you already tried to push in its sockets the ROMs chips (if any socketed)?
Well, the only socketed chip is the Z80. And I've tried to re-insert it.
Dunno if you tried already, but does inserting a game cartridge have any effect?..
No effect, tried it many times.
The RAM and ROM chips are not socketed..
Please, post here, a full resolution pic of the mainboard, but the soldering side
Will do. But I will have to take the machine apart completely. Kinda tricky thing to do: the board attachment to the box is not obvious: no visible screws / lips or whatever. And I hate to bend it or apply significant force.
Okay, finally some advancement!
The RAM cart and a similar machine (Hitachi H21) has arrived, so I had more choices to play with, and here's the result:
- I have replaced Z80 with the one I pulled from H21, and it worked! So, I assumed, Z80 was broken.
- Now, the interesting part: when I put "broken" Z80 to the new machine, it worked, too!
- When I put everything back as it was, the machine in question stopped working again.
( Oh, and, by the way, the RAM cart that I bought specificaly for this ocasion turned out to be broken. Or, I couldn't make it work. )
One possibility is that the MB-H2 uses a Z80 signal somewhere, and the H21 doesn't use that signal. Making that Z80 fail in one machine, but not in the other. For example I/O port decoding sometimes uses the /M1 signal, and sometimes doesn't. If Z80 fails for this reason, it's not 'broken' but broken.
Or it's something that looks unrelated, but still causes a problem (power supply, ???, many possibilities)
About that RAM cart, type:
with no RAM cart inserted, RAM cart inserted in 1st slot, and RAM cart inserted in 2nd slot. And see if this reports different values.
Anyway, good to hear you've located the problem. Am somewhat surprised it was the Z80 after all... I'd consider that specimen suspect even though it works in one of the machines.
...Or maybe there's a broken track or flaky socket (if there's any) that works from time to time.
Also, have you measured +5V in pin 11 in both boards?
If a different Z80 solves the problem, it is still most likely an oxidation problem, carefully apply so more pressure on it, is might sink a little deeper into the socket, alternatively, z80's are easy to get, so if a different z80 solves the problem, why not just replace it. the oxidation (if any) could be on the socket _or_ on the z80 pins, _or_ both.
type: PRINT INP(168) with no RAM cart inserted, RAM cart inserted in 1st slot, and RAM cart inserted in 2nd slot. And see if this reports different values.Yes, there are different values. I take it, the cart works, then?
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