Re: Is my Tek 468 beyond repair?


John
 

Well the good news is that some progress is being made. The board has been re-capped (except for the large can which I have been unable to remove) and the two 74S112 ICs have been replaced and mounted in sockets. Since it tested OK under a 470Ω load, the original crystal was replaced and the board returned back to the chassis and re-connected. The 5MHz clock is now stable and there is minimal ripple on the 5V line.

I have checked the wave-forms on the timer board and found that some are present, others are missing or incorrect.

For example, waveforms at 148, 149, 150, 151 and 157 (all directly associated with the clock) are all correct. A waveform is present at 147 but it does not match the diagram but is more like square wave. There is no waveform at 146/TP120 but the line is high. I couldn't reach some of the points on the board, for example 154 and 155, but nothing was happening at 156. Since be board is heavily interconnected with the CPU board, I am wondering whether this might be the issue.

I am still waiting for a replacement 8085. I checked and the 5MHz clock was present on pin 1 and 2.5MHz on pin 37 but still no ALE. Hopefully the replacement CPU will resolve that.

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