Hi! I just bought an old-school BK 1522 20MHz dual-trace scope on
eBay, and it works great -- at least, half of it. The trace for
channel 2 works great, but channel one gives no trace at all.

I've narrowed it down to the input circuit board for channel 1 -- the
input board on this scope contains the voltage scale switch and input
jack, a couple dozen passive components, a transistor or two, and
that's about it.

The scope contains two such identical boards. They each connect to
the rest of the scope via a 4-conductor cable. Disconnecting the
input 1 board entirely produces a straight line trace (as if it were
plotting GND); reconnecting it makes it disappear. Connecting the
input 2 board to the input 1 connector produces the expected effect --
the signal on input 2 displays on the input 1 trace.

There are no burned spots, no leaking / puffed out electrolytics, or
any other obvious defects. I've reflowed all the solder joints. How
can I leverage the fact that I have two identical boards -- one
working, one dead -- to find the problem?

Thanks a lot!
Ben

PS -- This was to be my first scope, so all I have is a DMM...