Re: 9825 restoration notes and questions

Jack Rubin

I replaced the plotter/memsize slide switch on the A24 board and things are a  little clearer although not necessarily better. It seems that the A25 board is invisible to the rest of the system. I show the same memory size whether or not the board is installed - if the A24 switch is set to select 64K memory and the '62 plotter, "list/execute" gives 31294 and with the '72 plotter selected, it gives 31260. If I enter "list 0/execute", the results are 31286 (plotter 62) and 31252 (plotter 72). The results are the same with or without the ribbon cables connected to A25. 

Setting A24 to 24K and selecting the '62 plotter, "list/execute" displays 22952 and "list 0/execute" displays 22944. 

I'm not sure how LIST works (does it show remaining memory?) or why adding a non-existent line consumes 8K of memory, but that's a question for another time.

Trying to RUN or STORE "asc", "for" or "mdec" all give "mnemonic unknown" errors, thus showing ROMs are not loaded.

All power supply voltages are in spec with minimal ripple and checking voltages on random 4116 RAM chips when the A25 board is installed shows correct values for +5, -5 and +12 at the chip. I've also cleaned and re-seated the various cable connectors. Everything looks good visually.


I'm not sure where to go from here. Trying to correlate Tony's schematics with the Processor Theory of Operation section of the service manual (09825-91030), I'm guessing that what the manual calls the Monitor Bus is what Tony labels Ext Ctrl. If the Monitor Bus connection is intact, I think I should find D2MapClk on pin 6 of U43 and DMapClk on pin 15 of the same chip. Does that make sense? Should the clocks should be 6MHz? 


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