Based on the behavior as well as some reading, my suspicion
was that a tantalum supply bypass capacitor had failed in a short mode. On removing the unit from the frame I
measured the resistance between A18(+15) and ground as well as B18(-15V) and
ground. Eventually it was identified
that the +15V was shorted to ground.
Fortunately, the problem was not with the YIG oscillator or first mixer (components
I had not hope of repairing or replacing).
After a lot of work the offending board was identified as
the A1300 board. This involved basically
pulling board connectors until the short is removed. Sounds easy until you note that: a) the thing
is assembled like a Swiss watch, b) the manual does not include a wiring
harness diagram. The A1300 board is
referred to as the “Phase Lock Board”, however, there is quite a bit of
functionality on it besides a PLL including the YIG driver and marker generator. With the +15V supply to this board removed,
the unit was tested in the frame with a successful display (flat line trace
with reference markers) and calibration signal.
The following picture shows the A1300 pulled up from the rest of the assembly
(yes, the middle boardset is on rails).
No capacitors on the board showed any visible indication of
failure (discoloration, charring) - apparently this is normal. I
started to make a list of candidates to remove, however, quickly realized that
part of the board was unpopulated but shown as populated in my manual
schematics. This tells me a couple of
things a) my particular unit is not as old as I thought it was, b) my
schematics were not entirely correct. By
going through the candidate list and removing the capacitors and measuring +15V
to ground at each step I was able to find the offending entity on the second
step. The two removed were bypassing
+15V for the PLL supply line. The
offender was a 100uF +20V, of which I seem to be out of. Based on the location of the capacitors and
artwork (under a shield limiting side access with several pads/traces under and
around) I could not just use some near value 0805 SMTs. In the A1300 picture above, the capacitors
are located under the shielded area on the lower right under the large cable
harness looping over it.
The unit was tested without the bypass capacitors (the
thinking being that I did not need the PLL working or working well for basic
use and first test). The unit was tested
with the calibration signal at various frequency spans and reference levels
with good results.
As part of this exercise, I decided early on that my
probability of success was low so I ordered an alternative that fit within my
budget and needs. This was an RF
Instruments SA0314. No one would confuse
it with an Agilent or Rhode and Schwarz with respect to size, performance or
cost. Based on initial testing it is exactly what I need and is performing
well. It provides better performance for
many measurements than older spectrum analyzers, fits in your pocket, and
exports digital data.
