Hi all,
I have been working on a 5110A synth driver with a problem of no
output. After getting a manual I diagnosed a failure of the Step
Recovery Diode in the harmonic generator and sure 'nough, it's open
circuited. (Why is it always these rare parts that fail - SRDs,
tunnel diodes , avalanche transistors...Murphy's list goes on and
on...)
Following advice on this list, I tried several diodes using clip-
leads. I was able to get nice spikes from a couple of different low
freq tuning diodes but still no output from succeeding stages.
The best result is with an unlabelled miniature tuning diode which
gives positive going spikes 30 nS wide and 1 volt in amplitude at the
cathode of the diode. These parameters would improve further if the
diode was properly mounted on the PCB. After passing through the HP
filter network however there is nothing visible at the emitter of the
pulse amplifier transistor and no output. The transistor is a zero
biased common base connected 2N1143 which is a Ge pnp with a Ft of
200 MHz - another unobtainable Murphy favourite :-( Removing it for
testing requires messy surgery.
Any ideas? Have any of the mavens here actually measured the pulse
parameters in this circuit? Is the transistor faulty or is there just
not enough drive to the amplifier? Unfortunately the manual is silent
on the subject.
On the bright side, I'm learning a lot (small consolation).
Any help appreciated,
Morris
