exxos's Atari, Amiga & retro forum

Surely it would be a good idea to look up the specs for the power transistor to see what its operating temperature is. A lot of those type packages have a surprisingly high thermal upper limit.

Was that transistor rusted or just over fried ?

Tom

@rubber_jonnie

stephen_usher wrote: 22 Sep 2022 14:03 Surely it would be a good idea to look up the specs for the power transistor to see what its operating temperature is. A lot of those type packages have a surprisingly high thermal upper limit.

With all the family stuff of late, I just hadn't looked it up, but it should be good up to 200C according to the datasheet. Currently running at about 74C.

The heatsink was not making good contact with the transistor, so all tightened down before resoldering. Pretty stable now and well within tolerance.

Tomswork wrote: 22 Sep 2022 15:13 Was that transistor rusted or just over fried ?

Tom

@rubber_jonnie

Just a bit grubby, I expect it was just grime burned on.

rubber_jonnie wrote: 22 Sep 2022 15:45 With all the family stuff of late, I just hadn't looked it up, but it should be good up to 200C according to the datasheet. Currently running at about 74C.

So, basically, the transistor only fails when the solder melts. :-)

stephen_usher wrote: 22 Sep 2022 15:53

rubber_jonnie wrote: 22 Sep 2022 15:45 With all the family stuff of late, I just hadn't looked it up, but it should be good up to 200C according to the datasheet. Currently running at about 74C.

So, basically, the transistor only fails when the solder melts. :-)

That's about the top and bottom of it, yep. The PCB might possibly catch fire too :fire: :fire: :fire: but that theory is currently untested.

Tandy picked a 12 dollar part and put toothpick amount of heatsink compound on it


Tom

Tomswork wrote: 22 Sep 2022 16:42 Tandy picked a 12 dollar part and put toothpick amount of heatsink compound on it


Tom

They certainly did. Interestingly, Q6 was also getting super hot to begin with, but I replaced it with a higher specced part and used paste on reassembly and it now runs totally cool.

Oh good, I kind of broke the TRS80...

I decided to do the first part of the 64k upgrade, which basically removes the 5v and 12v lines from the RAM sockets and moves the 5v to pin 8 rather than pin 9.

I stopped short as I just wanted to get to the point where I was able to replicate the 'hand added' bus bars but on the PCB, and the upgrade to 64KB does go down that route, my only exception being that I kept pin 9 at 5v to replicate the hand wired 41256 ICs so they would work as 16KB.

They were working fine before, so as long as I could keep pin 1 low, and have pin 8 and 9 at 5v it would replicate my hand wired setup.

Only once I dropped all my 41256's in, I got this:


Ok, let's revert back to the very original setup and my old handwired RAM. Same thing, so thinking maybe I now have bad video RAM. Oh well, it keeps me off the streets!!!

It is at least booting and the RAM size shows as correct, plus simple programs run too.

More to follow...

Looks like an address error. Check for solder blobs or the ribbon cable