[GreenKeys] M33 AC Receive Loop

[John Ball]

My teletype and UCC6 functions fine in Local but in Line it's the opposite.
I have a serial 20ma current loop adapter which uses 4100 and 4200
optocouplers and I could not get the machine to both stop marking or
transmit anything to the adapter when wired up. For testing I put it in half
duplex mode and bridged terminals 6 and 7 which caused the selector magnet
to energize and idle the machine. When I checked the voltage at the same
terminals I saw it was reading -0.5v DC and 6.6v AC. I understand the send
loop is AC but isn't the receive loop DC? I've looked at all the diodes on
the circuit card without desoldering them and none were obviously blown or
shorted. Is it worth it to pull them off and check anyways or should I be
looking elsewhere? I did have CR1 short out about a month ago for no obvious
reason. I replaced it and CR2 with matched values.

-John