Another dead C1541 on our desk. This one was reported as "not working". It was gutted for its 6502 and 6522 VIA chips, but supposedly even with those in place it wasn't working. So let's have a closer look... #commodore#c64#c1541#floppydrive
Opening up the drive reveals a quite early PCB 1540050 revision. The ICs are mainly from late 1982 or January 1983, placing this drive at somewhere early 1983 probably. The drive head measured fine, as is mostly the case with ALPS mechanisms. The Reset and Clock signal to the CPU look fine. I also inserted a 6502 and two 6522 from my other drive. With those in place the drive boots up and comes out of reset. However, it is not detected -- neither by cbmctrl nor by a C64.
This being a very early revision means that we are in bodge-city, Commodore. Loads of extra components, cut traces and even on the backside there are a couple of bodge wires.
As the drive isn't recognized we hook up the scope to the CLK and DATA lines on the IEC port. It turns out CLK is always high and DATA always low, even when there is activity on the bus. Not good.
Ok, so most probable reason is that the 74LS14 (UA1) is broken. It more or less drives the IEC lines directly, and if there is overvoltage or a static discharge during disconnect/connect of the IEC cable, you can fry this IC. I had another drive a few weeks ago with a dead UA1, but it still worked on the C64, as that was driving the bus very strongly, whereas the ZoomFloppy didn't have enough oomph. It seems this time the dead chip pulls very strongly!
While desoldering we see that two bodge wires lead to the suspect chip.
But we make quick work of this and indeed the test in the minipro TL866II+ shows that at least one gate in the chip is broken!
The chip is quickly replaced by a known good 74LS14 by Signetics, and suddenly the drive starts working again! It reads and writes disks without any issues!
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