TRS-80 model I problems

[simbalion]

I finally got my hands on a nice looking TRS-80 model I computer after years of looking for one today. Have had two monitors for years and an extra power pack, but no computer. This one is very complete with the manuals and looks pristine inside and out except the screws holding the case together have rusted for some odd reason. No signs of moisture on anything but those screws, must have been in a storage unit for a while. Anyways, the monitor is in good shape, but when I boot the computer up I just get garbage, diagonal lines across the screen and what looks like bit's of letters like it is trying to boot up. I pulled the unit apart and it looks fine inside, possibley even upgraded to level II from what I can tell. I found four adjusters on the board and fiddled with them a little, but no improvement. Anyone know what is causing this? If I have to, I'll get a picture tomorrow.

[JamesD]

Model 1s sometimes have RAM failures, the monitor might be bad, the power supply could be bad, the CPU could be bad... diagonal lines on the screen has me leaning towards a monitor issue.

You could test to see if the output was ok on another monitor if you could borrow one.

If not, the output from the TRS-80 is actually composite if you can build a cable for it.

Double check the pins on the monitor cable for corrosion.

Check for corrosion in the monitor but don't touch anything inside if you don't know what you are doing or you could kill yourself.

[simbalion]

I tried another monitor last night, but had the same issue. This unit was stored for 26 years before the guy got it out th sell and I am the second owner of it. It probably worked when he put it away as this guy didn't strike me as the type to keep something that was broken.

[JamesD]

You can find the schematics here:

http://incolor.inetnebr.com/bill_r/trs80_schematics.htm

 

Check the SYNC generator and video mixer circuits.

Look for Q2 and Q1 on sheet 2 part 3 for the mixer circuit and trace the SYNC line back to find the SYNC generator circuit.

[JamesD]

I tried to find a source for a repair manual or something you can used to relate the schematic to the board but couldn't find anything still available for download.

[simbalion]

Thanks for the help. If I can find someone to do some work on this beastie, that schematic should help. Sadly, to me it's all greek, though I can trace sections ok. My soldering skills would finish this computer off!