Compact Floppy Drive B problem

[X-Col]

After completing my Ace HD project, I thought I'd have a look at my Compact floppy drive problem.

So here is what I found:

Drive A has always worked fine, drive B does not read disks and spins irregularly.
I swapped the drives round and amended the jumpers accordingly, now the drive that didn't work as drive B works fine as drive A, and the drive that was working as drive A exhibits the same symptoms when used as drive B.

Hope that makes sense! 

Anyway, it proves that both drives are fine. Is there a problem with the motherboard?
Please give me some guidance, as I'm a little worried there is a more serious problem than just a bad floppy drive....

Thanks all!

[Dal]

All the logic signals are the same. The machine simply changes the state of the drive select line to address either drive.

Here's a to do list in order of most-to-least likely based on your findings:
Swap/replace the ribbon cable.
Check/swap/replace the power leads.
Use scope or meter to monitor the 12v and 5v lines while the drive is running. If there is evidence of voltage sagging, then you should replace the electrolytic caps.

[X-Col]

Voltages are ok, motherboard already recapped.

Here's a strange thing, both drives work fine seperated, but drive B (1) does not work (regardless of which floppy drive is used) when they are attached together.

Strange...

[Dal]

Did you replace the ribbon cable yet?

Also, you could use a scope or logic analyser on the drive select line to make sure it's behaving as it should.  Lo for Drive 0 and Hi for Drive 1 I believe. 

Are you absolutely sure the jumper settings on those drives are correct?  I only have an Ace so I can't confirm what they should be.

[X-Col]

Couldn't change the ribbon as these things are not the standard type, also could not swap them over as they're different lengths.

I have an Ace and they are a LOT more service friendly than the Compact models!

Anyway, I recapped both floppies. Put them back together and still the same. Swapped the floppies over and now they both seem to be working! Yeh!   

Thanks for your suggestions mate, P.S. Stick with the Ace, these Compacts are a nightmare....

[Dal]

How bizarre! Glad it's working - these old machines are a labour of love.