Seagate MFM Hard Drive Repair Notes
The following is based on a page by Steve Vagts, Editor or "Z-100 LifeLine" retrieved from earthlink.net.
Seagate Drive Spinup Problems
Hard disk spin-up problems are commonly attributed to Stiction, but an article in one of the computer magazines serving Heath/Zenith computer users suggested that adding a 47uF capacitor to the +5V line of drive could resolve some issues, due to poor regulation in the H/Z-100 power supply.
By experimentation, it has been reported that ST-251s and ST-138s with spin-up issues were repaired by replacing 22uF surface mounted capacitor on the +5V line with 47uF electrolytic capacitors. Of the drives that weren't repaired by this modification, some were so stuck that turning the platters by hand broke heads off the arm assemblies!
Capacitor Replacement
Use a 10V or 16V, 47uF electrolytic capacitor (with the two leads at the same end).
While there have been numerous controller boards on ST-251 and ST-138 drives, the capacitors are easy to locate as they are always located next to the power supply connector. Trace the nearest 22uF capacitor to the 5V supply pin using a multimeter.
Remove the surface mount capacitor, and replace with the 47uF electrolytic, noting polarity (the positive pin must be connected to the pad connected directly to the power connector 5V pin). The capacitor should be mounted so it ends up on its side against the board.
12V Capacitor Replacement
It may also be worth replacing the 12V capacitors, which have been reported to improve data transfer reliability. Use a small 25V, 47uF capacitor no bigger than 1/4" (6.35mm) in diameter.