DECstation 220 - Out of Ideas?

Rob Jarratt robert.jarratt at
Sun Jun 25 05:46:34 CDT 2017

> Thanks Ian. Unfortunately I don't know of any schematics. I suspect you are
> right about it being a wire-OR because the documentation for the P82C206 says
> its output is tri-state. As everything is soldered on rather than socketed (except
> the ROMs) I am not sure I can measure any currents, nor can I think of a way to
> check if the other chip (Olivetti GA099-B) is trying to drive the output the other
> way :-(. I will check the resistance between the outputs of the P82C206 and the
> ROM inputs and of the Vss pins.

I have done a bit more tracing of components and have come across a possibly anomaly. The signal that won't go low enough goes to the input (pin 2) of a 74LS125 buffer. The resistance across the input and output (pins 2-3) is a few Kohm. Other input-output resistances on this chip are much higher. The same goes for Vcc to the input, the resistance is much lower than Vcc to other inputs. It could be that the 74LS125 is faulty, or something else on that line has a low resistance path to Vcc, either way it would seem to be a possible reason for the signal never to go low enough.

I think I am going to have to get myself a cheap hot air rework station to allow me to remove the 74LS125 (it is surface mounted) so I can check it out of circuit.

However, I was looking at this machine with a view to passing it on anyway. Is there anyone in the UK who would be interested in it (unrepaired)? It would be just for the cost of post and packing, or you can collect from the Manchester area.



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