Toshiba HX-10, not working. | MSX Resource Center (Страница 2/3)

By llopis

Resident (55)

20-01-2019, 23:01

gdx wrote:

Capacitors and -5V.

Oh, where am I supposed to have -5V?

Are there any schematics out there by any chance?

By gdx

Enlighted (6221)

21-01-2019, 00:22

Look for the datasheet of the RAMs. The -5V is for them.

By Wild_Penguin

Hero (644)

21-01-2019, 12:01

Yes, it seems the VDP RAM (4116 chips) use -5v. I though these machines don't use it...

(Main RAM , 4864s, run on 5V only)

Question is, does non-functioning VRAM cause the CAS to be high all the time, or maybe something else is broken?

AFAIK there is no datasheet available for this MSX-Engine. Hopefully it is not the Engine which is broken, since there are no replacements around...

By llopis

Resident (55)

21-01-2019, 18:49

This model has VRAM D416C, which I see is equivalent to NTE2117. I checked the voltages and they're correct there (0, +5V, -5V, +12V).

At first I thought that maybe the VRAM shared the /CAS signal as well, but no, they're completely separate. As far as I can tell the /CAS signal only goes to the big, custom Toshiba chip (is that the "engine"?).

I guess there's little to be lost by pulling out all the VRAM chips and seeing if there's a difference in the /CAS line.

Any other suggestions what to look at?

By Spirantho

Supporter (15)

22-01-2019, 12:15

Hi,
I just found this thread while looking for stuff about the HX-10. I'm trying to fix an HX-10 with an identical problem; I've removed and checked pertty much all the chips I can. The VRAM I've left alone as it's isolated to the video chip. The CPU is confirmed good, the ROM I'm not sure about but I have an EPROM in there anyway. The problem is that pin 5 of the TCX-1007 is constantly high, and that's the /CAS line. I've hooked it up to my logic analyser and this is definitely the problem.
I'm going to try and find some TCX-1007 (Ha! good luck....) chips but I suspect I won't know until I can get my hands on an HX-10 to swap the TCX-1007 with. I'm not dissecting my HX-22, even if the chip is similar enough.

I'll update this thread as and when I find anything out as it looks like it may help llopis too.

By Spirantho

Supporter (15)

25-01-2019, 09:51

An update!

I've fixed the HX-10 I was working on - although the MSX-Engine wasn't generating a /CAS on pin 5 (and therefore looking like the TCX-1007 was faulty), it turned out that the chip is fine. Phew!
The fault was the ROM - I had already burned a replacement EPROM but it seems that the ROM dump I had of it was also faulty (not sure where I got it from!). When I got hold of a working HX-10 (Thanks to "Mark Fixes Stuff") I could tell the ROM was bad.

I'm puzzled as to why the ROM would stop the MSX-Engine generating a /CAS line and holding pin 5 high, but that's what it did, so to anyone who has a constant 5V on pin 5 of the MSX-Engine: Double-check your ROM!

Final fix: One 27C256A EPROM burned with the correct ROM.

By gdx

Enlighted (6221)

25-01-2019, 10:32

Congratulations!

By Wierzbowsky

Guardian (3608)

25-01-2019, 14:40

Nice! Basically any malfunctioning chip can bring a signal up or down, so you have to trace the problem carefully. If it's not possible to find out who is responsible for the bad signal, then changing all affected chips is the only option. I usually start from RAM, then go to ROM and then to logic ICs. In one case the culprit was the VDP.

By llopis

Resident (55)

27-01-2019, 22:38

That's great. Congrats! Any chance you could send me a copy of the ROM you used? I used the one from OpenMSX, but who knows if that's a bad one somehow. My email is my username at gmail.
Thanks!

Spirantho wrote:

An update!

I've fixed the HX-10 I was working on - although the MSX-Engine wasn't generating a /CAS on pin 5 (and therefore looking like the TCX-1007 was faulty), it turned out that the chip is fine. Phew!
The fault was the ROM - I had already burned a replacement EPROM but it seems that the ROM dump I had of it was also faulty (not sure where I got it from!). When I got hold of a working HX-10 (Thanks to "Mark Fixes Stuff") I could tell the ROM was bad.

I'm puzzled as to why the ROM would stop the MSX-Engine generating a /CAS line and holding pin 5 high, but that's what it did, so to anyone who has a constant 5V on pin 5 of the MSX-Engine: Double-check your ROM!

Final fix: One 27C256A EPROM burned with the correct ROM.

By RetroTechie

Paragon (1563)

28-01-2019, 04:41

Spirantho wrote:

I'm puzzled as to why the ROM would stop the MSX-Engine generating a /CAS line (..)

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.

/RAS on the other hand tends to follow Z80's /MREQ somehow. Meaning it will normally have active pulses on a DRAM's input, even if that DRAM isn't addressed.

Anyway good job getting the machine working again!