[TUHS] pray for us ! pdp11/34 salvation, continued.

[asbesto@freaknet.org (asbesto)]

Il Mon, Nov 03, 2003 at 09:49:16AM +0000, asbesto rigurgitava:

i quote myself, due to some imprecision (i have a really bad memory,
maybe i have some chip to change inside my mind :)))

> in these days we will try to change the broken chip on the pdp11/34
> RL01 controller board. 

No. the "broken" board is the M7891, 128Kx18 bit MOS memory. The
broken chip is E15, a simple SN74LS175.

here's the list of the messages in which i first described our 
problem; the messages start at Tue, 15 Oct 2002 07:46:35 +0000, 
and they can be find on the mailing list archive here, 
http://minnie.tuhs.org/pipermail/tuhs.mbox/tuhs.mbox

A more precise description of the problem was in a message
dated Date: Tue, 15 Apr 2003 00:00:37 +0000:
--------
the M7891 board (128K x 18 bit MOS MEMORY MODULE) have the D2 red=20
led light turned ON when turning on the CPU !

this means PARITY ERROR on this board 

checking the board we found 2 problems:

1) a 74LS175 chip named E15 on our schematic diagram, that seem
   phisically BROKEN on an edge (there is a fessure on the plastic
   DIP package). i can't change it now because i don't have a=20
   soldering station here to do a nice job, but looking the chip i
   think it may work...

2) a 5 Kohm 4-resistor bridge named R22 on our diagram, that is phisically
   BROKEN. this bridge give +5V on signals named DATA OC L, CSR OC L, OC L
   and X ADD OC L. i changed it with a 4.7Kohm 4-resistor bridge, without
   any result.

---------

Well

what i did for now is to cut the pin of the broken chip, without
desoldering. I will solder the new chip on the cutted pins, to avoid
desoldering the board. 

Some images of the work in progress here:

http://www.freaknet.org/asbesto/pdp11/

Note:

just touching the chip, part of the plastic box went out, as you can 
see in the images. Maybe this can identify a real fault on this chip ? 

stay tuned! :)

-- 
[asbesto : freaknet medialab : radio#cybernet : GPG key on  keyservers]
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[http://www.freaknet.org/asbesto IW9HGS http://kyuzz.org/radiocybernet]
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