Repair of a Sharp HB-8000 Brazilian MSX computer

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By tvalenca

Paladin (681)

tvalenca's picture

16-11-2017, 15:35

Argon wrote:

Hey guys.

My memory mapper is a "Memory-Mapper 256/512" from "R. & J.JANSEN"

When not using the mapper: noisy white screen.
When using the mapper: noisy white screen with a couple (about 4) horizontal darker lines.

So there's definitely a difference, but the computer still doesn't boot.

I will continue by socketing and replacing the RAM IC's and the 74LS138 as suggested by someone on facebook.
If that doesn't work, I will have to learn how to use a logic probe, never done that before Smile

Any other suggestions?

Maybe your memory mapper isn't suitable for MSX1 computers. they have to be self-initializable, because the BIOS is supposed to initialize Mapper registers and only MSX2 (and up) BIOSes have the code to do it.

but most important is: TAKE EXTRA CAUTION WHEN REMOVING ICs FROM HOTBIT's MOTHERBOARD. Its board can't take any extra heat, because it has fragile traces and vias. You have been warned!

By tvalenca

Paladin (681)

tvalenca's picture

16-11-2017, 15:37

Alexey wrote:

Try to remove all on-board RAM chips and see what happens when you start with a RAM cartridge. Without RAM chips the computer will use the RAM from the cartridge (assuming the original RAM was in the slot lower than the cartridge slot).

Alexey wrote:

Checked the MSX Wiki, the base RAM is in slot 3, so the RAM expansion in cartridge slots will be used by default. But still, removing the chips may be a good way to test that they don't mess with address or data lines.

Good point! A bad memory chip could mess with either Address or Data buses. But here's a tip:

A 64k MSX most likely have two 74LS157 as Address bus multiplexer. (at least the models that uses only discrete ICs, like the HB-8000 of this topic; Some MSXes have this multiplexer embedded on MSX engine or DRAM controller chip) If any DRAM IC gone wild and address bus become unstable, you just have to turn of these multiplexers. If you are skillful enough, this means disconnecting only VCC and GND pins from both of them. If you do this, insert a 64k RAM cart (or self-initializable mapper) on any slot and it boots up, you can safely assume that you have a bad RAM chip (without having to unsolder 8 heat-sensitive DRAM chips just to debug)

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