TRS80 Model 4 Fault

The TRS80 Model 4 had been languishing in the workshop for a few years and was finally due some attention.

Inspired by Ian Mavric's "Building the Super Micro" article I did a complete disassembly, cleaning and confirmed that the supression caps in the power supplies had been replaced before doing an initial test power up.

This was the result (sorry for the glare)....


While the characters would change with each power up the layout of the screen and behaviour was the same.  

The following steps helped me identify the problem and get the machine operational:

  1. Checked the power on pins 11 (VCC) and 29 (VSS) of the Z80 (U57).  This read 4.7 volts... a little lower than I expected but was consistent if I powered the main board from either power supply so I assumed it was OK.  At the lower end of the rated voltage range for the Z80 but probably OK.
  2. Used a frequency counter to check the clock on pin 6 of the Z80 (U57).  This was correct at 4 Mhz.
  3. Used a logic probe to look for activity on pins 27 (M1), 19 (/MREQ), 20 (/IORQ), 21 (/RD), 22 (/WR) of the Z80 (U57).  If the machine was running normally you would expect to see continuous pulses on these pins.  I saw no activity which suggested a problem with the Z80.
  4. Removed the Z80 (U57) and powered up the machine with no processor installed.  The result was the same display which further hinted that the problem might be the processor.
  5. Replaced the Z80 (U57) with a spare and was greeted with the Cass? prompt. 

It wouldn't be as much fun if the machines always worked.



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