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Re: MSX Computers

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TabajaraLabs:
Model: Sharp Hotbit HB8000 MSX1 computer
Symptom: No video, no caps lock light change when key pressed


The HB8000 has one of the worst PCBs I've ever seen, so take care repairing it


Usually the culprit is the 64K ram bank. Make a 64K SRAM memory cartridge and insert it on the top slot. If the computer boots, you can change the 4164s. It isn't easy to find the broken one. You can use the scope on the data and address lines and find something with a invalid logic level.

TabajaraLabs:
Model: Sony HB-T7 MSX2 computer (internal modem)
Symptom: Zed is dead.


This is a very interesting computer with an internal modem. Most problems are due to lightning on the telephone line, but this one got a different problem: CPU was stuck in wait state. There was refreshing of memories but no activity beyond that.


Someone left a cart on the top slot, and this cart had a battery. Somehow, electrolyte from the battery dripped on board in a very small spot near the cart conector, and corroded the /wait line.


After 4 hours looking for the problem, I figured there was a small discoloration on the board near the slot. Added a jumper, and the machine came back to life. Nice.

TabajaraLabs:
Model: MSX computers in general
Symptom: Zed is (as always) dead.


Some commmon problems in MSX computers which appears dead, in order of importance.
1 - DRAM bank - one of the DRAM memories goes south, so the computer. Test with a 64K SRAM cartidge (all data and address lines connected to slot, /rd and /wr too. /ce to ground, /oe to /sltsel on slot).
2 - Z80 - it is very common to have a dead Z80, even if the MSX has a fully buffered slot.
3 - Slot buffers (when applicable)
4 - Power supply - you read the correct tensions when there is no load, but tension dips when connected to circuit. Remember to recap the power supply and check for the regulation loops.
5 - corrosion - As everything who has a battery, corrosion play havoc on MSXs. Remember to substitute or to pull the batteries on every computer you get, ASAP. It is very common to have expensive 2+ and Turbo-Rs with batteries from 1980s and have it corrode the battery support, or worse, the board.


Interesting info: I fixed many MSX computers and NEVER EVER saw a S3527 or S1985 MSX engine broken. If you suspect the msx engine, point your guns to another chip. They are mostly unbreakable.

TabajaraLabs:
Model: Sony F900 MSX2 computer (applies to F700 and F500 too!)
Symptom: Disk Drive broken


Disk drives from Sony are a PITA.


You can use normal 1.44 drives from PCs (in 720K mode) if you follow these rules:
- Cut the wire of pin 34 of the cable, and join it to pin 33 (gnd)
- Measure the state of the HD sensor (left switch in the front of drive) without a disk. If it is open, desolder it and close it. If it is closed, just desolder it. It will left the drive in DD mode permanently.
- The cable has connectors for a pair of drives. In the connector for drive A: cut wires 10 and 12, and cross it (10 connected to 12 and vice-versa). PC drives are factory set to B: (in fact, DF1). This mod changes the drive to A: (DF0).
- You'll see the drives doesn't fit well on the drive cage. Use seven to eight screw washers in each screw that fixes the drive, so it will be higher on the cage and fit well on the front of the computer. I use to envolve the washers in sealing tape so it makes a "pack" and I can insert it at once on the screw

Don't forget to change the battery of the computer!

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