Dead? Model III

[Mos6502]

I found a Model III TRS 80. The CRT mounts were broken. I removed the top/cover and put the CRT into the proper position and it stays in place on its side.

When powed up the drives spin momentarily, but niether drive light comes on. The CRT powers up but does not display anything (if you turn the brightness up you see scan lines).

Is this machine toast, or is there a way to get it to work again? What's a probable cause for a blank screen?

Thanks.

 

[NeXT]

the Model III needs something to boot from first so it normally does that.
If I can recall, you can drop it down to BASIC though by holding the BREAK key and then press the reset button.

 

[Mos6502]

Tried that, but still blank. I'm going to check the connection to the CRT and make sure it's not loose next.

Any other ideas?

Thanks.

 

[NeXT]

Be careful with the CRT neck. Not only is there still a residual charge but it's quite easy to crack the seal and ruin the tube.
The presence of scanlines though when you power the computer on and crank the brightness is good. Same with the drives both checking for media as that means the disk controller is operating as well.

 

[Mos6502]

So everything looks more or less right. Somebody has obviously been inside before as a couple screws don't match.

I took the shielding off of the mother board and pressed on all the socketed ICs to make sure they were seated.

I noticed that the large chip, U77, had discolored contacts (they were black. I'm assuming they shouldn't be, haha). It says "mostek" on it, but I couldn't make out the number. Any idea what this chip is supposed to do? I'm hazarding a guess that it might be dead...

 

[NeXT]

The pins oxidize with time. If you are careful you can pull the chip out (note orientation!!!) and clean the pins with an eraser.

 

[deathshadow]

Or in severe cases tarn-x can really work wonders -- just be sure to rinse completely and don't leave it in the tarn-x for too long, or it will take the metal off completely... and be careful with it around the plastics.

 

[Mos6502]

Ok I will give that a try. None of the other ICs had any discoloring of the pins, so it seemed weird to me that one of them would be all black.

 

[Mos6502]

Are these symptoms indicative of a faltering cpu?

Or could it be a rom problem? RAM?

I might just try putting a new Z80 in if it seems like a CPU problem.

The CRT seemsto be functional,the disk drives seem to be functional (they spin when the system is turned on, though neither of the drive lights comes on), the keyboard seems to be working too, but nothing shows up on the screen.

Edit: I've also read somewhere that if the power supply to the ram is wrong the machine won't boot up. Any idea how to test this?