Need help repairing Toshiba HX-10

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By airamcruz

Supporter (7)

airamcruz's picture

01-10-2018, 11:43

Hello everybody, can anyone point me in the right direction?
I'm triying to fix my Toshiba HX-10. Power led lights up, also CAPS led work, no beep sound (CTRL-G) neither "typing" sound, all I can see is a blue screen with white horizontal lines (both RCA and RF) Funny enough, when a cart is in, the sound comes out right, but not the video (black screen with white lines)

I've changed nearly everything (Z80, ROM, VDP, LVA510 and the 8 4116 Video RAMs)

Any ideas?

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By Wild_Penguin

Hero (541)

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01-10-2018, 16:13

Hi airamcruz, and Welcome!

A picture of the fail (with the white lines) might be of use. I've been eavesdropping on threads troubleshooting similar errors. If you will post that, I'm guessing someone can tell right away which bit of the VRAM is not working (not me, I don't know such things from the top of my head Big smile ) EDIT: Also, is it still or moving? If it is still, but perfect horizontal lines consisting of pixels, remaining that way, definitely a VRAM/VDP problem

Do you mean you do not get keyclick sounds? Or typing command supposed produce sound do not produce any sound?

Do you get cassette relay clicks if you write "motor on" and "motor off" (be careful when you press enter to not mask the relay sound as it can be faint)!

Either the MSX has hung while booting BASIC (but CAPS LOCK still works?) - or it has not hung, but can not interpret any BASIC commands since VRAM is broken. Everything typed on screen in BASIC is first stored in VRAM and then red back from there if executed (correct me if I'm wrong).

EDIT: I suspect your CPU and main RAM is working since carts run (also the PSG). The problem is either in the VDP or VDP RAM. I guess BASIC part of ROM could be broken, but you should be able to run any cart game without that, if that was the only problem?

By airamcruz

Supporter (7)

airamcruz's picture

01-10-2018, 18:27

I've checked the voltages in the 4116's VRAM
Supposed to get:
-5V on pin 1
12V on pin 8
5V on pin 9
Is this correct?
I get a 0,43V on pin 1 on any 4116, so, if the above is correct, I have the -5V line broken. Any ideas from now on?

By airamcruz

Supporter (7)

airamcruz's picture

01-10-2018, 18:33

This is a photo taken without cart, after msx version info screen
https://drive.google.com/open?id=0B0MMbPSJ6bArbjd4VW51LWtibk04MnNSQW9LWDdkVE9tODlN

And this is a photo with a cart in
https://drive.google.com/open?id=0B0MMbPSJ6bArbWNxZ2FYTWFucDBxN1lUOE5ldzNiZ2NHM29r

Forgot to mention: I have a beeping sound now when pressing CTRL-G and I can hear the beeps while typing, don't know what I've done. LOL!

By airamcruz

Supporter (7)

airamcruz's picture

02-10-2018, 13:16

I tracked the -5V line and found a Zener Diode shorted (ZD3) Can anyone tell me what voltage and wattage is? the label just says "13x"

By Wild_Penguin

Hero (541)

Wild_Penguin's picture

02-10-2018, 13:32

airamcruz wrote:

I've checked the voltages in the 4116's VRAM
Supposed to get:
-5V on pin 1
12V on pin 8
5V on pin 9
Is this correct?
I get a 0,43V on pin 1 on any 4116, so, if the above is correct, I have the -5V line broken. Any ideas from now on?

This is most definitely the reason. That DRAM chip can not work without -5V. My guess is a broken regulator or something. A service manual for these computers would help, but last time I checked, it was not available online.

This is a very common model, I'd wait, maybe someone will reply who knows about the power circuitry on this machine (I have the SF model, I might take a look how the -5V is generated when I have time).

airamcruz wrote:

Forgot to mention: I have a beeping sound now when pressing CTRL-G and I can hear the beeps while typing, don't know what I've done. LOL!

This means that BASIC is working (i.e. only thing broken is VRAM)! If you can get correct reactions to input ("motor on", "motor off", 'play "C"'), this means that partly the VRAM is working. In your case, you should NOT get correct reactions, if our "diagnosis" is correct :-)

EDIT: Also, don't run this machine, at least not for prolonged testing / anything besides troubleshooting. I'm not sure that VRAM likes being run without -5V

By airamcruz

Supporter (7)

airamcruz's picture

02-10-2018, 14:12

Thanks Wild_Penguin

You're right, a service manual for this model would be very appreciated, unfortunatly it is not available online (did it ever exist anyway?)
I tried this basic commands: "motor on" "motor off" "play C" "CLS" "BEEP" "10 BEEP 20 GOTO 10 RUN" but nothing works
I will not turn on this machine for long periods, I think Video Ram is "killed" by the abscense of this -5V.
I've replaced the big 3 capacitors in the power section and I'm waiting for that zener diode (power regulator???)
My guess is 5.6V?!?!?!?

By airamcruz

Supporter (7)

airamcruz's picture

04-10-2018, 11:23

I found ou that the shorted zener is 13V
I've already ordered one, as soon as arrives I will replace it and post any news

By airamcruz

Supporter (7)

airamcruz's picture

04-10-2018, 22:58

Oh yeah!
The MSX is back to life!!!! Hannibal

I replaced the shorted zener and got my -5V back into the RAMS. The MSX works perfect now. Thanks Wild_Penguin for your advise.

By Grauw

Ascended (8318)

Grauw's picture

04-10-2018, 23:03

Congratulations! Always good to hear about an MSX restored to its former glory Smile.

Lucky that the RAM chips did not break in absence of that voltage. I think I’ve heard of that sometime (though maybe that was about the 12V line).

By Alexey

Guardian (2296)

Alexey's picture

05-10-2018, 09:46

Too bad we don't have a proper RAM tester to test every bit of RAM...

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