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Paul Dorman Paul Dorman is offline
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Default Question About Organ Service Manual Test Procedure

On 1/24/2021 7:04 AM, Mike Rivers wrote:
On 1/24/2021 6:03 AM, Paul Dorman wrote:

Â*Â*Â*Â* I have a standard Digital multimeter.Â* The test notes go up to
A#4, which is 466 Hz, which might be pushing the frequency response of
the meter.


Â*Â*Â*Â* The organ stop tabs used are Diapason 8' and Tibia 8'.Â* I am not
sure about the Fourier Transform of these waveforms, but hopefully they
are close enough to an ideal sinewave, that the measured RMS value
will be close enough.

Â*Â*Â*Â* But the harmonics of a 466 Hz fundamental, can be far beyond
500 Hz, and hence out of the meter's range.


Â*Â*Â*Â*Â*Â* This test procedure appears to be a simple level-
setting adjustment for volume, for the different groups of stop

tabs in the organ.Â* Some measurements are at the voice coil of the 12"
speaker, and some are
measured at a crossover network.


I think you're over-thinking this. Measurements with your meter will get
you well into the ballpark. At that point, take on the job of "organ
technician" and use your ears. If some notes stick out, adjust those so
they blend in with the rest of the instrument. Not all organs are voiced
perfectly, so get it close with your meter and then touch it up as needed.

Â*Â*Â*Â*Â* I just noticed that the service manual mentions that a model
870 Amphenol is an excellent choice for "measuring audio signals
as low as -50dB".


That's the kind of multimeter that was the standard bench instrument on
a radio/tv repair shop. Because measurements often must be made at the
grid of a tube, the megohms-input impedance of the VTVM, because of the
tube at the front end, wouldn't load down the signal that's there.

The input impedance of a modern DMM is pretty high, so you shouldn't
have any problem making measurements either at the speaker terminals or
the input of the crossover network, even if it's a tube amplifier.

In the Amphenol VTVM era, we didn't use dBu or dBV, and 0 "dB"
represented the voltage required to pump 1 milliwatt into a 600 ohm
load, which is the reference voltage for dBu.

[(0.775 V)^2 / 600 Ω = .001 mW]


If we calculate 1 milliWatt into a speaker 8 Ohm load, it will be:

(0.089 Vrms)^2 / 8 Ohms = .001 Watt

And 5dB above that would be .00316 Watts = (0.159 Vrms)^2 / 8 Ohms.

So it's obviously impedance dependent.


-50 dBu is about 2.5 mV. If this is a tone wheel organ, you'd find
voltages in that range coming right off the pickups (I think - I don't
know for sure), so unless the adjustment procedure involves mechanical
adjustment of the pickups to adjust the level going into the amplifier
section, you'll be fine with the sensitivity range of your meter.

Happy tweaking, Mr. Organ Technician.