Toshiba HX-10 Repair

Supporter (14)

03-10-2021, 13:56

Unfortunately after taking my MSX from storage it decided to unalive itself.
Now I have big fat short on +5V line to Ground.

I have checked big transistors on power supply circuit and zener diodes. Those seems to be fine.

All IC are stone cold, so I can't see obvious place of short.

Are there any known reasons for similar faults?

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By gdx

Enlighted (5330)

03-10-2021, 14:38

Check the capacitors.

By SkalTura

Champion (350)

03-10-2021, 15:57

gdx wrote:

Check the capacitors.

Yes, most likely a capacitor.
Check if you can see a bulge in the caps, or maybe even a leak.
If not, check what capacitors are in the 5V circuit (maybe you can find a schematic of the PCB), and de-solder them one at a time to measure if the short is gone.

Or just replace them all just in case...

By ArkadiuszMakarenko

Supporter (14)

03-10-2021, 16:41

Unfortunately there is no schematics for this PCB.
Fortunately there is not that many of them.

By Alexey

Guardian (3349)

03-10-2021, 18:06

Check the slot, as well as the bus expansion connector in the back. Also, check both joystick ports to see if there are no shorts there.

Then my bet would be on one of the transistors or voltage regulators. And then - caps. Even a ceramic cap can create a short between power and ground rails.

By ArkadiuszMakarenko

Supporter (14)

03-10-2021, 18:45

I have bad feeling.
I took 4164 ram and short from 3ohm went to 300ohms.
But power supply still can't produce all voltages.
I have feeling that PSU failed and took a lot of chips with it.

By ArkadiuszMakarenko

Supporter (14)

05-10-2021, 20:05

I have acquired motherboard replacement on eBay. So I will be able to continue my hardware tinkering

By gdx

Enlighted (5330)

06-10-2021, 13:22

I don't think the second motherboard is useful. What causes the problem in general are the capacitors, RAMs / VRAMs, video circuit chips or the micro-switch in the cartridge slot. It can also be a bad contact of the chips having a support or elsewhere. Check cables and welds.

About capacitors, verify if current does not flow through continuously with an ohmmeter (black to - and red to +), and if the pins do not touch a track or a pin nearby.

By ArkadiuszMakarenko

Supporter (14)

06-10-2021, 22:09

I will work a little bit more on it, when time allows, but I need to have reference board first, I need to do something on MSX!

I have removed some RAMS already, and after each one removal resistance between +5V and GND was rising a little.
I will try the same with VRAM, maybe I this will solve the issue?

I removed almost all decoupling caps I have only few left on 5V rail.

Arek