Toshiba HX-10, not working.

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By llopis

Resident (53)

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28-01-2019, 10:11

RetroTechie wrote:

DRAM control can be done in several ways (early write, hidden refresh.. it's complicated). But usually, /CAS signal(s) work like a chip select. So remaining high just means that DRAM isn't asked to write or output (read) anything. Which can happen if eg. a faulty ROM causes the machine to get stuck in a loop. Or some other issue makes the machine not get to the point where it searches for / uses the RAM.

But wouldn't a healthy Z80 always be fetching instruction data if it's getting a clock signal? Even if it's in a loop, it should be constantly fetching from memory, right?

By llopis

Resident (53)

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12-09-2019, 11:29

Just to give this some closure: I ended up tracking down a working Toshiba HX-10 and started reverse-engineering the circuit diagram and checking things on both. Everything checked fine and I didn't see any obvious problems with the faulty one. I also swapped all socketed ICs just in case, but nothing.
So I decided to desolder and socket the TCX-1007 (which is the big custom IC) and... bingo. That was the problem :-(
Anybody knows where I can buy a TCX-1007 or am I out of luck with that machine?

By Alexey

Guardian (3236)

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12-09-2019, 12:19

This chip seems to be the MSX engine of Toshiba. The only way to repair the machine is to get a broken motherboard with the working chip. I could buy the broken Toshiba from you if you decide to sell.

By Omega

Master (230)

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08-03-2021, 15:01

I would like to add to this:
I also had a black screen HX-10 on the bench to fix for someone. Similar symptoms as above: no main ram CAS activity. In my case it also turned out to be a corrupted BIOS ROM, i.e. CPU executing garbage instructions, never actually selecting the correct RAM slot, and hence no activity on CAS. A replacement EPROM fixed that.

By Gregory

Master (178)

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14-07-2021, 21:52

I also have a non-functioning Toshiba HX-10. Black screen,...
I have a few questions:

- What is IC16 on the board: it has a heatsink and the pins are coloured black...
- What is the purpose of the toggle switch in the cartridge slot. if i press it I loose my video output...
- how come the bios rom can be corrupted (MB83256). It doesn't have a window, so light can't erase bits, so how can this be...

By Gregory

Master (178)

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14-07-2021, 21:53

xriva wrote:

O.K, quick update, made myself an arduino based DRAM tester and tested all the D4164C chips(IC7 - IC14) and it turns out two of them were bad (IC8 and IC12). I have bought some new ones and put in chip sockets for all the DRAM.

Now all is well and I have a perfectly functioning HX-10, thanks for all the help and suggestions.

Could you tell something more about how this arduino tester works?

By Pencioner

Scribe (1478)

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14-07-2021, 21:59

Gregory wrote:

- how come the bios rom can be corrupted (MB83256). It doesn't have a window, so light can't erase bits, so how can this be...

Nothing stays forever...

By Gregory

Master (178)

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14-07-2021, 22:28

Pencioner wrote:
Gregory wrote:

- how come the bios rom can be corrupted (MB83256). It doesn't have a window, so light can't erase bits, so how can this be...

Nothing stays forever...

So I'll start by replacing the ROM on my board then.
I'll use the one that comes with OpenMSX.
Hope that's not corrupted...

By jltursan

Prophet (2559)

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15-07-2021, 15:29

引用:

- What is IC16 on the board: it has a heatsink and the pins are coloured black...

The HX-10 PCB differs between models; but it must be the VDP or the PSG. If it's the european model, most probably, the VDP.

引用:

- What is the purpose of the toggle switch in the cartridge slot. if i press it I loose my video output...

It mechanically resets the machine when you plug in a cartridge. If the machine is on, this serves to momentarily switch-off the power and avoid shortcircuits.

引用:

- how come the bios rom can be corrupted (MB83256). It doesn't have a window, so light can't erase bits, so how can this be...

ICs always degrade with time, only one "rotten bit" in a mask ROM and your mileage may vary, from not noticing anything bad, to making the machine unusable.

By Gregory

Master (178)

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15-07-2021, 20:21

Thanks for your reply.

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