My pdp-8i problem

[Philippe Sonnet]

Dear Tim,

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.
Good luck,

Philippe

----- 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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