Author Topic: EEVblog #645 - TRS-80 Model I Retro Computer Teardown  (Read 22261 times)

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Online EEVblog

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EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« on: July 29, 2014, 02:39:40 am »
Dave looks inside the most popular microcomputer of the 1970's, the Radio Shack / Tandy TRS-80 Model I
And also a look at the TRS-80 Model 102.
TMS4116 16Kb DRAM:
http://www.datasheetarchive.com/dlmain/Datasheets-37/DSA-731104.pdf

Level II BASIC Reference Manual
http://www.1000bit.it/support/manuali/trs/Level%20II%20BASIC%20Reference%20Manual%20%281979%29%28Radio%20Shack%29.pdf
Service manuals and Schematics: http://www.eevblog.com/files/TRS80model1/

« Last Edit: July 29, 2014, 12:10:04 pm by EEVblog »
 

Offline j3gum

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #1 on: July 29, 2014, 04:05:15 am »
Dave,
Thanks for the TRS-80 tear down!  I really loved watching it.

You may want to pull off one of the keys from the keyboard.  These switches were springs, and the hardware didn't have any debounce.  There also wasn't any in the ROMs.  Later there was additional software you could add that would do software debounce.  However, I followed advice to use WD-40 on a sheet of paper and insert it between the springs in the key and depress and pull the paper through.  This deposited enough goo on the springs to tame the bounce.  And it did work, believe it or not.

Also, I would love to see the RF output if you put some work on that Z80.  I bet it'd be significantly more noisy.  A simple loop should be enough:
10 goto 10

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Offline GeoffS

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #2 on: July 29, 2014, 04:10:34 am »
That takes me back!

I started working for Tandy when the Model I was introduced in Australia in 1978. They were looking for someone with an electronics background who could program in BASIC. I'd never heard of BASIC or had any computer programming background so I grabbed a couple of books on BASIC and read them several times before the interview. Luckily the interviewer knew less BASIC than me so I got the job.

I spent the next 7 years working on every subsequent model they introduced. Even got sent to head office for training - 2 weeks in Fort Worth, Texas in winter is quite memorable for all the wrong reasons  ;)
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #3 on: July 29, 2014, 04:11:18 am »
You may want to pull off one of the keys from the keyboard.  These switches were springs, and the hardware didn't have any debounce.  There also wasn't any in the ROMs.

Yes, I forgot to mention that, it was a major issues.
I didn't get an key bounce at all on this, but the owner thinks it's had the debounce mod done. Doesn't appear to be any hardware mod, so maybe in the ROM?


Quote
Also, I would love to see the RF output if you put some work on that Z80.  I bet it'd be significantly more noisy.  A simple loop should be enough:

I showed that, with and without a program running
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #4 on: July 29, 2014, 04:15:14 am »
I started working for Tandy when the Model I was introduced in Australia in 1978.

I did work experience in high school at the Tandy factory working on the Models III' and others. It was a dream, the Tandy factory was walking/bike distance from home.
Also, I used to drop by the factory showroom all the time, as they had a corner of the store where they would have a huge junk bin of stuff, products they were evaluating and didn't sell etc.
They were called Intertan in Australia.
 

Offline Lightages

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #5 on: July 29, 2014, 04:30:14 am »
I remember a friend of mine had a Model 1 and he wanted to to upgrade to 48K but not pay the silly prices Radio Shack wanted. So he decided to buy sets of 16K DRAM chips. He then piggybacked them and lifted the output enable pin and ran that separately to some logic. He inspected his work, deemed it perfect and powered it on. Nothing.

After asking me to look I discovered he had counted wrong and had lifted a different pin. All the chips cam on at the same time and fried each other. They cost, at that time, around $700 total. He looked at me, made me promise to not tell his wife and he went back next week and blew another $700 on another set. That time he did not make the same mistake.
 

Offline j3gum

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #6 on: July 29, 2014, 04:50:31 am »
Quote
I showed that, with and without a program running

Awesome!  That'll give me an excuse to re-watch it!  Thanks again.
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #7 on: July 29, 2014, 04:52:23 am »
I noticed the spliced in "Heavy" when talking about the MDO-3K.  Didn't want to burn too many bridges with TEK?
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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #8 on: July 29, 2014, 05:02:15 am »
I noticed the spliced in "Heavy" when talking about the MDO-3K.

Huh? What does that mean? :-//
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #9 on: July 29, 2014, 05:04:33 am »
I noticed the spliced in "Heavy" when talking about the MDO-3K.

Huh? What does that mean? :-//

When you were talking about how slow the controls were the audio jumped so it sounded like you started saying something along the lines of it being slow during processing, but then changed it to say slow during heavy processing.
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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #10 on: July 29, 2014, 05:07:42 am »
When you were talking about how slow the controls were the audio jumped so it sounded like you started saying something along the lines of it being slow during processing, but then changed it to say slow during heavy processing.

