exxos's Atari, Amiga & retro forum

stephen_usher wrote: 07 Sep 2022 09:03 I must admit that I was surprised how you did those RAM chips with bus wires. I always do my hacked chips individually so I can replace them independently. It must have been "fun" getting them all back into the sockets!

Actually, it was easy to do, because the wire is very flexible and the chips had already been in the sockets. A bit fiddly but not terrible.

TBH I was just being lazy 😁, as after doing the first two I couldn't be bothered to do the other 6 🤣🤣🤣

Hehe

stephen_usher wrote: 07 Sep 2022 09:04

rubber_jonnie wrote: 06 Sep 2022 23:14 It seems 'MEM SIZE' is the prompt you get at boot time on these machines so you can reserve RAM space. I just need to verify if the ? MEM command should come back with a result different to 15570. it's only 814 bytes short of the total it's supposed to have of 16384.

Does the Commodore 64 report the full 64K in the BASIC boot banner?... :-)

True :)

I just don't know how much it should show!

Ok, so I did a quick calculation based on the TRS-80 memory map and 15570 is just about right. I came out about 64 bytes different in the calculation, but I think that is close enough.

@PhilC Thanks for your kind offer of some RAM ICs, but I think I'll be good to go :)

So, fingers crossed the Zener will be in the post today (Strikes are on tomorrow and Friday) and I can finish up the power section restore, then I'll decide if I'm going to individually hack the RAM or leave my hack job in place but do a neater job.

I'll also give the keyboard a good cleanup, the case is already done.

Once it's reassembled I can start to look at options for RAM expansion and some sort of SD card solution.

The one thing I do need to try out before I reassemble is the lower case mod. I've tried using the shift keys to get lower case, but that doesn't work, though both shift keys do. I have to say, it's quite a nice keyboard.

More later :)

And I just realised, it's SepTandy, so what better month to have fixed this little fella :)

Good job there (as always). :D I hope the finishing touches go to plan.

rubber_jonnie wrote: 07 Sep 2022 10:53 And I just realised, it's SepTandy, so what better month to have fixed this little fella :)

Didn't know this celebration of Tandy exsisted.

I guess I will try to dig out some 80s Realistic gear and an old ~'85 Tandy Electronics components catalog to take to the hall.

mrbombermillzy wrote: 07 Sep 2022 12:20 Good job there (as always). :D I hope the finishing touches go to plan.

rubber_jonnie wrote: 07 Sep 2022 10:53 And I just realised, it's SepTandy, so what better month to have fixed this little fella :)

Didn't know this celebration of Tandy exsisted.

I guess I will try to dig out some 80s Realistic gear and an old ~'85 Tandy Electronics components catalog to take to the hall.

Cheers 😁

I think SepTandy is a relatively recent thing, but happy to be a part of it.

Those catalogs played a big part in my younger years, I spent a lot of time in Tandy.

Well it wouldn't be one of my threads if something didn't go a bit wonky.

So I discovered that despite being powered by the repaired internal power circuits, that I'd left the connectors from the bench PSU connected, even though the unit was powered off.

When I disconnected the GND lead of the bench PSU, the video signal dropped off. So I reconnected it and the signal reappeared!!

I typed some garbage on the screen and disconnected the bench PSU GND, then reconnected and the garbage was still there!! So just the video signal was dropping.

I then connected the GND of the machine to the GND on my oscilloscope (The 'scope was turned off) and that worked too.

So I'm off to have a look at the video circuit now to see if there is some issue relating to GND that I'm missing, or if there is a problem with the mains power brick.

Where did you connect the ground lead on the motherboard?

By the way, the oscilloscope's ground will be mains ground referenced, as will the PSU. Usually bench power supplies have a separate mains ground (green) which may or may not be bridged with the -tive output.

stephen_usher wrote: 07 Sep 2022 15:19 Where did you connect the ground lead on the motherboard?

By the way, the oscilloscope's ground will be mains ground referenced, as will the PSU. Usually bench power supplies have a separate mains ground (green) which may or may not be bridged with the -tive output.

I was able to connect it anywhere to GND on the mainboard and it worked. I was going to try connecting to the green connector on the PSU, but actually figured it out before doing so.

It turns out that the composite lead has a common GND on pin 2 of the DIN connector, and looking at the TRS-80 pinout, it doesn't follow that convention, and instead expects GND to be on pin 5. It also has a connector so that the main body of the DIN plug can be connected to GND if you wish. My lead doesn't use the barrel as GND, so basically there was no GND for the video lead.

I've solved the problem by connecting pin 2 on the socket (Not actually connected to anywhere on the TRS-80) to the nearby GND for the barrel of the plug, and it's all working as expected now, and there's a definite improvement in image quality.

I guess that a standard composite socket wasn't really a thing on this machine, and my modern lead just didn't have the right connections.

On a positive note, now I've now also been able to test the old RAM now the machine is working, and I have 3 pieces that work ok. I'll probably leave the 41256's in there though as they run quite happily off just the 5v, and I'll keep the other stuff as spares.

Still no Zener in the post though, so I can't close it all up just yet.