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.
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