Gregory's Serverone interesting issue is that there are a lot of corroded connections and switch contacts. this machine has less than 7k hours on it and it has sat in storage for 40 years.
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 here are two special tools i used to work on the machine. the one with the black handle is a contact burnisher. it removes oxides and corrosion from switch contacts without damaging the plating layer (unlike sandpaper). the other tool is a switch adjuster. you slide it over the metal contact tongue and twist it to adjust the alignment.  @tubetime oh wow I haven’t heard of a contact burnished (vs high-grit sandpaper). I looked up a little info on them and sounds like it’s the same idea as sandpaper though? You can still scrape through plating past the oxidation? What am I missing, is the burnished just a lot more gentle than even 2000 or 4000-grit sandpaper? So maybe you could damage plating but unlikely? Or a totally different sense of all this? @zackstern yes sandpaper tends to remove too much, too quickly. i guess you could use a fine grit, but a burnisher is easier to manipulate between the contacts. @tubetime how do you know how many hours it has? Is there a counter, like on an airplane engine? |
being electromechanical, it's full of a lot of relays, steppers, and other such mechanisms.
the schematic is a classic example of "ladder logic." the sides of the ladder are the power rails (in this case, 24VAC) and the "rungs" are individual control circuits, typically a relay coil on the left and a set of contacts on the right that, when properly engaged, turn on that relay.