exxos's Atari, Amiga & retro forum

Looks like TRS-80 Model 1 week...

stephen_usher wrote: 31 Aug 2022 19:04 Looks like TRS-80 Model 1 week...

Oooh, nice I know what I'm doing next :)

Oh I just watched it that's a classic symptom of the power being plugged into the video and popping the two 74 ttl parts cool

Fun fact the 5v out he mentions is for the opti coupler in the tv set is a hot chase set its a rca b and w set


Tom

Ok, so I did a sort of bodgy/sketchy thing to see if I could get the TRS up and running.

Using the suggestion from @stephen_usher I've plumbed in 8 41256 ram ICs with pin 8 bent up and connected to pin 9, and pin 1 bent up and connected to pin 16.

Here it is in all its sketchiness:


But still no luck, though it does seem to power up with just 5v, and I'm still seeing the video as @Tomswork described, video RAM initialised but not cleared.

I'm seeing address and databus activity and the clock is still good, but it's still not running.

However, I found another IC going nuclear, Z21, which is a decoder/demultiplexer and has a hand in ROM (CS lines, so probably not able to access the ROMS) and RAM activity. By nuclear I mean super hot, and once removed the overall current draw dropped considerably. It's a 74LS156, and wouldn't you know it, I had 157s, but not 156s. They're on order now. Testing the old one in the TL866 reported it as dead, which I guess I already knew, but nice to verify.

Whilst I was at it I let the machine run for a while and checked the remainder of the ICs, mostly 74LS, and nothing else was getting hot.

At least I know that the video RAM and circuit is working, so it should just be a matter of time before I can get things going.

It's just a little frustrating that I'm going away on Friday to a karate camp until Sunday early evening and then off to my parents until Wednesday, getting home late afternoon.

The optimist in me is saying that at least all the parts I have on order should be delivered by the time I get home :)

Tomswork wrote: 31 Aug 2022 21:04 Oh I just watched it thats a classic symptoms of the power being plugged into the video and popping the two 74 ttl parts cool

Fun fact the 5v out he mentions is for the opti coupler in the tv set is a hot chase set its a rca b and w set


Tom

About to watch it now :)

Let me know id the ttl is Fujitsu brand thats stuff gets burning hot and cooks itself

Tom

Some good tips

http://ts-inc.dyndns.org/Diagnose%20Rep ... l%201.html


Tom

Tomswork wrote: 31 Aug 2022 21:24 Let me know id the ttl is Fujitsu brand thats stuff gets burning hot and cooks itself

Tom

It's a Motorola. It was so hot I jumped when I touched it.

Tomswork wrote: 31 Aug 2022 21:28 Some good tips

http://ts-inc.dyndns.org/Diagnose%20Rep ... l%201.html


Tom

Thanks, I will have a look through.

Due to lots of parts still outstanding, I went ahead and just made a new keyboard cable in the meantime, using an old IDE cable:


Fingers crossed, when I return from my trip next week, all the rest of the parts I need will have arrived and I can fingers crossed get this fella booting, and if not, work out what's still wrong and needs fixing.

I am certainly considering fixing up one of the TRS-80 debug ROMs that Adrians Digital Basement has been creating if it is still dead after the next spell of fixing up :)