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

On 1/23/2021 11:26 AM, Mike Rivers wrote:
On 1/23/2021 3:24 AM, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example,
either at one of the speaker voice coils, or at the crossover.


An "audio VTVM" is a voltmeter with a high (usually greater than 1
megohm) input impedance AND has a flat frequency response over the full
audio range of 20 Hz to 20 kHz.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use
my regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?


It depends. Is this an analog VOM, or digital multimeter? And what
frequencies will you need to measure? Most analog meters will have
reasonably good frequency response over the full audio range. On the
other hand, most service-bench DMMs tend to poop out above around 500
Hz. They figure you only need AC volts to measure power line voltage.


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

The organ stop tabs used are Diapason 8' and Tibia 8'. 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.

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



My meter is NOT a True-RMS meter, so I know if the signal is too far
off from an ideal sinusoid, the RMS reading will be off, but I am
hoping it will be close enough.


That's true. It depends on what you need to measure when you're doing
your troubleshooting or tune-up. The meter is calibrated to read the
correct value for a sine wave within its working frequency range. If you
need to make voltage measurements of organ notes, your meter won't be
accurate - but that may or may not matter, depending on why you're
making that particular measurement.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no
signal?


I don't think this is talking about noise floor, but it depends on the
context.

"+5 dB" by itself is a ratio. It tells you how far above a reference
point your measurement is. For example, if you're measuring frequency
response, typically your reference is the voltage at 1 kHz.

5 dB represents a ratio of 1.78 [How do I know this?
http://www.sengpielaudio.com/calculator-db.htm] so if your reference
signal is 1.00 volts, +5 dB would be 1.78 volts.

Unless you're measuring power, in audio, for practical work, we use dBu,
which uses 0.775 volts as the 0 dBu reference. I won't go into the
explanation of that number here. But, for instance, if you measure 2.5
volts, that would equate to +10.18 dBu.

To fully answer your question, we need to know what you're measuring and
why. What does the manual say?


This test procedure appears to be a simple level-setting
adjustment for volume, for the different groups of stop tabs in the organ.

Some measurements are at the voice coil of the 12" speaker, and
some are measured at a crossover network.

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". I've never had this model (a bit before my
time!).