Nothing like having to fix your own test gear? Had to dig out the old Marconi Modulation Meter the other day. Been sat doing nout for ages. The FM section needed a good clean worked all OK, have for the fact that I could not get the calibration to work. That was a dirty potentiometer on the FM limiter section. Tried the AM section today to check a local broadcast AM transmitter. Hoping to see about 30% modulation, but nothing. Very long story cut short……This Marconi kit built in the 60s-80s is so well made. Even comes with an extension board inside the case to make repairs easier and a service manual with diagrams (schematics if you are from the US of A). After much bad language, eventually found an open circuit filter coil in the AM detector section. Why? God knows, when all it does is to deal with a few Mhz at almost zero current. Replaced with a near equiv. and away with the mixer. All as good as new. Okay, has no fancy displays, no LCD readouts, but tells you as it is.