Post Increment, TRS-80

System-80 working (sort of)

This poor System 80 has been subject to cruel and unusual punishment at the hands of previous owners.

I’ve got the Ready prompt (the equivalent of the TRS-80 Memory Size) but it’s been hard work. All of the 2102 video RAMs needed replacing, as well as several LS157 muxes. I managed to link A4 and A5 when replacing a mux – that cost me a few days.

Pressing Enter gives a screen of full graphics blocks – I think the video RAM read is not working fully. But to debug that, I need to understand what is left of a lower case support mod and (I think) software switch for 32 col mode

Post Increment, TRS-80

Restoring the System-80

The battle for the System 80 has begun!

I started with no video, no CPU activity, and a very bodgy looking 64k DRAM upgrade using 4164s

Fixed the CPU issue by replacing a dead electrolytic in the reset circuit. Video came back when I replaced the cable between the boards.

I’ve reversed the 4164 upgrade and am working on the Drastik.

The time honoured “ROM of NOP” is generating square waves on the lower 8 address lines. Now I need to find where they stop being square!

Drastik, Post Increment, TRS-80, Vintage Computers

Drasktik working on Model 1

Success!

The Model 1 is reporting 48k memory without a DRAM in sight. Only two solder wires required, plus adjustments to the two sets of jumper pads which I have replaced with DIP switches for ease of testing.

I’m still working on the solder-less 16k upgrade. May not be possible. Will look at this tomorrow (maybe).

Two minor issues with the board. Seems I can demux 14 bit DRAM addresses but have forgotten how NOR gates work. 🙂

Also had to rewire the keyboard cable as the janky job performed by the previous owner died when I opened the case.

I’ve also 3D printed some replacements for the rubber-ish board standoffs that crumbled to dust when I flattened the boards. If anyone wants the files, let me know.

Drastik, TRS-80, Vintage Computers

Static RAM for TRS-80 and System-80

I’ve designed a board that allows replacement of the 8 x 4116 on the Model 1 or System 80 (aka Video Genie) with a single static RAM.

It also allows an upgrade to 48k with no extra parts. A similar board for the Model 3 and Model 4 will follow, with support for up to 128k.

I’m finishing up the board design now. The System 80 has weird spacing on the DRAMs, so I have to adjust the board to accomodate.

I’m trying very hard to get the gerbers out to PCBWay this week. Finger crossed!

Post Increment, TRS-80, Vintage Computers

Conquering the Alps

The collective wisdom of the Internet comes to the rescue.

TRS-80 owners on the TRS-80 Model 1-4/4P Facebook group offers many suggestions to my keyswitch dilemma, including replating the contacts, applying conductive paint, and adhesive copper.

One suggestion stood out – not only because of it’s simplicity but because I have used it many times myself over the years. This procedure requires a very special tool, one that is nearly obsolete in this day and age. I wasn’t sure I had such a rare device.

I did find one, eventually, in the back of a drawer

The rare and timorous beastie

The mighty pencil eraser, handily attached to it’s applicator wand.

A few quick pokes on the offending contacts, and the resistance dropped to below 1k in most cases. In an hour so, all of the key switches had been reassembled and resoldered.

I did look at the contacts under the microscope but didn’t see anything significant. Perhaps the gold looked a little brighter, but that’s hard to tell on an $80 microscope with 1080p screen that is three inches across 🙂

We’ll see how long the fix lasts, but for now, I am declaring victory.