Sony HB-75B black screen

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By 8bitchipbits

Supporter (3)

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22-04-2015, 22:56

MSX newbie here and I've acquired a Sony HB-75B but it's stuck at black screen.

Only multimeter voltage tests done so far and power looks good and going to chips including ram OK also no visual damage anywhere and nothing running very hot etc.

Have switched out CPU with another but no change.

Mentioned in another thread that the eproms could have degraded over the years?

This appears to have 2 UV erasable ones HB EXP i and ii BASIC (there's no change of black screen if removed).

I do have an eprom programmer so can replace but would need good rom bins to compare them, either the originals or custom variants (ie can http://openmsx.sourceforge.net/ be used on the real thing?) if ever made but is that highly likely to cause black screen or would it be more likely other parts of the system?

I also have parts from a Toshiba HX-10 which has a corroded PCB and could be beyond repair in case there's any other swap out parts that could be of use for my diagnostics.

*Any* help extremely welcome.

Cheers

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By 8bitchipbits

Supporter (3)

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25-04-2015, 19:30

Will C-BIOS work, on Sony HB-75 models?

By Manuel

Ascended (13478)

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25-04-2015, 20:20

C-BIOS would work, but I wouldn't recommend it, because it doesn't implement BASIC. Just get existing HB-75B ROM images and reprogram the EPROMs, I'd say.

By collector msxxx

Resident (40)

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26-04-2015, 16:31

Tried to replace the roms with selfmade ones?

By 8bitchipbits

Supporter (3)

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27-04-2015, 17:21

Must be looking in wrong places as I've only found single file hitbit roms for 75p (nothing for 75b) but there's 3 chips - 2 are UV type with basic printed and the other an EEprom. I understand EEprom could be the sony-ized thing with personal data bank but are the basic UV one's generic to all MSX1s?

By Manuel

Ascended (13478)

Manuel's picture

27-04-2015, 17:51

Get the HB-75P ones to try it out Smile The only difference may be the pound key. I suppose, not checked that...

By Alexey

Paragon (1236)

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17-06-2015, 23:00

Actually I am also puzzled. I am fixing the hb-75p at the moment. The power supply has a fuse that is not in the schematics on the -12V circuit. I just wonder why there are 3 ROMs on board - 2 EPROMs 16kb each and one mask ROM 16kb. I found this nice site with various ROMs:

www.msxarchive.nl/pub/msx/emulator/openMSX/systemroms/machin...

There are only 2 files for my Sony there, and one of them is 32kb. So how is this ROM divided between two EPROM chips? First 16k in one and second in another or odd bytes in one and even in another (like in Atari)?

Alexey

By RetroTechie

Paragon (1414)

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19-06-2015, 04:21

Alexey wrote:

I just wonder why there are 3 ROMs on board - 2 EPROMs 16kb each and one mask ROM 16kb.

That'll be the BIOS/BASIC ROM split into 2 16KB blocks (0000-3FFFh and 4000-7FFFh). And another (EP?)ROM for the built-in utility in this machine.

It might be that the 32KB BIOS ROM is modified to 'need' the other 16K ROM, such that machine doesn't boot when the utility ROM is removed only. Probably the HB-75P (or -B) would run fine with 'generic' MSX1 ROMs from a machine like VG8020 - most differences between MSX ROMs are in keyboard layout, date format & such. You can find out which ROM does what by tracing the chip enable / output enable signals (in schematic, preferably Wink ).

As to original poster: you'd do better to find any of logic tester / frequency counter / oscilloscope @ this point. Otherwise you're just taking random guesses which parts to swap out. With just a few parts that would be doable, but in this case that's a waste of time.

First order of business: a) find out if Z80 gets a clock signal, b) check other clock signals like from VDP, c) test with an external RAM cartridge inserted, and d) same but with a sound-producing game cartridge in other slot & see if any sound is produced.

Btw did you do a thorough optical inspection of the board(s) ? Sometimes simply a good look is enough to find things like a fried component, a small crack in a pcb trace, leaking capacitors, ....

By Alexey

Paragon (1236)

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19-06-2015, 11:56

Yeah, agree with RetroTechie about the order of checks. However when I see image problems (corruption or black screen) I usually first check the voltages on VRAM chips. Those 4116 chips are crap. On my Sony HB-75P the fuse on -12v circuit was busted and the chips were missing -5v. So I always check whether 4116 chip has -5v on pin1, +12v on pin 8 and +5v on pin 9. If any of those voltages are missing, I check the power source. If the power source (voltage regulator or transistor) is ok and VRAM chips get unusually hot, I just replace all of them. Putting new ones in sockets of course...

So check the voltages first. If you don't have correct ones, next to check are voltage regulators in power supply. And of course the bloody fuse that is not shown in the service manual...

Alexey

By RetroTechie

Paragon (1414)

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19-06-2015, 21:58

I usually start with an optical inspection. Then checking voltages (on the boards themselves!) is always first, but 8bitchipbits said power looked good. Next is clock signals, if that's all present you move onto the function of individual (groups of) IC's.

4116 are crap indeed, and they die especially easy when their 'back bias' (-5V) supply is missing. So if any of a 4116 RAM's voltages are out of normal range, power off ASAP unless you want to unsolder / re-solder rows of chips. Evil

That said: broken VRAM chips shouldn't prevent an MSX from booting. And most probably, display something.

By Alexey

Paragon (1236)

Alexey's picture

28-06-2015, 00:58

I fixed my Sony. Had to bypass the broken fuse and replaced all 8 VRAM chips. The new chips cost me 20 euros. So now I have a Sony's MSX1 in my collection. Smile

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