Model 100 Basketcase

Try to resurrect a vintage Radio Shack TRS-80 Model 100 computer

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I've had this old TRS-80 Model 100 for years, and it's time I passed it on or got it working. Sure, I could just run an emulator for vintage fun, but that's not the same as remembering that I was one of the first kids to take notes on a laptop in my high school, and it was this very model that I did it on.

I've got the service manuals and a high school level electronics education. Let's do this.

Also, I need to find all the screws.

I've got a TRS-80 Model 100 portable computer just sitting in my basement, in parts. I t was given to me to see if I could sort out why it wouldn't run. There's a photo in this project that shows what it does on power-on: the LCD displays a single vertical line and nothing else happens.

There is nothing obviously wrong with the power supply, and I get the same symptoms if I use batteries or the wall-wart. The on-board battery still seems to be holding a good charge. There are no obviously blown or leaking capacitors. The first part of the service manual advises where to test for good voltages, which I've done. I've also followed the instructions to be sure the major chips are getting good voltage, and the power-on and reset line levels all seem good when tested at all the interesting inputs and outputs of those chips.

  • 1 × BitScope Micro A mixed signal generator and soft oscilloscope

  • Wait. There's more space-dust on this project.

    John Verne06/10/2017 at 12:44 0 comments

    Life got in the way, and all projects languished. There isn't much holding me back, except finding a few hours to clean off the desk in the shack and start probing again.

    I actually have a somewhat working scope, though I really should have purchased the unit with a scope adapter because sourcing that locally is a pain, and using the cheap leads is even more painful.

    Maybe I'll have a more interesting update this month.

  • Back to the shack for more hard-core (eventually)

    John Verne03/24/2016 at 14:20 0 comments

    Life got in the way, and I've done little more than verify my 'scope actually works. And now I have to clean up my shack before I can restart this work.

    In the meantime, perhaps others here would be interested in promoting a Retrocomputing StackExchange site? Such a site is being proposed right now, and it could use some love.

  • Now I Have a Project for the BeagleBone

    John Verne01/31/2016 at 06:52 0 comments

    Well, I finally ordered a BitScope Micro from a local reseller. Unfortunately, the reseller did not offer the probe kit(s), so it looks like I'm going to have to find that part elsewhere. I can get probes from Amazon pretty easily, but the probe adapters from Bitscope will have to be sourced somewhere else.

    However, this is a major step in the right direction. The direction where I find out if the LCD display board is totally toast, in which case this work gets a lot harder.

    But I'm glad I finally got this piece of kit. It adds a lot of basic functionality to my shack in a small package. Maybe I'll post pics of the signals I capture, assuming I can figure out how to make all this stuff work.

    The irony is not lost on me that the device I just bought is orders of magnitude more powerful than the dinosaur I'm trying to resurrect.

  • Basement Shack Seeks Scope

    John Verne01/23/2016 at 14:23 2 comments

    Wow, two Skulls and a like and a mention on the @Hackaday Twitter! This is the most internet famous I've ever been.

    I should probably follow through on this rather modest project...

    So, I'm shopping for an oscilloscope, and my budget and needs will probably be more than satisfied by something like a BitScope. I'm not sure what probes I'd need, though. I think I can get away with the 60 MHz 10:1 probes? If the 60 MHz refers to sampling rate and 10:1 refers to signal voltage amplification...? Given this project is somewhere around 8MHz at TTL levels.

    In an effort to leverage the community, consider this a request for any comments or advice regarding this or any other 'scope choices. My budget is tight, and while I have some access to local second-hand gear, I might not have the skill to know if the gear was any good.

    It's like my DMM: I'd love to be able to afford (or justify) a Fluke like my engineer friends. But I can't really, so I got a decent mid-priced college special. Is there such a thing in the oscilloscope market?

  • Let's do this thing

    John Verne01/19/2016 at 16:52 0 comments

    I found a scan of the M100 service manual, and printed up a big copy of the schematics. I worked my way through the service manual flowchart right up to the point where I need to 'scope the signals going into the display.

    Now I need a 'scope.

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John Verne wrote 01/24/2016 at 01:42 point

[Looks like we reached the limit of threaded conversations on]

@K.C. Lee  has pointed out that I was looking at the bandwidth of the probes, not the device itself, which is indeed 20MHz. And I've learned that this is a rating related to a -3dB roll-off, which is excellent information. This should all be more than sufficient for this rather modest equipment I'm testing.

  Are you sure? yes | no

andres ortiz wrote 01/23/2016 at 16:11 point

A 10:1 probe attenuates your signal, it doesn't amplify it. Basically it lets you measure a bit higher voltage and you compensate on the oscilloscope by multiplying the reading by 10 or if it's a nicer scope, setting the channel settings to 10x.

  Are you sure? yes | no

K.C. Lee wrote 01/23/2016 at 17:07 point

Actually the reason why they have 10X probes are to reduce the probe loading - capacitive and resistive to the circuit under test.  The higher voltage is a nice side effect.  Most of the better scopes would have a probe setup menu that deals with the probes.  Some like certain Tek scopes might have extra contacts for sensing their own probes.

Those $10 logic analyzers from China might be more useful for looking at digital signals.

  Are you sure? yes | no

andres ortiz wrote 01/23/2016 at 17:19 point

Much appreciated, something to add to my handy info folder. As for the cheap $10 logic analyzers, i have a knockoff saleae logic analyzer. 8 channels, works surprisingly well for the cost, also if i recall it works with the logic software so you don't have to pirate the official saleae one.

  Are you sure? yes | no

John Verne wrote 01/23/2016 at 21:46 point

The BitScope I mention is a multi-channel logic analyzer, but has a scope mode, too. I need a scope to look at the signals going into the display board, according to the service manual.

Anyway, it sounds like a 10:1 probe would do just fine for the TTL levels I'm looking at, and none of them are particularly fast changing. I assume further that a "60 MHz" scope can look at a a ~60MHz signal reasonably well -- that is, the speed of the scope is what it can see, not the sampling rate?

  Are you sure? yes | no

K.C. Lee wrote 01/23/2016 at 22:26 point

Some say you want a ratio of 3 or 5 times the signal of interest.

Not that it matters a lot as the Bitscope has a bandwidth of 20MHz.

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Arduino Enigma wrote 01/23/2016 at 14:48 point

I have a soft spot for this laptop as well. Good memories.

  Are you sure? yes | no

John Verne wrote 01/23/2016 at 16:07 point

It was a surprisingly capable computer for its day. I'll call this
project a win if I can just get it to boot its ROM. But a real win would
be to run my own Forth programs on it. And visit the hackaday website

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Arduino Enigma wrote 01/24/2016 at 03:13 point

I am surprised to find out they are still available...

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