I don't know, just one of my usual edits. Maybe I misspoke and did another take, that happens. I don't recall exactly. Nothing implied or hidden.
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #11 on: July 29, 2014, 05:14:37 am »
Didn't want to burn too many bridges with TEK?

If I was avoiding "burning bridges" with Tek, I:
a) Wouldn't have mentioned it at all
b) Wouldn't have got into a public twitter debate about it with Tek deliberately copied in
c) Wouldn't have ranted about it again on today's Amp Hour

If there is one person who doesn't give a toss about burning bridges, it's me.
 

Offline GeoffS

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #12 on: July 29, 2014, 05:19:52 am »
I did work experience in high school at the Tandy factory working on the Models III' and others. It was a dream, the Tandy factory was walking/bike distance from home.
Also, I used to drop by the factory showroom all the time, as they had a corner of the store where they would have a huge junk bin of stuff, products they were evaluating and didn't sell etc.
They were called Intertan in Australia.

At Mt Druitt? The Intertan name started in 1986. prior to that they were just part of Tandy Corp.
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #13 on: July 29, 2014, 05:28:42 am »
At Mt Druitt? The Intertan name started in 1986. prior to that they were just part of Tandy Corp.

Yep. I think it was 1987 I was there, in the service department.
Might of even still had Tandy on the building, I can't recall.
Tandy was always the public brand, but the company behind it was Intertan.
Oz Tandy products had "Manufactured in XXXX for Intertan Australia Ltd" on the back sticker.
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #14 on: July 29, 2014, 06:02:32 am »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?
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Offline free_electron

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #15 on: July 29, 2014, 06:04:28 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?
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Offline GeoffS

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #16 on: July 29, 2014, 06:22:49 am »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?

Black and white video,  no colour information so no PAL (or NTSC).
 

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #17 on: July 29, 2014, 06:30:01 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?

I'm letting the electrons settle to the top of the case, it helps rejuvenate the performance. Poor thing spends most of it's life the right way up and gravity isn't kind.
 

Offline Stonent

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #18 on: July 29, 2014, 07:13:09 am »
I'm curious about the video interface.  Since this was an early model, assembled in Fort Worth, it just seems odd that they have a PAL version that they made in the US for overseas use. 

Or maybe the video type doesn't really matter and it just scales itself to the 60 or 50 Hz input?

Black and white video,  no colour information so no PAL (or NTSC).

I didn't think about that.  So for just black and white video there is no difference between US and Europe besides the 50/60 thing?
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Offline free_electron

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #19 on: July 29, 2014, 07:18:20 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?

I'm letting the electrons settle to the top of the case, it helps rejuvenate the performance. Poor thing spends most of it's life the right way up and gravity isn't kind.

i thought you were letting the liquid crystal flow back to the top of the screen...
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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #20 on: July 29, 2014, 07:26:25 am »
i thought you were letting the liquid crystal flow back to the top of the screen...

That would be silly!  :-DD
 

Offline Richard Crowley

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #21 on: July 29, 2014, 07:28:16 am »
what's with the upsidedown TDS 4 channes tekscope on the background ?

I'm letting the electrons settle to the top of the case, it helps rejuvenate the performance. Poor thing spends most of it's life the right way up and gravity isn't kind.
Are globes down there in the antipodes made with the South Pole at the top?  For a better view of the planet south of the equator?
 

Online German_EE

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #22 on: July 29, 2014, 07:31:52 am »
Dave, a big THANK YOU for this video. I had a TRS-80 in 1978 and I typed my thesis on it so I have happy memories of that wonderful keyboard. Some extra points:

1) To save costs the keyboard was memory mapped and the ROMs scanned the keyboard area on a regular basis looking for key presses.

2) There was a PAL version of the Model 1 but I can't remember much of the details apart from Tandy in the UK selling a UHF modulator so that we could use a 625 line TV.

3) At some time Tandy introduced a lower case mod because, as I said, I typed my thesis on it using the Scripsit word processor package. After it was finished I took the cassette tape down to the local Tandy showroom where they printed it on a daisywheel printer for me.

4) That warranty sticker was a serious issue because if it was broken the local Tandy repair center doubled your bill. I don't think they liked people to tinker.
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Offline ggchab

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #23 on: July 29, 2014, 07:37:29 am »
The characters generator could produce upper and lower case. But the video memory had only 7 bit, the missing one (bit 7, I think) was "rebuilt" with a logical gate. It was possible to obtain the lower cases by simply adding a 1 Kbit chip, cutting some traces and adding 2 wires  ;D
 

Offline ggchab

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Re: EEVblog #645 - TRS-80 Model I Retro Computer Teardown
« Reply #24 on: July 29, 2014, 07:49:35 am »
The "Memory size" question when booting up is to define a part of the upper memory that will not be used by the basic. This memory could then be used to store a kind of "Terminate and Stay Resident" (TSR) program (a bit like MS-DOS !!). One example is a small debouncing program for the keyboard.
 


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