[sebhc] H90

Carroll Waddell carrollwaddell at sc.rr.com
Fri May 11 15:24:37 CDT 2007

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.
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