[sebhc] H90

[Carroll Waddell]

Lee Hart wrote:

> Robin England wrote:
>
>> Hi Carroll
>>
>> Bearing in mind the blown caps I would go straight for the ROM(s) on 
>> the CPU board. The earlier TI chips were always quite vulnerable to a 
>> bad +5 supply.
>
>
> I find that blown tantalum caps usually means that someone plugged in 
> one of the I/O board up/down off by a pin. This can also blow one of 
> the on-board regulators (the -12v and -5v ones seem most vulnerable).
>
> So, replace the capacitors and check the voltages from the regulators 
> along the top of the board to be sure they are all working.
>
> I would get the CPU board working by itself, without any memory or I/O 
> boards to confuse things. You'll have to set the DIP switches 
> accordingly. If you had a Z89-37 or other disk controller with 
> dangling cables to the CPU board, you'll have to pull that cable and 
> replace the missing chips on the CPU board for it to work without that 
> disk controller.
>
> Likewise, if you remove an expansion memory memory board, you'll have 
> to remove its flat cable and re-install the missing chip where it 
> plugs into the CPU board.
>
My current situation is this. I'm using different parts from the 2 CPU 
boards I have. I currently have only the CPU board with 48K RAM on it. 
No expansion boards. One of the boards had a bad Z80. Swapped the Z80 
(By the way, I'm only trying to get 1 board working). Now the address, 
memory refresh, and M1 lines are pulsing where they were dead with the 
other Z80. However looking at the 8 data lines, only 2 of them are 
pulsing from 0 to 5 volts. The other 6 only change from 0 volts to about 
2 volts. Not sure yet if something is loading them down.
All the voltages on the board are correct. I swapped the ROM chips that 
were the same from the other board, - no change in symptoms.
I'll keep looking, and thanks for the advice. Keep it coming. This board 
is hard to troubleshoot because it's so hard to get to in the machine.
Carroll
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