My pdp-8i problem
sonnet at sols.ucl.ac.be
Wed Mar 1 16:10:57 CST 2006
It is very hard to answer your question. I also spent hours fixing my
pdp-8/L, in which almost all circuits had to be repaired. It took me two
years. The only way : use a logic probe and have a set a spare flipchips
which you know are OK. I created a system to test the flipchips using a
digital acquisition card on a PC (see www.sonnet.be). Collect all possible
schematics, study them, have a confortable seat, stay calm and systematic
and use a card extender to have better access to all the lines.
----- Original Message -----
From: "Tim" <tradde at excite.com>
To: <cctalk at classiccmp.org>
Sent: Saturday, February 18, 2006 11:30 PM
Subject: My pdp-8i problem
> I was hoping someone on this list might have some input on this. I have
> also posted this to alt.sys.pdp8. I have a pdp-8i with a memory problem.
> Here is the post from alt.sys.pdp8:
> Quite some time back while scoping out a problem I shorted a pin in my
> 8i backplane and lost memory control. Since that time Vince and I have
> been trying to figure out where the problem is to fix it. I don't know
> what pin I shorted as at the time I did not realize
> I had damaged anything. The +5v breaker tripped though.
> The problem is I can not deposit or read back any memory locations.
> The MB is displayed correctly on a deposit. Scoping signal has shown me a
> very odd signal on B FIELD 0 (1) when the machine is in run mode. It has
> a wave form on it rather than just a normal
> voltage. The oddest thing is the high point of the wave goes up to +6v
> with a bit of the beginning of the top portion going above +6v. To me
> this should not be. How can I get
> +6v from a system that is using +5v as it's input voltage? We have
> some theories but nothing concrete as yet. The other thing is if I remove
> all 3 G228s (A36, B36, B37) the
> signal goes away. I do not have parity memory so nothing is in A37.
> We believe there is a bad ground somewhere, but I have not been able to
> determine where. I do not have a wiring list so I may have to trace some
> signals manually. I think I have to find all the places the B30L1 (B FIELD
> 0(1)) goes to.
> Any ideas or suggestions from anyone? This has been bugging me for a
> long time. I look at it for awhile, and then put it down for awhile. It's
> also not easy to look at this in the location it is in. It's too low to
> kneel and really be comfortable. Too high to lay on my stomach. :)
> Thanks for any input on this. I should say that part of the system
> still works. Examining or depositing memory while it does not appear to
> load/store the data increments the PC
> as it should. I can sort of run, which was my original problem. It
> runs but stays at location zero. It is not an interrupt causing this. So
> the front panel is mostly operational
> so at least some of the machine is still working. I just have to find
> the problem with the memory.
> I know most people on this list do not have a pdp-8i, nor may know
> much if anything about them. Any suggestion will be taken and looked
> into if it seems to have hope of leading to a solution. Also I will
> answer any questions people have to help describe the problem more or
> give more details into what Vince and I have looked at.
> Tim Radde
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