Rob, I have a cheap hot air re-work station that you are welcome to borrow. You might need to order a nozzle as I do,t have many Dave
> -----Original Message----- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Rob Jarratt > via cctalk > Sent: 25 June 2017 11:47 > To: 'General Discussion: On-Topic and Off-Topic Posts' > <cctalk@classiccmp.org>; 'Ian S. King' <isk...@uw.edu> > Subject: RE: DECstation 220 - Out of Ideas? > > > > > 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. > > Regards > > Rob
