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John Williamson John Williamson is offline
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Default Question About Organ Service Manual Test Procedure

On 23/01/2021 08:24, 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.

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?

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.

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?


Given the figures you quote, 0dB will be the "standard for the period" 1
milliwatt in to a 600 ohm load, which corresponds to 0.775 volts RMS, so
5dB in power above that is the given 1.4 volts RMS. The reference was so
ingrained in audio electronics at the time, nobody even considered using
any other voltage reference without specifying it.

As for specifying the use of a VTVM, the standard input impedance for
one of those was as close as could be got to a purely resistive 10
megohms, and as the circuit may be high impedance with either a
capacitive or inductive component, you need to match that on the meter
you are using for accurate results. (I take it the organ uses valves
(tubes)?) If it is a valve based unit, then all voltages will probably
be specified as measured by a VTVM, and if so, using a moving coil meter
(usually with an input impedance of 20 kilohms per volt) will give
incorrect readings, unless the manual specifies the meter impedance. The
manual should tell you which type of meter to use when prodding round
the circuit.


At the speaker, meter impedance is not critical, though using a VTVM is
still specified as most moving coil meters of the period did not have
either true RMS capability or a level frequency response, as well as
having a variable impedance at the test leads, depending on the range
that was set.

(I grew up working with that generation of gear and its limitations,
goodness, I feel old now.)
--
Tciao for Now!

John.