I'm struggling to give enough information to be useful without telling you my entire life story, but here goes ...
Short version: I have a BBC model B with two issues - it won't recognise any ROMS in sockets IC88 and IC100 (IC52 and IC101 are fine), and my cheese wedge Z80 co-pro will boot CP/M, but I get the same pattern of repeating characters on the command line every time I reboot. I am relatively confident the co-pro is fine and the problem is at the Beeb end. I can see that the data and address lines on the tube connector seem to connect to the data and address buses via sockets IC88 and IC100. Do these sockets and the tube connector have any other components in common that might explain this issue?
Long version: Last year I picked up a BBC model B, dual Cumana drives and a Z80 cheese wedge co-pro. The Beeb motherboard was quite rough - patch wires from a cpu replacement and quite a lot of corrosion on the bottom of the board as it seemed something nasty had dribbled onto it while it was in storage with the previous owner. Despite this, after a PSU recap, all was fine for the first few months and I managed to get the drives working and have a good play with CP/M on the co-pro. Eventually the Beeb started having increasingly frequent glitches to the point where it wouldn't even boot a diagnostic ROM. The corrosion and my ham-fisted attempts at diagnosis and repair mean that this machine will never work again.
Having now caught the Beeb bug, I had a quick trip to eBay and bought Beeb2. This machine doesn't have the bodge wires (well, just one) or the corrosion, but It was very dirty with a layer of black grime over the top side of the board. I'm too much of a coward to dunk it in the sink, so I've been cleaning as I go with IPA. After replacing a bad serial ULA, hunting down a couple of bent pins, reflowing the joints on all of the 40 pin sockets and the keyboard connector (all 40 pin chips on the board have been replaced/socketed) and scrubbing all the sockets with contact cleaner I now have a system which runs for hours and is rock solid. I have installed the chips to support the disk drives and it is almost all good ...
I have two remaining problems ...
1. ROM sockets IC88 and IC100 were sketchy and needed quite a lot of persuasion for the ROMS in these sockets to be visible (IC52 and IC101 are fine). I initially thought this was a dirt problem as scrubbing with contact cleaner would eventually get them working - I no longer think this is correct. I've tested continuity on the data, address, 5v and ground lines with the other good ROM sockets and it's all fine. Equally none of the lines seem shorted with each other. I get roughly 5k-ohm resistance from the data lines to ground and several M-ohm for the data lines. All the ROM sockets (good and bad) read the same.
2. The Z80 co-pro successfully boots CP/M, but after a few minutes characters start appearing on the command line as if I had a stuck key. This always starts with a lower case 'c'. If I hit return, CP/M correctly identifies a bad command and I then get a stream of 'd's followed by e, f, g etc. I get the same sequence each time I reboot ...The co-pro cable is in good condition and I have checked continuity back to the Z80 co-pro board, and re-flowed the connector on the board (as it looked hand-soldered). This made no difference.
I noticed that the tube connector on the Beeb MB seems to connect to the data and address lines via IC88 and IC100 I reflowed the tube connector and the two ROM sockets. I still have the same problem with the Z80, but now the two ROM sockets don't work at all.
I'm thinking of replacing the two ROM sockets, but thought I would check here first as I managed to kill Beeb1 by randomly ripping out components.
Any suggestions greatly appreciated,
Ian.
Short version: I have a BBC model B with two issues - it won't recognise any ROMS in sockets IC88 and IC100 (IC52 and IC101 are fine), and my cheese wedge Z80 co-pro will boot CP/M, but I get the same pattern of repeating characters on the command line every time I reboot. I am relatively confident the co-pro is fine and the problem is at the Beeb end. I can see that the data and address lines on the tube connector seem to connect to the data and address buses via sockets IC88 and IC100. Do these sockets and the tube connector have any other components in common that might explain this issue?
Long version: Last year I picked up a BBC model B, dual Cumana drives and a Z80 cheese wedge co-pro. The Beeb motherboard was quite rough - patch wires from a cpu replacement and quite a lot of corrosion on the bottom of the board as it seemed something nasty had dribbled onto it while it was in storage with the previous owner. Despite this, after a PSU recap, all was fine for the first few months and I managed to get the drives working and have a good play with CP/M on the co-pro. Eventually the Beeb started having increasingly frequent glitches to the point where it wouldn't even boot a diagnostic ROM. The corrosion and my ham-fisted attempts at diagnosis and repair mean that this machine will never work again.
Having now caught the Beeb bug, I had a quick trip to eBay and bought Beeb2. This machine doesn't have the bodge wires (well, just one) or the corrosion, but It was very dirty with a layer of black grime over the top side of the board. I'm too much of a coward to dunk it in the sink, so I've been cleaning as I go with IPA. After replacing a bad serial ULA, hunting down a couple of bent pins, reflowing the joints on all of the 40 pin sockets and the keyboard connector (all 40 pin chips on the board have been replaced/socketed) and scrubbing all the sockets with contact cleaner I now have a system which runs for hours and is rock solid. I have installed the chips to support the disk drives and it is almost all good ...
I have two remaining problems ...
1. ROM sockets IC88 and IC100 were sketchy and needed quite a lot of persuasion for the ROMS in these sockets to be visible (IC52 and IC101 are fine). I initially thought this was a dirt problem as scrubbing with contact cleaner would eventually get them working - I no longer think this is correct. I've tested continuity on the data, address, 5v and ground lines with the other good ROM sockets and it's all fine. Equally none of the lines seem shorted with each other. I get roughly 5k-ohm resistance from the data lines to ground and several M-ohm for the data lines. All the ROM sockets (good and bad) read the same.
2. The Z80 co-pro successfully boots CP/M, but after a few minutes characters start appearing on the command line as if I had a stuck key. This always starts with a lower case 'c'. If I hit return, CP/M correctly identifies a bad command and I then get a stream of 'd's followed by e, f, g etc. I get the same sequence each time I reboot ...
I noticed that the tube connector on the Beeb MB seems to connect to the data and address lines via IC88 and IC100 I reflowed the tube connector and the two ROM sockets. I still have the same problem with the Z80, but now the two ROM sockets don't work at all.
I'm thinking of replacing the two ROM sockets, but thought I would check here first as I managed to kill Beeb1 by randomly ripping out components.
Any suggestions greatly appreciated,
Ian.
Statistics: Posted by simmo1010 — Sun Feb 04, 2024 5:39 am