Commodore 1541 Drive Repair Part 1

I recently attended a repair workshop at the System Source Computer Museum in Hunt Valley Maryland. This was my first time attending one of these events so I didn't bring anything with me to work on, I figured I might find someone else that needed help, which is exactly what happened. I met someone who had brought a bunch of Commodore 1541 drives and was hoping to find someone who could help repair them. Even though I had never worked on  a 1541 drive I knew they were 6502 based to I thought I would take a crack at it. 

Most of the drives would not even get through their power on self test, indicated by the activity LED flashing. One started to emit a burning smell when we powered it up, so we put that one aside. Another would not power up at all. We swapped chips on a few but didn't make any progress that way. There was one that powered up properly but as we were trying to do further testing with it connected to a C64, the computer failed. So, we didn't make much progress there but I took one of the drives home to work on.

Before I could dig into the drive I needed some documentation. I found this site which has a good collection of schematics for various versions of the drive:

http://www.zimmers.net/anonftp/pub/cbm/schematics/drives/new/1541/index.html

I also found a couple good books on Archive.org:

Commodore 1541 Troubleshooting and Repair Guide

The Anatomy of the 1541 Disk Drive

The first thing I noticed was the C21 tantalum capacitor had burned up. This cap is between the +12V supply and ground so was probably acting as a filter cap. The +12V supply still looked good with the cap burned out so I continued to work on the drive until I was able to get a replacement. 

The next step was to troubleshoot the power on self test problem. In a previous post I talked about the IF65 in-circuit emulator that I used at a previous job. Troubleshooting this sort of failure was a perfect application for this device. I started by connecting it to a known good drive to make sure everything worked ok. 

With the IF65 setup verified I re-connected it to the unit under test. I started by testing the ROM on the bad unit, and that seemed to be ok. Next I ran a RAM test and immediately saw a failure across the whole RAM address range. The errors were pretty random so it didn't look like just a bad data or address line. Next I turned to the IF65's address trap functionality and checked out the chip enable lines which looked good. The addresses lines also looked good, but I was getting very odd signals on the data bus. 

The RAM chip on my good drive was soldered in, so swapping was not an option, so I removed all the other chips on the bad unit that were connected to the data bus, but the RAM test still failed. I started to look through my surplus chips to find an equivalent RAM chip and quickly noticed the problem. The chip in the RAM socket was actually a ROM from another drive and not a RAM chip! Not sure if this happened when we were swapping chips at the repair day or happened during a previous repair attempt. I did find an equivalent RAM chip in my collection, put that one in the drive and it passed the power on self test. 

This was not the extent of the problems with this drive, I will cover more of the repair process in my next post.