Andrew,
I have read all. I will recommend to remove the serial, floppy and 16K
memory board. YOU HAVE TO SET THE VARIOUS DIP SWITCHES FOR THIS
CASE !!! (write down at first the settings) - Keep in mind that you
have to select the correct memory size - in case you are removing the
chips - you need for the initial setup (H: prompt) ONLY 16K! In case
you don't get the H: with this minimum, you should check the addresss-
and data bus with the scope, if there is any activity. May be that one
or more of the lines are pulled low or high because of a faulty bus
buffer (74ls245, 74ls244, 74ls373 ???). There also two or three PALs
(programmed logic arrays) which differ with the part number according
to the part-number of the monitor ROM ! Read in the manual the various
chip numbers (customer specific - PALs and EPROMs) with the various
jumpers on the board. The first H88/H89 had eproms with 3 supply
voltages (+5,+12,-12V) and the later issues had single supply (+5V).
Check the wire jumpers on board in accordance to the EPROM types.
Concerning the 16K memory board for having total 64K you should notice
that the cable connection uses one of the ram sockets on the cpu
board!!! If you want ONLY 32K you have to remove one of the ram-chips
on the 16k board and you have to put it in the socket on the cpu
board.
REDUCE THE HARDWARE TO A MINIMUM! The system MUST come up to the H:
prompt with only 16K ram and without all intercaces (serial, floppy,
16k upgrade to 64k). In my time as technician in the Austrian Heath/
Zenith representative I had made all repairs on h8, h/z89/h90, z100
and I managed ALL faults. Only once I had a faulty CPU (Z80), but
PALs, memory chips and standard logic devices can become fault. The
supply voltages +5, +12 and -12V can easily be checked and then you
should check the reset on the cpu, the conditions on the address and
data-bus.
Good luck ... will watch your comments here
peter
> [snip]
> > > Andrew Lynch
>
> > > PS, if anyone has a known good CPU board they'd let me buy or borrow,
> > please
> > > let me know. I plan to fix this unit but some known good parts would
> > surely
> > > help fault isolate with some good old fashioned "swaptronics". :-)
>
> > PS. Swaptronics is a good diagnostic tool. When I used to diagnose large
> > IBM mainframes, I used "swaptronics" a lot.
> > Carroll
>
> [snip]
> [AJL>]
>
> Hi Carroll,
> Well, I worked on the Z-90 today and found out a few more things. First, I
> have seen the reset circuit is working. It outputs a brief pulse to reset
> the CPU rather than holding the CPU /RESET line down as long as the keys are
> pressed as I was expecting. I was able to catch occasional glimpses of the
> reset pulse on my oscilloscope but the logic probe detected it reliably.
>
> I have been focusing on the power supply and tantalum capacitors but so far
> haven't found any bad ones. Have also reseated and cleaned all the IC's but
> no change. Rechecked all the jumpers and noticed the Z-90 CPU board was
> configured to boot from an 8" drive that I don't have so I changed that
> around but otherwise they were OK.
>
> The system EPROM is a 4K 2732 at U518 which made for some interesting jumper
> selections. It also taps A11 off of the bus using a jumper wire which took
> quite a while to figure out. I removed the EPROM and verified it was OK in
> my EPROM programmer. The copy seemed to work the same as the original but
> neither has brought up the MTR yet. Sometimes the EPROM chip select has a
> signal, sometimes not. The CPU board does not seem to start up
> consistently. There is chip at U520 which I presume is a 2K Floppy ROM from
> the documentation. I didn't recognize the chip number so I presume it is a
> mask ROM of some sort.
>
> One thing I have noticed though about this computer I don't really like is
> that the peripherals modify the operation of the main board more
> significantly than I would have expected. For instance, the floppy
> controller inserts itself into the interrupt circuit and if you remove the
> FDC, the /INT line goes low and stays there indefinitely. I have to replace
> the FDC in order to get the CPU to work properly.
>
> I'm using the classic technique for finding CPU board problems by just going
> around the CPU, memories, etc examining states looking for anomalies. There
> certainly is one on the Data #2 (D2) line. It seems to be operating in some
> weird tristate mode or something is dragging it down. The maximum voltage I
> am seeing on that line is less than 2 volts which makes me think it may be
> coupled to something. I have removed all the parts I could find searching
> for a broken input or output gate but no luck yet. Definitely something is
> weird on D2 but I don't know what. The address lines seem to be operating
> normally.
>
> I suspect someone spilled something on the CPU board since all the chips
> were stuck in the sockets much more than normal. Also there was a residue
> towards the top of the board. I wiped it off but who knows where it seeped
> into. The liquid probably went under the sockets and so finding the sneak
> circuit is going to be a challenge.
>
> Next, I am going to try swapping out some of the chips on the data bus to
> see if I can find the one pulling D2 down.
>
> Thanks!
>
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>
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