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64K Repair - deanclaxton - 04-10-2022
This will be the thread where I tackle some initial repairs to my recently aquired 64K machine.
Current Status - seems to work in that I get a prompt, but there are a heap of dead keys.
To Do :
- Replace missing key caps (Brad has sent me some)
- Remove, clean and reinstall ALL key switches
- Understand the purpose of all the hardware mods on the main PCB
- Clean up the mods - some pretty awful soldering there
- Get the floppy drive unit (SBC01) tested and perhaps install a Gotek
- Fully test the machine with software loaded from disk/gotek
- Investigate options for a ROMX type product for Microbee (https://theromexchange.com)
Some pictures of the machine as it is today:
The rear of the board - lots of mods/bodge wires :
Some close ups :
I noted that a couple of IC's had legs removed, with some of the mods attaching the signals elsewhere. One of my objectives is to understand the mods and clean them up. I believe there is a hardware reference book that covers off the modifications that were published?
This is a rev F board so perhaps some of the mods were included in later hardware.
Anyway - fun times ahead - my first Microbee
cheers,
Dean
RE: 64K Repair - ChickenMan - 05-10-2022
Looks like a very modded bee indeed. In the Repository Documentation/Manuals/Technical I'd suggest you grab the microbee_hardware_notebook_with_updates.pdf and the microbee_technical_manual_1986.pdf, I think everything you will need is in those 2 docs.
RE: 64K Repair - MbeeTech - 06-10-2022
The Thick brown wires look to be repairs for the keyboard matrix.
Some of the blue wires (at the Font Rom) are mods to take the 4732 dual font rom.
The black wires near IC33,34 are most likely the mods to allow the baseboard to work with the DRAM coreboard (CIAB upgrade).
The Blue wire near IC30,32 is most likely due to the 12Mhz -> 13.5Mhz clock upgrade (Cpu 2Mhz -> 3.375Mhz)
The baseboard looks to have been part of a kit bee from the soldering, although from the component side it looks like a factory build.
As to the miscellaneous resistor changes, other links, perhaps look at the hardware notebook in regards to upgrades to the cassette interface, serial port, parallel port
and video output circuitry.
RE: 64K Repair - deanclaxton - 06-10-2022
Excellent - thank you both for the info
I'll grab those documents from the repository and take a look. Are the equations available for the PAL chips on the DRAM board? I know the JED files are there but wondering if the equations are published anywhere?I'm curious what the different colour codes represent functionality wise (probably published in one of those documents) but also interested in the equations to duplicate the functionality in another device plugged into the socket. I'm sure it can be worked out from JED and schematic but curious if its published.
cheers,
Dean
RE: 64K Repair - MbeeTech - 06-10-2022
(06-10-2022, 04:05 PM)deanclaxton Wrote: Are the equations available for the PROM chips on the DRAM board? I know the JED files are there but wondering if the equations are published anywhere?
No, as a policy, we do not release the source code for any Microbee software or (in this case) proms.
(06-10-2022, 04:05 PM)deanclaxton Wrote: I'm curious what the different colour codes represent functionality wise (probably published in one of those documents) but also interested in the equations to duplicate the functionality in another device plugged into the socket. I'm sure it can be worked out from JED and schematic but curious if its published.
As far as the colours on the PALs go, it was just to identify which fuse file it was programmed with in the factory.
Red = 64k DRAM (only 32k banks 0 & 1 supported)
White = 128k DRAM (4 x 32k banks supported)
Silver = 256TC DRAM control with 8 x 32k banks supported, and
Gold = Video control PAL for the Premium & 256TC
RE: 64K Repair - deanclaxton - 06-10-2022
Thanks Ewan - that makes sense. So to clarify, if I was to populate the empty RAM sockets I'd need a white PAL? I'm not sure whether these are OTP type, or reprogrammable - will check it out.
RE: 64K Repair - deanclaxton - 06-10-2022
I've been able to generate the equations for both the Red and the White PALs, using the JED files in the repository. The only difference is the handling of the pin 21 output (RAS1).
Cool
RE: 64K Repair - Ernest - 07-10-2022
Now to burn up a GAL20V8B or similar, drop-in another Bank of RAM and away you go.
Ernest
RE: 64K Repair - MbeeTech - 07-10-2022
We have white PALs in stock.
Contact via the website and I'll send one out if you want one.
RE: 64K Repair - deanclaxton - 08-10-2022
Thanks Ewan,
I'm going to have a play around with a board that plugs into both the PAL socket and the ROM socket first up - with a flash chip and CPLD on it. Initially I'll replicate the PAL logic, with the stock ROM image on the flash chip, but then I'll see where that leads.