Repair of a Sharp HB-8000 Brazilian MSX computer

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By Argon

Paladin (1005)

Argon's picture

12-11-2017, 11:22

Hey fellow MSX lovers.

I obtained a Sharp HB-8000 in bad state, it was completely dead on arrival Smile

I was able to repair the power supply by recapping all capacitors and replacing the coil.
And I was able to repair the modulator by fully recapping it.
(Both repairs are confirmed working by testing them in another HB-8000 I own)

However, the computer still doesn't work.
I just get a black screen as output.
I tried "piggy-backing" the RAM chips, but that didn't do anything either.

Anyone has any other ideas?
What should I check next?

Thanks!

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By Alexey

Paragon (1229)

Alexey's picture

12-11-2017, 14:06

First is the visual check of the board. Any cracks, corrosion, loose parts, miscolorisation (result of overheating) and other visible defects should be checked. Then of course the presence of correct voltages - some computers have -5v for RAM chips. If that voltage is missing, chips are usually ruined after a while. Then I would check if any components are abnormally hot.

Then goes the part that needs special tools like oscilloscope. You need to check correct frequency on VDP and Z80 and also check what's output from the video-out connector. This may give you some clues. If 3.58 MHz generation works, but there's nothing on video output, then you need to diagnose the video circuit. Check for activity on data and address lines when you start the computer. Check whether the RST signal correctly changes after power on.

If you have video signal, but black screen, then there is a problem with startup. Usually this means that the RAM is bad. Try to use external RAM or game cartridge to diagnose. In some cases the black screen could mean a partially fried VDP or VRAM.

If everything seems to work, but the computer doesn't start, you need to get the logic analyzer and start debugging the circuit deeper. For example I hook the analyzer to data lines and CS signal of ROM and try to see how BIOS initialises and where it gets stuck. If there's no meaningful startup code, then I check the logic chips and replace them if I see something weird or no activity at all.

In any case the diagnostics of a broken computer is a nice and difficult quest for amateurs including me. So if you get stuck, always ask for help from professional electronic engineers.

By fernando.collazo.5682

Master (199)

fernando.collazo.5682's picture

13-11-2017, 02:18

By tvalenca

Paladin (674)

tvalenca's picture

14-11-2017, 15:04

Some HB-8000 have been sold with a bad batch of 74LS74 and 74LS138 TTL ICs, so it's not uncommon to find one with problems on refresh generation circuit or slot select logic. But I had one with a bad VDP (fryed /INT signal) and another with a bad reset buffer as well. Without an oscilloscope it's tricky to find the problem, but you may want to check for activity on Slot Select signals, RAM /RAS and /CAS signals, and /RESET and /INT signals on Z80.

If you have a RAM cartridge, (preferably a SRAM one) plug it on any slot and if it boots you may start checking /RAS and /CAS signal generation. Using an Oscilloscope (or logic probe) start on RAM /RAS and /CAS pins and walk through it using its service manual (alternative link) or schematic until you find activity. look at this article (english google translation) to get more info.

By Argon

Paladin (1005)

Argon's picture

15-11-2017, 16:41

Hey tvalenca, what is a "RAM cartridge"? A memory mapper?
SRAM? Maybe an FM-PAC?

Thank you.

By Grauw

Enlighted (6255)

Grauw's picture

15-11-2017, 19:10

Either a plain 64K RAM cartridge, or an MSX1-compatible memory mapper.

Memory mappers which are not MSX1-compatible won’t work, because the MSX1 BIOS does not initialise the registers. Though maybe one that is not MSX1-compatible will let you boot to the Basic prompt, it will crash if you load a program though.

By Argon

Paladin (1005)

Argon's picture

15-11-2017, 19:16

I have a 1024K memory mapper and a Philips 16K (or is it 64K?) one I think, would any of those do the trick?

By Grauw

Enlighted (6255)

Grauw's picture

15-11-2017, 22:06

Give the 1024K one a go. The 16K one won’t work, it’s in the wrong memory area. 64K would.

By Argon

Paladin (1005)

Argon's picture

16-11-2017, 09:37

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?

By Alexey

Paragon (1229)

Alexey's picture

16-11-2017, 10:05

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).

By Alexey

Paragon (1229)

Alexey's picture

16-11-2017, 11:50

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.

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