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PDP-8/M restoration - Jerry Walker

jackrubin said:
Jerry has started a very nice series on restoring an 8/M. So far, a detailed power supply repair and timing card testing. Thorough and well done with clear description of the logic used in testing.

View on YouTube: https://youtu.be/WQyaQorPxc0
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I'm watching the first one, and it's nicely done so far.

One thing that seems odd to me is that it looks like the CPU and memory were inserted backward when he got the machine?? It looks wrong in the video, and he hasn't commented on it. Probably just as well he inspected it before powering it up!

Vince
 
Very nice. I noticed the backward cards too. Glad he did also.
 
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The order of the cards actually don't matter at all. DEC documentation have an order given, but putting them in backwards, or placing them elsewhere on the Omnibus works equally well.
 
jackrubin said:
It's picked up in one of the user comments below. He's very methodical and plodding in a good way.
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I coudn't see the comments, as they seem to have moved them to the very bottom, and I have to try to scroll past the bottom of the page; then they suddenly appear.

I don't know who designs websites these days, but I they just get less and less intuitive.

Vince
 
bqt said:
The order of the cards actually don't matter at all. DEC documentation have an order given, but putting them in backwards, or placing them elsewhere on the Omnibus works equally well.
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I'll agree about the order, mostly. But you do *not* want to insert cards with the component side facing the rear.

Vince
 
vrs42 said:
I'll agree about the order, mostly. But you do *not* want to insert cards with the component side facing the rear.

Vince
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Uh... Say what? I didn't look at the videos, but that would indeed be very bad. But it also sortof don't work so well. The flip chip connectors don't fit properly if you insert things the wrong way.
But, well... People managed to push multiple 3.5" floppies into the drive at the same time, so I shouldn't assume anything.
 
My approach to restoring the machines at the RICM is very much like his. We document every step in a text only restoration blog that we put on the WWW page for each machine. This has been very helpful if a past problem shows up again. It has also been helpful for others that are restoring the same machine and want to know how to approach the restoration.

His lengthy videos are much more compelling than our text blogs, and show a lot about how to use the test equipment, not just fix the machine. We need to add some lengthy videos explaining what we have done during a restoration, or how the machine works, instead of just a few seconds of video showing a machine running diags. The PDP-9 will likely be the first one that we document with lengthy videos.
 
I've found the RICM blogs to be very helpful but clearly there is a lot of work involved in taking detailed notes and then transferring everything to the blog. It never gets any easier. I've downloaded the 8/I and 8/E blogs as .pdf files that I can search as needed.

The videos show a lot more of the actual analysis and test/repair/test process but they are very hard to use as a reference since there is no equivalent to "OCR" for videos - no way to search for a specific bit of information. I've got a stack of 8/M power supplies to repair and I ended up taking notes and writing down part numbers as I watched. I generally go through the videos at 1.5x speed and then go back to specific time stamps for info.

CuriousMarc now has a wiki that he maintains in addition to his videos but that is even more effort to setup and maintain. He's pretty much gone professional at this point, so really that's a whole different level.
 
jackrubin said:
I've found the RICM blogs to be very helpful but clearly there is a lot of work involved in taking detailed notes and then transferring everything to the blog. It never gets any easier. I've downloaded the 8/I and 8/E blogs as .pdf files that I can search as needed.
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Thanks for the comments on our blogs. We will continue to make the blogs even if we add some videos.

From watching a lot of technical videos it looks like the better ones involved a lot of video editing and post production to make them look professional. I am not sure that I want to invest the time to learn how to use a video editing program to make nice looking videos. Posting unedited videos from my phone would probably look pretty crappy.

jackrubin said:
I've got a stack of 8/M power supplies to repair
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We made an LTspice model of the power supply in the DW8E-PC Omnibus Expansion Chassis so we could understand how it works. I think that is the same power supply in the 8/M and in some PDP-11 chassis. We would be happy to share the power supply LTspice model if you want to play with it. We also made LTspice models of the PDP-9 accumulator because it uses more than two voltage levels on some of the signals. That helped our understanding and debugging a lot. Now we are making LTspice models of Wang LOCI-2 boards so we can understand how the BCD encoded registers work and how data is transferred between registers. Some of the register behavior is not obvious when looking at the schematics.
 
jackrubin said:
The videos show a lot more of the actual analysis and test/repair/test process but they are very hard to use as a reference since there is no equivalent to "OCR" for videos - no way to search for a specific bit of information.
Click to expand...
You could use something like DownSub to download subtitle files from many video sites. YouTube will auto-generate subtitles on request, but the more technical things are, the more likely it is that the auto-generator will make a mistake. If you download in SRT format, you'll get the text as well as the time in the video where it appears.
 
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