CBM 1541 drive faults

Jules Richardson jules.richardson99 at gmail.com
Sun Dec 13 09:48:19 CST 2015

On 12/12/2015 08:27 PM, Mike van Bokhoven wrote:
> Locked on LEDs and spindle motor would suggest VIA to me, though it's been
> many years since I've looked at one. First thing I'd check is that there's
> activity on the address/data buses.

I don't have a lot of test gear, but I suppose poking at A0 with my DMM on 
frequency setting should give a sign of whether anything's going on...

> If things are going on, and there are 6522s at hand, I'd try a quick swap.

I can certainly borrow ones from one of the good drives - I suppose given 
that the PSU rails are OK, it's unlikely that there's anything in the 
faulty drives that will damage them.

>> I read somewhere that ROM faults aren't unheard of, ditto with 6522 VIA
>> failures. Also the 74ls14 at UA1 (particularly if someone's unplugged the
>> drive from the host with power on), but I think that affects CPU reset,
>> which appears to be working in my units.
> Can you swap ROMs in from one of the booting drives? Worth a go, though
> I've never seen a ROM failure.

Can do, I just wasn't sure if the ROMs are tied to a particular board 
revision (so I need rev A ROMs for a rev A board etc.). Looking at my Rev A 
and Rev C boards though it would seem that they have the same part numbers, 
and I assume they'd be different if the contents were.

>> Also, has there been a worse external drive in the history of floppy
>> storage? Slow, complicated, expensive and unreliable seems to have most
>> bases covered ;-)
> I sort of agree, though I have to say that of my pile of about 30 1541s and
> 1541 IIs, all but two ran up fine on first test, after having been in
> storage for 25 years. One had a bad 6522, and the other had an alignment
> issue. They were all still amazingly slow though!

:-)  Gotta love 'em, just for how quirky they are - I really don't know 
what Commodore were thinking when they designed them (even if they'd worked 
as intended with a higher transfer rate).

I do have a drive unit for a Research Machines system designed along 
similar lines (on-board CPU and serial connection to host), but at least 
that is a dual-drive unit and uses an off-the-shelf FDC.



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