TRS-80 Model 1 Level 1 4k

Seek advice, give advice or tell others about your repair and restoration projects

TRS-80 Model 1 Level 1 4k

Postby tezza on Sun Feb 22, 2015 7:40 am

In 1977 three consumer-friendly microcomputers were released...the Commodore Pet 2001, the Apple II and the TRS-80 Model 1 (Level 1 BASIC, 4k RAM). Up until now I never had any of these three "original" models but rather variants that came a year or two later. Until now.

I've been given a 4k L1 TRS-80 Model 1. It comes with the original screen and tape deck and it's in good cosmetic condition. These are quite rare as most of these original models were upgraded to L2, 16k. The computer doesn't function though and I'm working my way through what's wrong.

The problem seems to be with power. The power pack is ok, but so far I've determined the 12 v line is shutting itself down. That in turn shuts down the 5v line. The -5 volt line seems ok.

I conclude that either one of the power transistors isn't working or there is a short on the board, Hopefully not the latter as these can be hard to find. Anyway, here is a pic of the fine looking circuit board. This repair project will progress in fits and starts as there is a lot going on in my life at the moment. I'll post updates though.

I'm expecting there to be other problems lurking once I get the power going. There always is.. :)

Image
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2293
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: TRS-80 Model 1 Level 1 4k

Postby Audronic on Sun Feb 22, 2015 12:45 pm

Hi Tezza


A Quick and dirty way is to PROGRESSIVELY remove all the Socketed chips whilst monitoring the power lines.
Please take photos before and after removal of chips so you can put them back correctly.
Probably start with the 1) Ram, 2) Video Ram 3) -- etc including the wet fingertip test (temperature).

A Good pickup.

Best of luck

Ray W_____________ (across the Ditch)
Audronic
 
Posts: 70
Joined: Mon Sep 30, 2013 11:08 pm
Location: Williamstown Victoria 3016 Australia

Re: TRS-80 Model 1 Level 1 4k

Postby tezza on Thu Feb 26, 2015 10:44 am

Just a progress report. Andrew Quinn (TRS80)'s helped me with this and the chief suspect is a power regulator IC. Luckily JayCar has a replacement. I've cut the old one out, inserted a socket and now just need to get the part on the weekend and plug it in.

Here's hoping!
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2293
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: TRS-80 Model 1 Level 1 4k

Postby SpidersWeb on Thu Feb 26, 2015 6:07 pm

Nice work :) Looking forward to seeing what happens.
Wanted - Dead or Alive - Reward $$$: Compaq Deskpro 8088 / 286 / 386 - IBM RT 6150/6151 parts - AT&T 3B2 parts
VC Twitter
SpidersWeb
 
Posts: 1130
Joined: Tue Jan 24, 2012 8:38 am
Location: Wellington

Re: TRS-80 Model 1 Level 1 4k

Postby tezza on Sat Feb 28, 2015 10:03 am

Well, that worked. It WAS that Power regulator IC (thanks Andrew). With a replacement I now have all voltages as they should be (and the power light now goes on.).

So, on to the next challenge. No Video. The voltages on some of the pins on the two video transistors looked suspect but swapping these out didn't do anything. Resistors are all within tolerance. So, working backwards I'm now at Z41, The signal output on pin 5 seems to show the right voltage BUT a check with a scope shows no waveform at all compared to my reference model 1. So this chip is now suspect. Input pin 6 shows a correct signal. I've haven't checked the patterns on the other input pins yet just to make sure the problem doesn't go even further back. That will be a job for another day.

I suspect I'll need to import this IC if I want to replace it. :(

Image
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2293
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: TRS-80 Model 1 Level 1 4k

Postby Audronic on Sat Feb 28, 2015 4:50 pm

Hi Tezza

Have a look at Z5 (74C00) with the CRO and see if it's got some pulses on it ??
Audronic
 
Posts: 70
Joined: Mon Sep 30, 2013 11:08 pm
Location: Williamstown Victoria 3016 Australia

Re: TRS-80 Model 1 Level 1 4k

Postby tezza on Sun Mar 01, 2015 8:57 pm

Yes, I will do more tests later this week. Thanks for the advice.
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2293
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: TRS-80 Model 1 Level 1 4k

Postby tezza on Sat Mar 07, 2015 9:27 pm

Ok, I managed to grab some time today and repair this thing.

Actually it wasn't Z41. That did have a signal after all, although it was hard to see with the scope.

No, Audronic was onto it. On some more reading I got very suspicious of Z5 (a 74C00) as it feeds the base of a transistor (Q2) in the video circuit. This transistor wasn't switching on because the voltage coming from this chip to the base was too high. When I checked the voltages on Z5, input was ok but output on one of the gates was screwy. Replacement with a spare 74C00 fixed that problem and lo and behold, video appeared!

So in summary there were two issues with this board, a power regulator IC and a 74C00 Quad 2-Input NAND Gate on video sync circuit. The arrows show the offending pieces of silicon.

Image

Reassembled and running, it looks like this...

Image

Image

Yes, it definitely is Level 1 BASIC. Given L1 machines are not common (most were upgraded) I'm pleased to have it. It also came with a genuine TRS-80 screen although not the one shown here. The one this Level 1 unit came with is, ironically, more modern (and modern looking) than the one pictured. The former's in good condition though and works, so I'll use that screen with my Level II machine and this one (which is the original design) with my Level 1 unit.
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2293
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: TRS-80 Model 1 Level 1 4k

Postby tezza on Sun Mar 15, 2015 7:36 pm

The full writeup for those that might be interested.
http://www.classic-computers.org.nz/blo ... L1-fix.htm
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2293
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand


Return to Repair and Restoration

Who is online

Users browsing this forum: No registered users and 3 guests

cron