Restoring The Cheapest TRS-80 At The Swap Meet

We don’t know if you’ve looked into it recently, but the prices for vintage computers are through the roof right now. These classic machines are going through something of a renaissance at the moment, with even relatively commonplace computers commanding several hundred dollars if they’re in good condition. For those looking to start a collection without breaking the bank, you may need to accept some specimens that have seen better days.

That’s the situation [Vlado Vince] recently found himself in — he wanted to get his hands on a TRS-80 Model I, but wasn’t willing to spend eBay prices. So he waited until the Vintage Computer Federation’s swap meet in June and was able to snag a “fully functional” example for $95. Unfortunately the seller must have been using some form of that phrase which we were previously unaware of, as it took a considerable amount of work to get it back online.

Getting the machine home [Vlado] was only able to get a very faint video signal out of it, and even then, it had no horizontal sync. Upon opening it up he found considerable corrosion, in addition to signs of previous repairs and bodges. Doing some research he found that one of the corroded ICs was indeed part of the video circuit, so it was carefully extracted and replaced with a socket to take the new chip. He also removed a hacked together memory upgrade that was likely installed in the 1980s, figuring it being there would only complicate the repair.

This still didn’t give him a useful video signal, so he started a closer examination of the board. It turns out there were subtle modifications all over the place, with traces cut and whole components removed for reasons unknown. With the board schematics in hand, [Vlado] started undoing the changes and returning the circuits to their stock arrangement. He also repaired the badly damaged keyboard connector, as the previous owner’s bodge wires didn’t seem to be up to the task. These efforts paid off as he was finally able to get the machine to boot, though the video was erratic.

At a loss for the continuing display problems, [Vlado] started replacing components in the video circuit hoping to luck out. He swapped out the electrolytic capacitors and variable resistors, and even tried replacing components in the power supply, but the issue remained.

He eventually got the oscilloscope out and started poking around the board, looking for anything that seemed out of the ordinary. He found that the signal coming from that very first video IC he replaced was out of spec; as it turned out he had replaced the original 74C04 with a 74LS04, but what he actually needed was a 74HC04. With the correct chip in place the image stabilized, and the machine was finally in the “fully functional” condition that he was promised in the first place.

Ultimately [Vlado] had set out to get his hands on a TRS-80 so he could get a better understanding of how it inspired Voja Antonić’s design of the Galaksija. Considering the amount of effort he put into this repair, we’re willing to bet he got all the insight he was looking for. Especially when combined with the experience of building his own Galaksija replica.

Incidentally, if you take a look at our video from the VCF swap meet, you can actually see the TRS-80 [Vlado] ended up buying at around the 0:50 mark. If you happen to have grabbed one of the other computers covered in that video and got it back up and running, we’d love to hear about it.

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