PDP-8E diagnostic help needed

[Tony Duell]

> 
> Hi,
> 
> I've got a PDP-8E which I've almost got working.
> 
> Can anyone here help me figure out this remaining problem?
> 
> As I examine memory, the address lights count up to 01111, then
> go back to 00000, instead of 10000. I can manually enter an address
> higher than 1111, but the 10000 and 100000 bits don't stick -- they
> go low, as soon as I hit the examine switch to step to  the next 
> memory location.
> 
> I can manually load an address 1000000 or 10000000, and hit examine to
> see 1000001, 1000010, 1000011, etc..., but once I reach 1001111, it's
> back to 1000000.
> 
> It was recommended to me that it might be the carry between E52 and E37,

It's been soem time since I've been inside a PDP8/e, but I would have 
thought that a carry prolem would have caused all the its to 'stick' (or 
latch), but only the bottom <n> to increment.

> or the E38 input multiplexer for bit 7 (on M8300), so last night I 
> socketed and replaced all three of those ICs, but I still see the same 
> symptoms.

I repeiared a PDP9/e with a similar-ish problem years ago, and it was one 
of the latch chips I think.

> 
> I have extender boards, so can access M8300 during operation. I
> measured the carry line between E52 and E37 go low when I reach 1111, but
> the light for line 10000 doesn't light on the front panel on the next 

OK, can you find where the 10000 bit comes from? Look at the inputs to 
that chip -- should the appropriate output be being asseted? If not, 
which of the input(s) is wrong? Trace that back, and so on. 

This is not a complciated processor from what I remember, so it shouldn't 
be too ahrd t odo that.

-tony