DECstation 220 - Out of Ideas?

Dave Wade dave.g4ugm at gmail.com
Sun Jun 25 14:26:13 CDT 2017


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-bounces at 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 at classiccmp.org>; 'Ian S. King' <isking at 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




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