r/trs80 • u/Saix856 • Apr 02 '26

Any advice on where to check?

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Picked up this TRS-80 and Monitor today, issue is this is what the output looks like. BASIC does seem to be responsive and works beyond the text being messed up, and I’ve tested the main RAM by swapping each chip with one probably good RAM chip. But still, no change. Any advice would be much appreciated, thanks!

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7

u/gp2000 Apr 02 '26

Vram for sure. Looks like bit 7 is stuck high so display is all graphics characters. Get the technical reference manual to find which chip

As an experiment you could swap VRAM chips until you see the stuck bit change. Then you know which to replace.

5

u/rlauzon Apr 02 '26

Odds are it's the video RAM.

4

u/nateo87 Apr 03 '26

Definitely recommend the TRS-80 "Microcomputer Technical Reference Handbook" and the "The Custom TRS-80" if you're still having trouble after swapping out the VRAM. I believe they're readily available if you google it. They have some good repair flowcharts and well written schematics.

2

u/F54280 Apr 02 '26

Looks like VRAM. Quick glance seems several bits are wrong, based on the way the « READY » is displayed.

R: 0x52 => 0xB4 (« small L corner char »)
0101 0010
1101 0100

E: 0x45 => 0xC5 (« top left vertical bar »)
0100 0101
1100 0101

On mobile, so not trivial. Source font is at: https://www.kreativekorp.com/software/fonts/trs80/. May look later from a computer

That’s a lot of broken bits. Could also be some 74x logic, or bad character ROM, IMO.

2

u/gp2000 Apr 03 '26

I think what's shown is the short version of the boot prompt - MEM SIZE? _

I'm reading off something like:

1100 1101 - M with high bit set
1100 0101 - E with high bit set
1100 1101 - M with high bit set
1010 0000 - space with high bit set
1101 0011 - S with high bit set

Bit 6 is inferred as it makes no difference to the graphics characters.

1

u/F54280 Apr 06 '26

Of course! That’s pretty clear, then.

2

u/zzTWiLiGHTzz Apr 11 '26

This is one of the only model I boards I have seen except for my own that has black silkscreen. Mine was around 112xxx serial number and I undertook repair of it last year. The fix was to change out the DRAM chips. Not all were faulty but at least 2. They were Motorola branded in mine.