Re: 128DCR internal drive

From: Marko Mäkelä (
Date: 2001-11-19 08:38:02


As I don't have any 128DCR (only a 128D and a flat 128), I cannot answer
these questions.  I'll pass them on to the cbm-hackers mailing list.


On Sat, 17 Nov 2001, Raymond R. Carlsen wrote:

> Marko,
>      I have two repair jobs on the bench... both 128DCR motherboards (PCB
> 250477) with the same problem: no boot in any mode and the drive LED stays
> on. Tracing the fault was easy. Both boards had the reset line held low
> because of a logic low out of U108 pin 32, (the drive R/W amplifer chip)
> connected to the reset one-shot U27 pin 8 (input trigger). Disconnecting
> that line allows both computers to boot in all modes. The internal drive
> reads (no write) with one motherboard and will not even read with the
> other. Of course external drives work fine with the internal disabled.
>      My question is: what is that line between the chips for? It is not
> even shown in my Sams Photofact schematic, but it is on the schematic on
> My guess is it's there to prevent the chip from going into write
> mode during boot time. I opened that line in my own working 128DCR and
> it made no difference... read, autoboot, format all work fine. In the bad
> boards, the DC resistance of U108 pin 32 measures about 100K to ground, so
> the chip must be generating the logic low all the time... but why? Can I
> have two bad SMD chips here (what are the odds?) or is something else
> causing it? Take your best shot... I'm all ears. Thanks in advance!
> If I do have two bad chips, is there any hope of finding a MC2871A
> anywhere? 
> Ray
> PS  Both boards belong to some guy in England, and he shipped them all the
> way to me in the USA for repair.

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