Another Crazy Idea: disc drive controller swapping

[Tony Duell]

[Sorry for the delay in replyinjg, I think my ISP has been having mail 
problems.]

> >> Let's assume for a minute that the heads in the EME232 drives are
> >> undamaged. That means all the issues are related to defective circuitry
> >> on the PCB. So, how about this for an idea: move the controller chip and
> >> surrounding circuitry (or even the whole board) to the EME232, then wire
> >> it into the existing drive mechanics.
> >
> > Where are you getting this ASIC (or board) from? If it's another simialr
> > drive, then no problem
> 
> I was thinking a 3.5in drive (with the DENSITY switch hardwired to 720k) 
> or a 5.25in 1.2MB drive similarly jumpered.

Right. I might be inclined to try to build the whole thing from scratch 
based on the schematics of a 5.25" or 8" drive using SSI/MSI chips. That 
would be quite a fun project, I just might give it a go sometime...

As an aside, the drive to avoid for things like this is the origianl SOny 
full-ehight 3.5" _single head_ one. It has a head that produces a very 
small signal and a step-up transformer (!) on the board before the read 
amplifier. No other floppy drive (AFAIK) did it this way. The similar 
double-head SOny drives use much more conventional circuitry.

> 
> Reverse engineer the circuitry (i.e. disassemble and trace out on 
> paper), then etch a new PCB to fit the Amstrad drive shell.

You do realise that schematics or serive manuals exist for some drives. 
Teac ones, in particular, if you want to use a floppy drive ASIC as part 
of the design.

My experience is that the areas where different floppy drives most differ 
is the the motor circuitry, both the spindle motor and the stepper. 
Steppes can be common + 3 wires to senquence (3 phase), ditto but with 4 
wires, or 2 independant coils (which you have to drive from full-H 
drivers). Spindle motors are likely to be brushless things now, but just 
how the control circuity  is arranged depends a lot on the drive. The 
easiest drives to mess about with had a motor that just needs power (12V 
or 5V) and has a logic-level (or maybe open-collector-driven) 'run' 
input. Older drives may have brush-type DC motors (speed control may be 
intenral, on a separate PCB, or on the main logic PCB), or even AC 
motors (8" drives). 

> > One other though, which I mentionedsoem weeks ago. The periperhal
> > componens to the read amplifier seem to be those that I would expect for
> > an MC3740 IC. I wonder if yo could use that IC and a little external
> > circuitry to replace the defective read amplifier secion (and the rest of
> > the ASCI seems fine).
> 
> I take it you hammed the part number, and that you actually meant the 
> MC3470?

I did...

> 
> That would probably do it -- the catch, of course, is that the MC3470 
> seems to be obsolete, and as rare as hen's teeth. Again, unless someone 
> in the community has squirreled away a rail of them.

This is the first time I've heard the Apple Disk II being describved as 
'rare as hen's teeth' :-). YEs, there's one inside...

Since the Amstrad ASIC is working apart from the read chain (or is it the 
head switching cirucity?), I wonder if you could keep that for things 
like head positioning and kludge on the nevessary bits to fix the broken 
part.

-tony