[vcf-midatlantic] Another great workshop this weekend 1/8 & 1/9, 2022

[jsalzman at gmail.com]

I brought a portion of my TRS-80 Model I collection to figure out what
works and what doesn't. I had four Model I computers. Three were in
my collection for a while. One was a unit I got at the recent VCF or
Festivus (I forgot which...). I also brought two expansion boxes and an
official TRS-80 monitor.

One Model I had Level I BASIC and did NOT have any kind of keyboard
debouncing modifications done to it. That meant that my Hello World test
program looked like this:

100 PRRIINT  "HEEELLLO";;
20 GOTTO 10

...so that one I set aside for another day.

Another Model I had a strange raster display that looked like a detuned TV
signal. I eventually determined that the 5V source was "pulsing." It would
go from 0V to 5V, then drift down to 4V over two seconds, then drop to 0V,
repeating again after a few seconds.

...didn't have the patience for that one, so I set it aside for another
day, too.

A third Model I booted right up to Level II BASIC and typed just fine. At
least one of my TRS-80s was working. 🙂 So I hooked up one of the two
expansion interfaces I brought.... NOTHING. The Model I would still power
up fine, but I could not see the extra memory from the E/I. So... I hooked
up the other E/I.

Still no change... So I opened up the second E/I to troubleshoot.

After poking around for an actual service manual online, I found a
troubleshooting guide. I had trouble getting 5V in the E/I. All the parts
looked good. Nothing was blown. Just wasn't getting 5V. So after closer
examination of one of the 723 voltage regulator chips, I discovered that a
pin was broken off the chip. I pushed in the remains of the pin into its
socket and heard the click of the E/I's system relay. Well, since I did not
have a replacement 723 regulator, I just rigged in a fix for the pin and
now the E/I was working. When I plugged it back in with that Model I, I saw
the extra 32K added to the system.

YAY!!! Now I can test out the external floppy drive I brought with me. But
wait... I brought the wrong floppy drive data cable... 😠😠😠

So... I decided to work on the fourth Model I I brought with me.

I powered it up and it looks good, except for an odd pattern of dots on the
display. Everything seemed to work fine on it as a Level II BASIC system
that properly connects to the working E/I, but I couldn't get rid of the
dots on the display.

The dots didn't seem to affect computer operations. They just took over
their character spot on the display where other characters were expected to
be. I wrote this test program to check and see what happens as I fill in
video RAM with data:

10 PRINT "X";:GOTO 10

I get a screen full of "X" characters except for where the dots are. I
assumed there was something wrong with one or more of the 2104 Video RAM
chips, but the locations of the dots would indicate the problem exists on
several chips. I will have to read the technical details to try and match
actual circuit operation with the pattern of failure seen on the screen.
There could also be a timing or glitch issue in the video chain circuitry.
I did not have time to research all of that, so I set that unit aside for a
future date.

I mainly brought these systems to do triage of their unknown condition. At
least I had one full working Model I w/ E/I, albeit without knowledge of a
working floppy drive at the moment.

Now onto the PET stuff...

Recently, I acquired a CBM 2040 Dual Floppy drive and a CBM 2022 dot matrix
printer. I wanted to interface them with my working PET 4032. However, I
did not have the proper PET to IEEE interface cable. Although the PET
supports IEEE-488, it uses an edge card connector instead of the normal
IEEE-488 connector. I was hoping the hosting organization would have one I
could borrow in case the adapter I ordered from Retro Innovations did not
arrive in time (It didn't). Without that cable, I was unable to directly
test those peripherals.

I could not find a suitable cable from VCF and their museum. I was able to
test the printer in self-test mode. The ribbon was still in pretty good
shape, and the print was strong. I did have to clean and oil the carriage
rails. And in the future, I will need to replace the sound deadening foam
that's beginning to rot away in it.

I'll have to wait until I get the PET-to-IEEE adapter to test the disk
drives.

Meanwhile, I loaded up the SD2PET adapter on my 4032 and played around with
all sorts of PET programs I downloaded from the Internet. This was the most
time I have ever spent on my PET computer since I acquired it years ago. I
also tried to see if my SD2PET would work on the VCF Museum's PET 2001, but
the status light on the SD2PET keeps flashing in an amber color, and not a
steady green when it's on my 4032. I don't know what that means, but I will
figure it out. The SD2PET won't run on an original stock 2001, but is
supposed to run on a 2001 that has either the BASIC 2.0 or BASIC 4.0
upgrade. The Museum 2001 has BASIC 4.0, so there is probably another issue.
Maybe the expansion port contacts need a good cleaning. I did not want to
do that during museum operating hours on the last few hours of the workshop
weekend. I'm not a glutton for punishment over engaging in 11th hour repair
projects. 🙂



On Sun, Jan 9, 2022 at 11:55 PM Jeffrey Brace via vcf-midatlantic <
vcf-midatlantic at lists.vcfed.org> wrote:

> This past weekend there was a smaller turnout due to the cold and snowy
> weather, but we still got a lot of stuff done and enjoyed ourselves in a
> safe way.
> Jeff Salzman worked on TRS-80 and Commodore PET 4032 stuff.
> Ian worked on TVs and a Oscilloscope.
> Connor worked on a lot of various things
> CJ worked on TVs and other items.
> Ryan B worked don a Macintosh SE
> Gregg worked on an emulator.
>
> Perhaps they can elaborate on what they did.
>
> I took care of some maintenance stuff in the musem.
> Not sure what others were working on.
>
> So the next repair workshop is @ System Source Jan 29 & 30. Next e-mail
> will give details.
>
> =========================================
> Jeff Brace
> VCF National Board Member Chairman & Vice President
> Vintage Computer Festival East Show-runner
> VCF Mid-Atlantic Executive Director
> Vintage Computer Federation is a 501c3 charity
> http://www.vcfed.org/
> jeffrey at vcfed.org
>