Olivetti M24 onboard FDC fries connected floppy drives

[vintage_cassy]

Hello,

I'm new here so for the first I bought an Olivetti M24 computer (my first computer that days) on eBay. At the first glance, the computer was in Ok shape. No massive corrosion due the battery leakage. But the floppy drive was in a bad condition. One the capacitors was missing/ripped of/exploded and maybe caused a short circuit, so a replaced all capacitors with newer ones but it didn't worked either. So I bought an other drive on eBay and it worked for some time. But than some TTL on the graphics card gave up (the 74S74 for H-Sync) and I replaced it. And the floppy stopped working as well.

But now every floppy drive connected to the onboard controller will be fried (I tested two known working 3.5" drives which went bad afterwards). I took a look on the schematics and saw two TTL 7436 as driver chips for the IO lines and replaced them. But the next drive went bad as well and I suspect the z765 FDC as a cause.

My next attempt is to unsolder the z765 and replace it with an other one. Or does someone has some suggestions? Before unsoldering I want to connect a logic analyzer to the FDC and check the signals for something suspicious... Or does someone has some suggestions to look at another direction?

Yours Alex

 

[lucasM24]

Ciao Alex!

The Olivetti M24 was also my first introduction to computing...

This is not knowledge, just memories from deep memory banks without source, so don't consider this necessarily true:

»If the BIOS of the M24 is too old, it not only doesn't work with 3,5" floppy drives, it causes damage.«​

I know it sounds strange, maybe it's nonsense. Afaik, BIOS Revision 1.43 supports 3,5" floppy drives. Which versions do you have?

Lucas.

 

[Chuck(G)]

I doubt that it's the 765. Take a look at the schematic on sheet 13, which is the FDC section of the M24. Note that all FDC signals pass through other ICs.

If this were my unit, I'd check the +5 and +12 levels at the floppies.