VU meter adjustment (We’re nearly there!!!)

[nevetslab]

These Yamaha ones are definitely of the type that g1 describes: fixed centre screw which doesn’t turn with the zero adjustment lever. And yes, I would assume that the sealed screw applies tension on the bearing. In fact, I’ve seen a YouTube video where someone releases this centre screw (melts the lacquer with a soldering iron) to restore movement to a stuck needle.

And I think Helmholtz is right about how these meters are calibrated: I really cranked it up and did get the meters to deflect all the way, but it needed a line level source at a instrument level input setting to achieve this! Some variation on sensitivity between the two meters so there could be some drift on those caps...

I’m used to using meters on desks to set input levels, so these are of limited value for that!!!

g1's quote from the User manual confirmed my suspicions on the meter calibration. Now, looking at the L-Pad made by the 24k input resistor and the 330k shunt resistor, that has very little attenuation. The schematic is blurry, but looking at the PCB layout drawing, I see they clearly ARE 330k following the 24k input resistor. I hear what you're saying about the meters would be better suited as signal level meters, as we're used to having on a mixer, rather than measuring the signal level arriving at the speakers. It may be the relative sensitivity of the meter movement itself isn't sensitive enough to modify the front-end circuit to them, and feed them from the line output of the mixer, ahead of the power amp. I'm guessing these meters are a DC meter movement, requiring the two signal diodes to full-wave rectify the AC signal being applied from the power amp. An 'easy way' (tedious, no doubt) would be to move the input cable signal lead to the junction of the 24k/27k/330k resistors and see what the signal level is to get 0VU. Not sure if you have the test gear to do this. An oscillator with clip leads and an AC Voltmeter to monitor the oscillator's output level would be needed.

[Rhod]

Really interesting, thank you.
My son has oscillators a plenty (the desk will take the output from some synths - the really nice overdriven tone of the mic preamps on this desk is one of the things he's after, as well as the spring reverb and eq) so in theory we could try what you've suggested, nevetslab. On the other hand there may be some merit in relying on other gear to set levels (it will be connected to my son's audio interface which can give a reading for the signal that is being sent to the desk and/or that which is coming from it) and just enjoy the aesthetics of whatever deflection those poor old meters can manage...