Hitachi MB-H2 not working

Page 1/8
| 2 | 3 | 4 | 5 | 6

By Demiurgo

Master (154)

Demiurgo's picture

27-06-2015, 15:41

I have a H2 with a problem... power reaches at least the power led and the tape, because when I turn it on, led lights up and tape starts running by itself. Problem is, I get no image at all, and I tried RF and RGB connectors. Any idea about what to check?

By the way, the innards of this computer are a real chaos... oO

Login or register to post comments

By Alexey

Guardian (1930)

Alexey's picture

27-06-2015, 20:13

I haven't found a service manual for this computer. Did you try to insert a game cartridge? Does the computer produce any sounds during the game or on keypress?

The first things to check visually are capacitors. Blown capacitors are usually visible and can be easily replaced. Any loose wires or burnt/broken/disfigured components must be fixed.

Then the DC voltages coming from a PSU or voltage regulators on-board must be checked. There should be +5v, +12v, -12v and also -5v if VRAM uses 4116 chips. If every necessary voltage is present at power regulators, next thing is to check voltages on RAM and VRAM. The 4116 chips must have -5v, +12v and +5v on pins 1,8 and 9. The 4164 chips must have +5v on pin 8. Ground must be connected on pin 16 of both chips.

After that comes the time for the oscilloscope checks. With it you can verify whether the CPU clock is correct (pin 6 of Z80), whether sync siglan is produced correctly and so on.

During my last few repairs I had to replace all 8 VRAM chips (4116) in old computers (MSX, Sinclair) because they are usually first to break if voltage supply goes wrong or missing. Bad chips are usually hotter than good ones, but there's no 100% guarantee.

If you want to sell this computer, I would be interested.

Alexey

By Demiurgo

Master (154)

Demiurgo's picture

27-06-2015, 21:50

Almost three months ago I stop fighting with the computer, but I can confirm that turn it on with a cartridge makes no difference at all. Indeed I find strange the tape starts same time as I turn on the computer. I bought (and sell) another model with exactly the same symptoms.

There is no visible damage. No burnt wires, no blown or swollen capacitors. Temperature was not abnormally high in any IC. I replaced the Z80 and it was working fine in another computer. I made some checking on the IC, and I took these notes:

RAM (8)
IC7, IC8, IC9, IC10, IC41, IC42, IC43, IC44
TMS4164-15NLJ

VRAM (8)
IC13, IC14, IC15, IC16, IC17, IC18, IC19, IC20
NEC D416C-3

ROM (2)
IC11, IC12
HN613128P, HN613256P

VIDEO
IC23
TMS9928ANL

AUDIO
IC26
AY-3-8910A

PPI (PROGRAMMABLE PERIPHERAL INTERFACE)
NEC D8255AC-5

I also took some pictures, because maybe you can see something I can´t... how can send them to you? And finally, if you are interested in the computer, how much do you offer? Cool

By Demiurgo

Master (154)

Demiurgo's picture

27-06-2015, 22:12

Here are some pictures:





By Alexey

Guardian (1930)

Alexey's picture

27-06-2015, 22:59

Wow, what a mess! Very ugly design comparing to my Yamahas. And a lot of metal too. If you are in the Netherlands, I might be able to pick it up myself, otherwise shipment will be over 40 euros, which is no go for me. And I won't pay more than 50 euros for a defective MSX1 computer. So it's probably not a good deal for you anyway, sorry.

Start checking the voltages, a busted voltage regulator may be an easy thing to fix.

By RetroTechie

Paragon (1475)

RetroTechie's picture

28-06-2015, 00:39

1. Thorough optical check
2. ALL supply voltages present & within normal range? Check on the board(s)!
3. Clock signals
4. Caps lock response
5. Try with a known good external RAM cartridge
6. Same as 5) but with also a game cartridge inserted & check for sound output
7. Test individual chips in other systems, read out ROM contents in a EPROM programmer, etc, etc.

Demiurgo wrote:

Almost three months ago I stop fighting with the computer,

At which of above points did you get? All other points before it checked?

By Demiurgo

Master (154)

Demiurgo's picture

28-06-2015, 01:25

1. No damage found at optical check.
2. I tried what I was able to, but had problems to access to VRAM with boards assembled (computer has three different boards, plus a smaller one for heatsink) so they were not checked.
3. No idea how to. I don´t have oscilloscope. Just a logical probe, and never used it yet.
4. No response to Caps o kana keys. They have their own led, and no response. Anyway, the computer has a speed control, the blue button on the right side, and it activates they own led when pressed.
5. I have no RAM cartridge.
6. I have no RAM cartridge.
7. At least, Z80 works. ROMs are soldered to board, and I am afraid to toast the board trying to remove them...

By the way, I found in this page a datasheet for the computer:

http://www.msx.org/wiki/Hitachi_MB-H2

But I found a difference. There says video processor is a TMS-9918A, but my unit has a TMS9928ANL.

By Alexey

Guardian (1930)

Alexey's picture

28-06-2015, 01:51

Hmm, this capacitor doesn't look right:

Is it properly connected?

By dhau

Paragon (1542)

dhau's picture

28-06-2015, 02:07

I usually start by supplying 5v from a trusty old pc at psu. Most MSXes work with that. If it works, check fuse and recap electrolyties in power supply part

By Demiurgo

Master (154)

Demiurgo's picture

28-06-2015, 10:56

Checked Z80A, graphic chip TM9928ANL, sound chip AY-3-8910A and the NEC D8255AC-5 chip and all have 5 volts. oO

By Demiurgo

Master (154)

Demiurgo's picture

28-06-2015, 11:44

Alexey wrote:

Hmm, this capacitor doesn't look right: Is it properly connected?

Yes, I think so. It must be bent because lack of room in this cramped case... oO

By the way, if you want any close-up picture, just let me know.

Page 1/8
| 2 | 3 | 4 | 5 | 6
My MSX profile