19 Nov
2001
19 Nov
'01
7:46 p.m.
On Tue, 20 Nov 2001, Richard Erlacher wrote:
From: "Tothwolf"
<tothwolf(a)concentric.net>
I figured the model 3 used the same controller as the model 1, but it
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
the last time I pulled it out, tho I'd like to find a manual for it too,
since I imagine I will have to service it at some point in the future.
On Mon, 19 Nov 2001, Richard Erlacher wrote:
The model 3 has a 1793, which is very similar, but not identical.
The 1771 has an inverting data bus, and the MFM-capable version with
an inverting bus is the 1791. I do believe I saw a message that
indicated that BG Micro has the WD1771 for $4.95. I doubt there's a
version that wouldn't work in the Model 1. You can get one for cheap if you go down to the
thrift store and buy an old
TRS-80. That will cost less than a new WD1771, I'd bet.
That conflicts with my ideals and goals on collecting/repairing these
machines. The expansion interface for the model I also seems to be
somewhat hard to find, as I haven't seen one in a thrift store in quite
some time. The model 3 and 4 have a different 1771, which I seem to
remember is not an exact replacement. You're right, in that the model 1 expansion box is
not a really common
item. I've not seen one in quite a while. They've probably gone to
the great beyond by now.
Kinda sad too, I used to see this kind of gear in the resale shops all the
time. Over the last 4-5 years, very little of it seems to be showing up in
the places I used to see it in.
The 1771 was quite popular, and had several
second-sources, so perhaps
the NS part is worth a try. In the Model 1's single-density
application, almost any 1771 should work fine, though working fine
doesn't really mean working reliably.
I've heard a few stories about reliability problems with the expansion
interface's floppy controller. I sure hope not all of them were true ;P
What I'd suggest is that you synchronize the data
from the drive with
the 1771 by interposing a pair of 74HCT74's between the FD data in
(after the schmidt trigger) and the 1771, clocked with the fastest
harmonic of the FD clock that you can find on board. That forms a
dual-rank register and synchronizes the data in a way that has a good
chance of improving the system performance. You might confer with Tony
Duell, as he's apparently persuaded the M1 FDC to work fairly well.
An upgraded data separator was quite a popular enhancement.
The original owner of this model I did add an external data separator
board between the expansion interface board and the 1771. Would that board
have provided a similar circuit?
-Toth