I have repaired and adjusted a 2756P and a 494P.
Apart from replacing all old capacitors as recommended by KE5FX
document I also checked all RF boards for "suspicuous" components.
One thing to look out for is that there are several resistors with very
high resistance. Old Allen-Bradley, 4.7, 10 MOhm, etc. that has aged far beyond
their rated tolerance. In my case > 50% (!). One is located on the span attenuator:
R1046 and that one was some 16 MOhms.
There are also low resistance Allen-Bradley types used in the decoupling networks.
They too had drifted and was replaced even though I cannot say that they were
faulty.
Before replacing these resistors, I was able to execute the internal calibration
procedure without any problem only to discover that the analyzer did not
lock the next time it was used in narrow band modes.
The batteries were dead on both analyzers and I replaced them with modern
types. On the 2756 with a larger capacity battery that was placed outside
the PCB (plenty of space in that instrument).
There are also electrolytic capacitors that should be checked and replaced
in the phase lock assembly modules.
After having replaced most axial and radial type electrolythic capacitors, a number of
A-B resistors and having re-run the calibration routine, the analyzers
became stable, not needing any further adjustments.
And a bit OT. I mentioned, before in another thread: The Air inlet holes
for the 2756P are IMO way to few/small. The small Buhler fan ran at full
speed and the air that came out from the PSU was very very hot.
After having drilled a lot more holes on the bottom side close to the
front, the fan now runs at lower speed. I have not noticed any drift or other
instabilities on the analyzer after that modification.
Ulf
SM6GXV
