Re: HP-71B + 96KB Port 5 == short battery? Message #9 Posted by don wallace on 2 Sept 2005, 1:59 a.m., in response to message #8 by Howard Owen
Howard.
When you get a round tuit ;-) you could always take photos and
e-mail them to me or post them here. That would help.
The approach I would take is to find out if the chips are
socketed (unlikely) and swap chips. Failing that, if possible
write software which tries to read and write to particular bytes. You won't see much with a 'scope, let alone a multimeter, but if the chips are socketed or other wise can be electrically disconnected (disconnect the "chip select line from the pc board and tie it "off" [usually +5 volts=logic 1]") that takes it out of circuit. A meter is good for checking power supply is good n all memory chips, though (first step I would take).
Assuming four 32kB chips (big assumption), doing this to each chip, one notes what memory capacity the machine reports or which bytes don't exist when peeked and poked ;-). When you find a chip which doing the above on gives no change, you have found your problem.
The other (perhaps better) way to do things is to use an electronic
multimeter in OHMS mode (NOT the continuity test mode!), to check to see if all the common buss lines are intact from chip to chip.
(In ohms mode, the meter generates 0.3 volts across the tips, quite safe. In continuity mode it puts 3 volts across the tips with more current behind it. The logic chips may not tolerate such "abuse".
I am being cautious, but that pays...)
In general do tests with all equipment power off and batteries removed! When working on the logic, touch a GROUND buss first to put yourself at the same potential as the logic to avoid static damage.
For example, one can check the address buss one line at a time,
then the data buss, power supply rails and also the chip enable control lines. Laborious, but effective.
Also, if you had a practised eye you could look for "cold solder joints". I used to fix WYSE WY-60 terminals and they were
notorious for really bad solder joints to the stainless steel
jumper wires that were used in copious numbers to avoid the cost
of manufacturing a double sided PCB. They were a real pain in the ass, so I found it better to do all the joints that looked even slightly suss in one hit. That always worked...
Also, chip sockets sometimes give trouble.
Hope this helps.
DW
|