On Sat, Aug 19, 2017 at 6:14 PM, Rob Jarratt via cctalk <cctalk@classiccmp.org> wrote:
> For the time being I have decided to take a look at the other spare board, > which works less well in that it does not produce any kind of video output > at all, but on the basis that the 82C206 seems to be OK, it might be easier > to repair, and I can compare it to the other board before I try replacing > the 82C206. I have noticed on this spare that there is something wrong with > the monitor sensing circuit, which feeds the PVGA1A. On the spare there is > no activity on this pin, while on the partially working board there is > activity. I traced it back to the inputs to a comparator (LM339). This > comparator is comparing the RGB outputs of an Inmos G176 to a reference > voltage. The behaviour is different for the two boards. I noticed however I wonder if this is used to detect termination resistors in the monitor, which would pull the voltage down. That could distinguish between no monitor, a mono monitor (only pulls one signal down) and a colour one. > that these RGB pins are also connected through an SMT component (x3, one per > colour) to the RGB pins of the VGA connector. The SMT component has no > markings and when tested with a multimeter appears to be shorted across the > terminals. It is labelled "FEn" (n=1,2,3). Is this just some kind of fuse? > If not what could it be? How many connections does it have? If just 2 then it could well be a fuse. Not really to protect the machine in the event of flashover in a monitor putting high voltages on the lines, but rather to meet the legal requirements in some countries that any device linked to a modem needs such protection (the UK had such rules at one point, there were boxes containing 24 fuses and 48 zener diodes to connect between the DB25 connectors on termnal and modem). The other possibility would be some kind of interference filter, but (a) those would normally have more than 2 connections and (b) filtering a high speed video signal is not generally a good idea. Are any of these fuses (let's suppose they are that for a moment) testing as open-circuit? If not, I think we can ignore them for the moment. -tony
