You should indeed have activity on the address lines, take another look as the
machine comes up, they should be toggling, even briefly. Could be that data
lines are stuck, and/or it is getting a halt somewhere. Another trick would
be connect a 1k to +5, and touch it to the reset line, you should see the
address lines blip a little then. If they never change state the processor is
dead or the mux buffers stuck (74245's?) .
If you can build a little proto board to plug into the processor socket so you
can disconnect the address lines from the board, you should be able to 'free
run' the processor, and see activity. If those address lines dont toggle with
clock, power applied and reset low, I would suspect the processor.
Randy
Date: Tue, 22 Jan 2008 19:26:49 -0800
From: jwstephens at
msm.umr.edu
To:
Subject: Re: Dead IBM PC 5150 motherboard - help troubleshooting
Richard Beaudry wrote:
<snip>
6) One thing that seems odd is that the ADxx (Address/Data) lines seem
"stuck". The scope does not show a waveform, and measuring with a meter
shows some lines right at +5VDC, some at about +4VDC and some at 0VDC.
Perhaps the processor is indeed HALTed???
On several of the CPU's if you hold the processor reset, you actually
get the reset vector address out. So on the 8088 8086 80186, you get
FFF0 out. I don't know if 8088 has all 16 lines out, or is multiplexed,
but on the 186 we could troubleshoot a board we had with a 186 on it for
a support processor by setting a flipflop that held it reset, and then
look at the lines with a scope.
Jim
I did some troubleshooting of some of my 5150's using the "Sam's IBM PC
Troubleshooting and Repair Guide Book". I noticed that there is one on eBay right
now, item #190193033366. Might help.
david.