A brief report on progress (or lack thereof) before the CPU surgery begins.
Taking hydrophilic's advice I have taken a look at the activity on some A- and D-pins.
For reference: this is what it looks like on the working '71 a few seconds after power on (powered using the same lab supply as when working on the broken one):
Address line 13. A typical "running" binary signal.
Address line 11:
Data line 0. The signal is dipping below 0 volts att times; Vmax=4.89. This could be because of me using poorly calibrated/compensated probes. Other than that I have seen stranger TTL level signals.
Now for the broken 1571:
Address line 13. Using a different times/div. Looks good but slightly less "neat" signal.
Address line 11. Constantly high (approx 5 V) squiggly signal.
Close up of the above. Could be a bad capacitor somewhere I guess. I didn't replaced the ceramic ones:
Data line 0. Now this is interesting... This is the constant wave form and I have no idea what it is trying to do:
Not knowing where in the program loop the 6502 is I don't know if the CPU is trying to send data out or in (never bothered trying to trigger with R/W or RDY).
The squiggly A11 could mean that the program counter simply stops at some point or problems with continuity of the A-bus as per hydrophilic's suggestion.
But I'll go ahead with the CPU replacement anyway and this will be my final attempt before giving up on the board. The new 6502 has arrived from Germany and I will start snipping pins and soldering in a socket soon.
I will try this as soon as I can. It never occurred to me that the clock could be bad. If it is it could probably be replaced if I can find the right crystal. I have an (old and crummy) oscilloscope and I will take a look at the clock-pin and see if I get a decent square wave.
A 6502 without a good clock-signal would exhibit the same behaviour as a broken one I guess (none at all).
I am trying to repair a broken 1571 that I scored for cheap (this is the first time I have tried anything more advanced than changing the filter caps and regulators of a 1571 PSU).
The symptoms are: constant green light and motor constantly spinning.
Looking at the service manual and the notes on Ray Carlsen's home page (http://personalpages.tds.net/~rcarlsen/cbm/1571/) i have tried these initial repairs:
* All 74LS-IC's replaced.
* The 74F32 was replaced with a 73HCT32 (the "fast TTL series" is hard to find these days and looking at the data sheets the HCT series *should* work as a replacement if you take care to ground the unused pins).
* All electrolytic caps have been replaced (ceramics are probably OK, they don't degrade the same way electrolytics do).
* The ROM has been replaced with a working one (from a working '71).
(The drive is hooked up to a lab-supply.)
But the symptoms persists.
The next step is replacing the 6502 and I'd rather not do it until I have exhausted all other possibilities.
Is there anything else I should check/try first? Are there any diagnostics I can do "inboard" for a 6502?