For troubleshooting possible RAM problems, it's really easy simply to hook a
64Kx8 SRAM on top of the DRAMs, grabbing the addresses before the decoder and
multiplexers, and use OR'd nCAS to the DRAMs as the output enable to the SRAM.
You simply disable the data from the DRAMs by pulling the pins out of the
sockets. The result is that your system works using the SRAM, yet you can poke
around all you like in the DRAM array and the associated timing logic.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, November 21, 2001 1:13 PM
Subject: Re: 1771 floppy controller questions
> >
> > On Wed, 21 Nov 2001, Tony Duell wrote:
> >
> > > > makes sense that they changed that part of the design. I own both a
model
> > > > 3 and 4, neither of which currently work. Any ideas on where a service
> > > > manual can be found for either of these? I also have a model 2 that
worked
> > >
> > > I have at least schematics for the M3 and non-gate-array M4. What's the
> > > problem with your machines? What do they do at switch-on?
> >
> > It has been a number of years since I powered them up. Right now I think
> > they are both sitting disassembled in a couple of large boxes. I'm in the
>
> When yopu have time to get them out again, let us know (or let me know),
> and we can discuss troubleshooting them.
>
> > process of construction of a building just for my old hardware, so once
> > that is done, I'll pull both of them out and bench them and see what their
> > problems were. I think the model 3 would start up, but just plain lock up
> > about the time it was going to boot. I think it may be a ram problem. The
>
> Mybe RAM. Most disk problems on the M3 and M4 are either the 1793 chip
> (I've had to replace a couple) or more likely that infernal ribbon cable
> between the CPU board and the disk controller. If you don't care about
> your machine being unoriginal, you can use 40 way IDC cable and
> connectors to replace it (just use 1 row of pins in the connector to
> replace the 20 pin SIL socket on the CPU board or disk controller).
> You're only using 20 wires in the cable, which is something of a waste,
> but it's the easiest fix I've found [1].
>
> > model 4 didn't power up at all if I remember correctly.
>
> In which case check the outputs of the power supplies -- both of them --
> before doing anything else.
>
> [1] For the HP97 owners on this list, 26 way IDC cable/connectors and pin
> headers can be made to replace the ribbon cable between the logic board
> and the card reader. It cures a lot of card reader problems :-)
>
> -tony
>
>
Received on Wed Nov 21 2001 - 14:46:34 GMT