROMS this would be easy. These parts will be more challenging.