VintageVic
Experienced Member
This is for someone possibly strugling with odd 'searching for...' -stuck failure of the 1541 drive.
The board in question is 1540050 pcb. Initially it was a runner, no booting, but spindle running
indefinitely with the red light on. I checked the obvious thing first, all socketed chips, but they tested
ok with a working pcb.
Someone had done fix to repair factory defect. It can be seen with added resistors to the UB1 chip.
However, the UB1 seemed to be known bad quality MOS7707 chip. It was a bit more work to remove that
chip, but it had to be done. I installed IC socket and redirected the factory fix resistors under the IC socket.
Now it was much more easier to put known good 7406 chip to UB1.
Yes! The drive booted now - great. However, next issue was that the drive gets stuck at load attempt
with screen saying 'Searching for...'. With that on the screen the drive did not respond at all to the load command.
Now, that UB1 is typical suspect for that failure, but I put a known good chip there so now what?
I double checked all traces to UA1 and UB1, since they were tampered with and broken trace could be an issue, but traces
were good. Checked traces to UC3 as it could be the cause, but they and the 6522 chip was ok too.
I kept chasing traces of different chips and eventually realized that for some reason, UC5 and UC7 gnd pins had
some 700-800 ohm resistance to the ground. Now that was odd! And not good for the drive. Soon I found out that
one of the gnd trace under the other VR nut had been broken. It had just enough contact for the drive to boot, but
not enough to operate correctly. Second image shows fix to the broken trace.
After the trace was fixed, the error was gone and the unit again works correctly.
Another 1541 pcb fixed!
The board in question is 1540050 pcb. Initially it was a runner, no booting, but spindle running
indefinitely with the red light on. I checked the obvious thing first, all socketed chips, but they tested
ok with a working pcb.
Someone had done fix to repair factory defect. It can be seen with added resistors to the UB1 chip.
However, the UB1 seemed to be known bad quality MOS7707 chip. It was a bit more work to remove that
chip, but it had to be done. I installed IC socket and redirected the factory fix resistors under the IC socket.
Now it was much more easier to put known good 7406 chip to UB1.
Yes! The drive booted now - great. However, next issue was that the drive gets stuck at load attempt
with screen saying 'Searching for...'. With that on the screen the drive did not respond at all to the load command.
Now, that UB1 is typical suspect for that failure, but I put a known good chip there so now what?
I double checked all traces to UA1 and UB1, since they were tampered with and broken trace could be an issue, but traces
were good. Checked traces to UC3 as it could be the cause, but they and the 6522 chip was ok too.
I kept chasing traces of different chips and eventually realized that for some reason, UC5 and UC7 gnd pins had
some 700-800 ohm resistance to the ground. Now that was odd! And not good for the drive. Soon I found out that
one of the gnd trace under the other VR nut had been broken. It had just enough contact for the drive to boot, but
not enough to operate correctly. Second image shows fix to the broken trace.
After the trace was fixed, the error was gone and the unit again works correctly.
Another 1541 pcb fixed!
