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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 10:25 AM, Paul Dorman wrote:
On 1/23/2021 2:02 AM, John Williamson wrote:
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.


Â*Â* Ok, so that's practically the same as the Radio Frequency/Microwave
standard of dBm, where 0 dBm is 1 milliWatt, but usually into 50 Ohms.


Actually, it's not the same, because the characteristic impedance
is different (50 instead of 600 Ohms), so you'll get different Volts RMS
values for the same Wattage.

But the zero dB point is the same 1 milliWatt.



Â*Â* And +30dBm is 1 Watt.



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)?)Â* 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.


Â*Â*Â*Â* This service manual is for a Conn transistor organ (no tubes).

Â*Â*Â*Â* It says "Audio VTVM" for type of meter used in the measurement.


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.)