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   Re: (Ultra Slow Speed?) Re: 5 1/4 hd disks usuable?
   Author:   Nicolas Welte <welte@chemie.uni-konstanz.de>
   Date:   1999/03/20
   Forum:   comp.sys.cbm
   
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John Iannetta wrote:
>
>     John/Lori said,
>
> "Wouldn't it be simpler to try to bump up the write current?
> perhaps there's a spare bit in one of the CIAs that could be used for
> switching the write current for different disk media?".
>
>     Good idea!  Here's something that might work.

[circuit diagram removed]

>     The circuit is for the "short board" 1541; UC1 is the PLA, and UC3 is
> VIA1.  CA2 (pin 39) of UC3 is not used by the 1541 DOS, and is here used to
> control "HD" mode (High Density, active low).  That pin is controlled by
> register $0C of VIA1, which DOS writes to only on reset (hardware or software
).

A similar mod can be made for the 1541-II and the 1571cr (128DCR
internal drive). Both drives use a 44pin SMD R/W amplifier which has a
special 'increase write current' input pin. It is normally connected to
the +5V supply, write current can be increased by setting this pin to
LOW level. This is pin 27 on the chip, just disconnect it from the board
(or cut the trace) and solder a wire to above mentioned VIA1, pin 39. It
has a different component location, just search for the bus controller
VIA.

Of course this is untested, but it might work.

Nicolas
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