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G8MNY > TECH 14.11.25 17:12l 113 Lines 4885 Bytes #8 (0) @ WW
BID : 45026_GB7CIP
Subj: Marconi Distortion Meter TF2337
Path: JH4XSY<N3HYM<VE3CGR<K1AJD<GB7BED<GB7JED<GB7CIP
Sent: 251114/0809Z @:GB7CIP.#32.GBR.EURO #:45026 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Correction Sep 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Level Meter Distortion Meter
ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ
ｳ--ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ--ｳ I have been working on one of these
ｳ ｳ� ｳｳ� ｳ ｳ old AF automatic distortion meters.
ｳ ｳ '. ｳｳ '. ｳ ｳ It is quite different to the usual
ｳ--ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ--ｳ fiddly variable Wien Bridge deep
ｳ ｳ notch type. So it is ideally suited
ｳ 0 ｳ for setting up tape machines, where
ｳ Function Distort ｳ the playback level and frequency will
ｳ [][][][] [][][][] ｳ never be that steady.
ｳ O o ｳ
ｳ [][][][][][][][] ｳ
ｳ O � ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
In Level Range Mains
Level Ranges:- 30V, 10V, 3V, 1V, 300mV, 100mV, 30mV, 10mV.
:- 30, 20, 10, 0, -10, -20, -30, -40 dBV
Response :- 10Hz - 250kHz @ -3dB
Distortion Filters 400Hz, 1kHz & External.
Distortion Ranges: 30%, 10%, 3%, 1% min to 0.2%
& dBs -10 -20 -40 -40 min to -55dB
Harmonic Response: 1.5x Filter to 130kHz @ -3dB
Mains 230V. odd 3 pin 1cm square socket.
The automatic refers to the distortion measurement does not need the zero level
reference level to be set first. (But is must be reading 30-100% on the level
meter scale.) This is done not by a very low distortion AGC system as you might
think, but a cleaver meter movement with 2 coils and no return springs. One
coil is fed with rectified DC from the input level detector 1 and acts as the
return spring and the other is fed from an 2nd detector from a high pass filter
that removes all of the fundamental, so it just the harmonics. The resultant
meter deflection is the distortion fraction.
In ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ ACﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ
oﾄﾄｴLevelﾃﾄｴInputﾃﾄﾂﾄﾄﾄﾄｴLevelﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｴLEVEL METERﾃﾄﾄﾄﾄﾄｿ
ｳAttenｳ ｳ Amp ｳ ｳ ｳDet 1ｳ DC ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ ｳ
ﾀﾄﾄﾄﾄﾄﾙ ﾀﾄﾄﾄﾄﾄﾙ ｳ ｳ ﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ ｳ
ｳ ﾀﾄﾄﾄﾄﾄﾙ ﾚﾄﾄﾄﾄﾁﾄﾁﾄﾄﾄﾄﾄｿ
ｳ ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ ｳ DISTORTION ｳ
ｳ oﾄｴ400Hz HPFﾃﾄo ﾚﾄﾄﾄﾄﾄｿ ﾚﾄﾄﾄﾄﾄｿ ｳ2 COIL METERｳ
ｳ ﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄｴ ｳ Set ｳ ｳLevelｳ ﾀﾄﾄﾄﾄﾂﾄﾂﾄﾄﾄﾄﾄﾙ
ﾀ>oﾄｴ 1kHz HPFﾃﾄo<ﾄｴGain ﾃﾄﾄｴDet 2ﾃﾄﾄﾄﾄﾄﾄﾄﾙ ｳ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ ｳ Amp ｳ ｳ ﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
oﾄ External ﾄﾄo ﾀﾂﾂﾂﾂﾂﾙ ﾀﾄﾄﾄﾄﾄﾙ DC
^ﾄﾄﾙDist Range
It just uses simple RC transistor Amps and bridge diodes for the electronics.
The notches are sealed LC high Q units. (M derived?)
There are only 3 presets:- Level gain, Distortion gain, & input amp Bias.
DISTORTION MEASURING PRINCIPLES
f1 f2
ｳ f1 0dBﾄｴ | ﾚﾁﾄﾄﾄﾄﾄﾄ ｳ f1
Levelｳ ｳ f2 ｳ | ｳ High ｳ f2
ｳ ｳ ｳ f3 ｳ | ｳ Pass ｳ ｳ f3
ｳ ｳ ｳ ｳ f4 f5 -60dB_ｳ___|_/ Filter ｳ ｳ ｳ f4 f5
ﾀﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾁ ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ ﾀﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾁﾄﾄﾄﾁ
Distorted Signal Filter Filtered Signal
The high pass filter must completely remove the fundamental and all LF hum etc.
but leave all the harmonics intact. The detectors (especially the distortion
one) should be a true RMS type, as the waveform will generally be very complex
and nothing like a sine wave. The ratio of f1 to all the harmonics f2+++ is the
distortion factor.
Calibration is done by adding in a 2nd tone (e.g. 3-10x freq) via high value Rs
at say 10% of the level to indicate 10% distortion. The distortion meter
bandwidth can also be checked with this circuit.
1V 1kHz ﾄo-oﾄﾄﾄ10Kﾄﾄﾂﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄｿ
Osc 1 o ｳ ｳ ｳ
_ｳ_ ｳ ｳ Distortion
ｳ DVM Meter
100mV ｳ _ｳ_ _ｳ_
2-20kHz ﾄo-oﾄﾄﾄ10kﾄﾄﾙ
Osc 2 o
_ｳ_
To check the HPF filter is working OK a very pure sine wave is needed, so an LC
tuned circuit after a signal generator is used. But I did find that > 0.5V @
resonance on the meter did make the inductor distortion rise > 0.01% !
Osc 88mH 500mV
50mV 1kHz ﾄﾄﾄ((((()ﾄﾄﾂﾄﾄﾄﾄﾄｿ Hi Z
low Z ｳ Distortion
ｳ 0.28uF === Meter
_ｳ________________ｳ_____ｳ
MODERN ANSWER
Of course with a good quality PC sound card and a Furrier analysis program that
can add up the harmonics distortion measurement should be easy.
See my bul "Marconi 2331A Distortion Factor" & "Simple 1kHz AF osc".
Why don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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