Woo hoo - Collection Completed!! TRS-80 inbound

[viking272]

I see there are some TRS-80 operating system (os9?) chat over on AF...

https://www.atari-forum.com/viewtopic.php?f=14&t=11918

@rubber_jonnie
Did yours come with disks for the OS?

[rubber_jonnie]

Ok, so it looks like CR1, the crowbar for 5v is actually dead.

I connected all the capacitors one by one and checked the current draw each time. With all the socketed chips removed (CPU, ROM, RAM etc) it's pulling just under 900mA at 5.25v.

As soon as I touched CR1 to the pad at the end I'd removed it from, there was a sudden drop to about 1.2v and the bench PSU went into current protection mode and maxed at 900mA.

Testing the diode once removed, it's no more than a piece of wire!!

So I need to get a new 1N4735 for the crowbar circuit now. It will run without, but I'd like to have it in there for the purposes of overvoltage protection once the power circuits are back together.

But at least without it and with the bench PSU I can carry on testing.

[rubber_jonnie]

I see there are some TRS-80 operating system (os9?) chat over on AF...

https://www.atari-forum.com/viewtopic.php?f=14&t=11918

@rubber_jonnie
Did yours come with disks for the OS?

Mine is unfortunately a TRS80 Model 1, so no disks, just BASIC in ROM.

OS9 is for the Color Computer and I believe also works on the Dragon 32. I have just upgraded my Dragon to 64K and mean to see if I can make 0S9 work at all.

[rubber_jonnie]

Ok, so it seems this machine also has at least 2 bad RAM ICs.

I hooked up 5v and 12v from my bench PSUs, and plugged in the A/C adapter to generate the -5v. The built in 12v and 5v circuits are currently out of service, so there is nothing generated from either via the PSU.

All 3 voltages reach the RAM sockets just fine, and though 5v is a bit low, it is getting there.

I tested the RAM one at a time and turned on power, and 2 are going nuclear! So, off to order some replacement RAM now as well, but sadly my parts didn't arrive today and tomorrow the posties are on strike. Fingers crossed for Thursday, but then I'm away for a week!

Oh well, still feels like progress. I think I need to hook up to composite too, see if there is any signal at all.

[stephen_usher]

If you have some spare 4164 chips it's easy to hack them to fit 4116 sockets and you only need the +5V rail to run them (and they run cooler).

Convert 4164 to 4116

[rubber_jonnie]

If you have some spare 4164 chips it's easy to hack them to fit 4116 sockets and you only need the +5V rail to run them (and they run cooler).

Convert 4164 to 4116

Sadly not, I just used them in the Dragon! The thought of using 4164s had also crossed my mind too.

[rubber_jonnie]

Just did a quick check, and there is no clock signal where there should be.

The clock is based round a 10.6445Mhz xtal and a 74LS04, so that would be easy enough to replace as I have spares in my parts drawers.

This machine is not happy.

[stephen_usher]

Sounds like voltages have gone where voltages shouldn't have. Oh dear.

41256 RAM can be used too. You just need to lift pin 1 and solder to pin 16 in addition to lifting pin 9 and connecting it to pin 8.

[Tomswork]

The expansion interface is always handy to have you can get cf card options as well for the trs80

Link to upgrades http://bartlettlabs.com/MIRE/


Tom

[rubber_jonnie]

Sounds like voltages have gone where voltages shouldn't have. Oh dear.

41256 RAM can be used too. You just need to lift pin 1 and solder to pin 16 in addition to lifting pin 9 and connecting it to pin 8.

That's a good idea, I've got a load of 41256 chips.

Having replaced the 74LS04, I now have a clock signal 😁 Testing the old one in my TL866 shows it as very much dead.

And a video signal too!

PXL_20220830_161746340.jpg

So feeling pretty happy about it at the end of the day 😁