Sony Hit-Bit 75 not starting.

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Por ian.hollands.56

Supporter (9)

Imagen del ian.hollands.56

24-12-2013, 21:28

Hi all,
I wonder if anyone can give me any ideas on how to fix this sony HB 75. I bought it maybe ten years ago not working.
I get a rf signal but just a blue screen, reset also clears then back to blue screen.
I have checked +5v and +12v and all is ok.
z80 is on halt but getting data in out signals with logic probe.
Any ideas gratefully received.
many thanks
Ian

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Por hit9918

Prophet (2886)

Imagen del hit9918

25-12-2013, 13:08

I just took RAM chips out of emulator, and screen stays black.
But your machine gets till setting colorregister to blue.
So it looks like cpu and RAM work.

And when I set breakpoint to 0x38, it looks like first interrupt comes when screen is still black.
Looks like VDP interrupts work.

Well then it should get to the beep? Does it beep?
I got no idea whether a broken PSG can make it stuck. I guess it would blindly continue.
When VDP is broken so that no display is coming, it still should beep. And then make click click as you type.

Can ROMs get bad maybe?

Por Daemos

Paragon (1672)

Imagen del Daemos

25-12-2013, 23:51

Roms can get bad. EPROM techinology is known to get leaky after 20 years orso.

Por edoz

Prophet (2207)

Imagen del edoz

26-12-2013, 00:46

Color 15,4,4 ?

Por ian.hollands.56

Supporter (9)

Imagen del ian.hollands.56

26-12-2013, 01:09

Thanks for the replies, Power on. jagged lines then blue screen and thats it.
Checked on my toshiba, power on. jagged lines then blue screen then a quick white flash and basic prompt.
So im guessing that its not completing the reset circuit. same result with the reset button. tried with rom pack still the same result. No caps lock light or any sign of life.
It came with a load of Msx stuff i bought maybe 10 years or so ago. Its in mint condition so i guess it failed many years ago as the accompanying toshiba shows signs of use.
I'll just have to keep checking with my logic probe and maybe pull the z80 and bend the reset leg and try manually reseting it and see where that leads.
There is a couple of smaller Ic's that are socketed so thats normally a sign of high failure for quick repair so i guess there worth checking as well.
Maybe the roms but then i think it would crash or show signs of corruption.
thanks

Por Eugeny_Brychkov

Paragon (1127)

Imagen del Eugeny_Brychkov

28-12-2013, 00:54

@hit9918: it will work without PSG. I agree that RAM should be ok, as well as VDP interrupts.
@Daemos: agree.
@ian: please share hi-res image of the board from both sides. It may also help if you make small video of how it starts (does not start).

Por ian.hollands.56

Supporter (9)

Imagen del ian.hollands.56

28-12-2013, 06:53

Hi, I'm working away from home for a couple of days but when i get home ill stick some pics up.
many thanks

Por pseen

Resident (60)

Imagen del pseen

01-02-2016, 17:06

Hi, I have a Hit Bit 75P with the very same problem. Here is a short video showing the problem:
https://www.youtube.com/watch?v=8GthkGrHjNM

@ian.hollands.56 Did you manage to repair it?

Does anyone else have any suggestions?

Por Alexey

Guardian (2626)

Imagen del Alexey

01-02-2016, 22:09

pseen wrote:

Hi, I have a Hit Bit 75P with the very same problem. Here is a short video showing the problem:
https://www.youtube.com/watch?v=8GthkGrHjNM
@ian.hollands.56 Did you manage to repair it?
Does anyone else have any suggestions?

Could be anything. Not enough info to diagnose. Does it work with any game cartridge? Does CAPS go on and off when you press it? Do you hear a click from audio output when you press keys?

The most common problem in those old machines is bad RAM or VRAM chips. Do you want to sell this MSX or you want to repair it? Can you solder?

Por pseen

Resident (60)

Imagen del pseen

03-02-2016, 13:45

Thanks for your response, @Alexey!

I'm planning on repairing it myself. I have some experience with a soldering iron.

To answer your diagnostics questions:
- CAPS does not light up when it's pressed
- No keys generate any audio clicks when pressed
- No game cartridges boot up (tried six different ones)
- No difference between RGB or RF connections to TV

My working HitBit shows first a dark blue screen when it's booted up, then a light blue screen with the MSX SYSTEM V1.0 info. The broken one is stuck at the dark blue screen.

Por Alexey

Guardian (2626)

Imagen del Alexey

03-02-2016, 14:31

Ok, thanks. Seems that the problem prevents the system from starting at early stage. So what you can do next:

- check for obvious short circuits between power and ground lines
- check main voltages coming from the power supply circuit (+5, +12 and -12)
- check voltages on all RAM and VRAM chips
- check if some RAM or VRAM chips get quite hot (this usually indicates the failure)
- check if there's 3.58mHz on pin 6 of Z80 (requires oscilloscope or advanced multimeter)

This should be enough for primary diagnostics.

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