Greetings, gentlemen,

This should have been an easy fix given everything I have on my side,
however it's been anything but. I'm about all out of ideas as to how to
proceed.

In this section of a board there's a fault:

https://www.dropbox.com/s/o4tybr81ce...A_amp.GIF?dl=1

The fault is somewhere in the "voltage to current input amplifier" part.
This is the x-amplifier board that generates the horizontal sweep (a
"sawtooth") for the x plates of the CRO display of an RF spectrum
analyser. Normally I would simply probe from one stage to the next until
the waveform vanished and then I'd know which stage was at fault. I can't
do that with this circuit due to two big obstacles:

The stages are all directly coupled.
They're in vertical pairs with shared supplies.

This makes them all *interdependent* on each other. Wherever the fault
lies is impossible to isolate because owing to this interdependence, ALL
6 transistors are showing 'impossible' DC voltage readings such that none
of them can be expected to function properly and pass a signal on to the
current-to-voltage section.

Here's another real kicker: the instrument uses a *Y* amplifier board
*identical* to the X one in every way. They're interchangeable. So I've
tried doing comparative passive resistance checks between the two boards
expecting to find the faulty area that way, but even the resistance
values on the faulty board are all over the place as well! I would have
thought having an identical board to compare with would have saved my
arse, but it seems not!

Any suggestions? I don't really want to pull individual transistors at
random for out-of-circuit testing on a 40 year old board if I can avoid
it.



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