I've been doing a lot of disk-related activity lately (transferring disk images via ADTPro, formatting and testing disks) which has worked fine until recently.
The first thing I noticed was that I got a strange disk-seek sound (a "wiggle" of sorts I can describe it like that), and several disks (transferred from my Mac via ADTPro) gave that behaviour and couldn't proceed loading the content. So I disconnected that drive (I have two 5.25" drives), but now I'm getting it on the other drive as well with some disks. At the moment I only have one drive connected to the drive card.
While some disks work and others don't (disk images recently transferred to the II+) and those problematic disks checking out fine when checking the disk surface (I think I used Copy ][+ for this, or some other copier) I find this behaviour erratic and strange.
Oh, I've also checked the disk images that I've had problems with on my Mac, using the Virtual ][ emulator. They work fine there.
I know capacitors go bad after a while, so the first suspect is the one and only electrolytic capacitor on my Disk ][ controller card. It's marked C2 on the PCB and it's value is 22uF/16V. It'son the left hand side of the PCB as you can see in this photo I found in the forum's media browser archive (don't mind the switch -I haven't figured out how to insert photos here from the web) :
My C2 capacitor reads around 56 uF with my digital multimeter's capacitance mode which obviously doesn't sound right (I removed one pin off the PCB to get a correct reading). Could this be the reason for the strange behaviour?
Unfortunately I don't have a correct replacement here (and ordering online takes forever) so I wonder how critical this part is? I do for instance have a new 100 uF/25V capacitor -is it suitable for this use? I probably have others as well, but I'm aiming for new, unused capacitors I may have.
By the way, I replaced the two power transistors (Q2, Q3) a few days ago as they had previously had their legs bent a few times too many and (rather messy) resoldered as a quick fix, so new ones were in order. That probably rules out them as the problem source as well.