03.28.2015 00:02

Troubleshooting Tandy Model 100 Hardware

The clock signal going into the 81C55 appears ok. Power is measured to be 5.12 V. The CPU also has adequate power and a clock signal that appears good enough.

Good Clock

The signals comming out of AD0-AD7 are very noisy, as shown in the following image. The keyboard signal out of PA0-PA7 appears dead.

Bad Signal

I've tried to reflow all the leads of the 81C55 as well as all of the connected chips but only got a momentary glimpse of keyboard response and garbled LCD output. A reset made the keyboard non-responsive and the LCD screen blank.

Posted by Jovan Trujillo | Permalink