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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 4th Aug 2021, 4:39 pm   #1
JohnJones
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Default TRS-80 Model 1 case screws

Hello All

I've just got myself a TRS-80 Model 1 which needs some fixing up.

One thing it's completely lacking is the six case screws. Does anyone have the specifications for these – length, width, tpi/pitch – so I can get some replacements?

Many thanks

John
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Old 4th Aug 2021, 5:38 pm   #2
barrymagrec
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Default Re: TRS-80 Model 1 case screws

Being an American designed piece of equipment the screws are likely to be Unified sizes - should be easy enough to work out the actual size by looking at the case threads and checking against a thread chart.

https://journeymans-workshop.uk/unified.php
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Old 5th Aug 2021, 4:47 am   #3
TonyDuell
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Default Re: TRS-80 Model 1 case screws

According to the exploded diagram in the technical manual, there are 2 each of 3 types of screws.

At the rear : 6-32 UNC, 2" long (oddly, it also says 5cm for the latter dimension)

In the middle : 6-32 UNC 1.75" long (4.5cm)

At the front 'Thread Forming' screws 1" long (2.5cm)

From memory they have a black oxide finish and Phillips heads.

Do you have the rubber sleeve spacers that go between the CPU PCB and keyboard?
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Old 5th Aug 2021, 8:10 am   #4
JohnJones
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Default Re: TRS-80 Model 1 case screws

Thanks very much – that's exactly what I needed. I'll also need to seek out the technical manual, obviously.

Indeed, all the spacers are missing too, but I found 3-D printable replacements on Thingyverse so they are done.
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Old 5th Aug 2021, 4:48 pm   #5
TonyDuell
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Default Re: TRS-80 Model 1 case screws

I have the original technical manual on paper on my bookshelf, but I am pretty sure you can download it from some site or other.

The original spacers were bits of rubber tubing. The idea was they held the CPU board down to the lower case but also pushed the keyboard up through the bezel. Rigid spacers (which I assume 3D printing will produce) will not work as well. You're better off finding some tubing and cutting it to length.

The cable between the 2 boards is quite fragile. And there's at least one wire such that if it open-circuits the machine will not start up. Don't ask how I found that out.

Do you have the power supply brick and the monitor?
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Old 5th Aug 2021, 8:25 pm   #6
Slothie
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Default Re: TRS-80 Model 1 case screws

Here's a link I have for TRS-80 Model 1 documentation - just checked its still there!

http://www.akhara.com/trs-80/docs/model1/
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Old 7th Aug 2021, 2:57 pm   #7
JohnJones
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Default Re: TRS-80 Model 1 case screws

Quote:
Originally Posted by TonyDuell View Post

The original spacers were bits of rubber tubing. The idea was they held the CPU board down to the lower case but also pushed the keyboard up through the bezel. Rigid spacers (which I assume 3D printing will produce) will not work as well. You're better off finding some tubing and cutting it to length
Many thanks for this tip. Yes, you're right, the 3D ones are rigid, so I'll go with the tubing.

Quote:
Originally Posted by TonyDuell View Post

The cable between the 2 boards is quite fragile. And there's at least one wire such that if it open-circuits the machine will not start up. Don't ask how I found that out.
The cable between the two boards has been replaced before I got it, I'm guessing by a Radio Shack engineer as it has been done well and there is a Service Center sticker on the base dated March 1985.

Quote:
Originally Posted by TonyDuell View Post
Do you have the power supply brick and the monitor?
Power supply, yes, and it's pumping out the right voltages. Monitor, no, but (perhaps strangely) I was looking out just for the keyboard/computer as I'm lacking space for the full set-up.

Thanks for the help.
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Old 7th Aug 2021, 2:58 pm   #8
JohnJones
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Default Re: TRS-80 Model 1 case screws

Quote:
Originally Posted by Slothie View Post
Here's a link I have for TRS-80 Model 1 documentation - just checked its still there!

http://www.akhara.com/trs-80/docs/model1/
That's great stuff, many thanks.
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Old 7th Aug 2021, 3:01 pm   #9
JohnJones
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Default Re: TRS-80 Model 1 case screws

Quote:
Originally Posted by barrymagrec View Post
Being an American designed piece of equipment the screws are likely to be Unified sizes - should be easy enough to work out the actual size by looking at the case threads and checking against a thread chart.

https://journeymans-workshop.uk/unified.php
Many thanks.
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Old 7th Aug 2021, 4:41 pm   #10
TonyDuell
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Default Re: TRS-80 Model 1 case screws

About the monitor. Three pins on the DIN connector are used -- +5V, composite video, ground.

You can normally drive a composite input monochrome montor from that. It's roughly US television scan rates, 60Hz vertical, 15kHz-ish horizontal.

You can, of course, ignore the 5V output. It was needed for the original monitor -- and only in 115V countries like the States. Let me explain that. The original monitor was an RCA portable TV with the tuners, IF strip PCB (a plug-in board) and audio bits removed. A cable went it through the hole that was designed for the volume control, the 'V' stood for 'video' just as well. Anyway, this TV was a live chassis thing. So in place of that IF strip was a PCB containing an optoisolator. The input side of that, the side that was isolated from the mains, had a transistor amplfiier to drive it. That needed the 5V to power it.

Why only in 115V countries? Well, that TV chassis was desiged for 115V mains only. So to run it over here you needed a step-down transformer. And they did it properly, they fitted a proper double-wound isolating transformer. Meaning the video input no longer needed to be isolated, the replacement PCB was just a simple transistor amplifier. Which was powered from the TV chassis.

In case you're wondering my second computer was a TRS-80 model 1 (my first was an MK14). I did quite a bit with it, I still have a soft spot for this series of machines and have its grandson, the Model 4, set up on a desk here.
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