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G8MNY  > TECH     24.11.23 08:13z 115 Lines 4872 Bytes #16 (0) @ WW
BID : 60924_GB7CIP
Subj: Marconi Distortion Meter TF2337
Sent: 231124/0800Z @:GB7CIP.#32.GBR.EURO #:60924 [Caterham Surrey GBR]

By G8MNY                                   (Correction Sep 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

  Level Meter  Distortion Meter
--旼컴컴컴컴컴왯컴컴컴컴컴커--   I have been working on one of these
  뇌          납               old AF automatic distortion meters.
   '.        납 '.             It is quite different to the usual
--읕컴컴컴컴컴牡컴컴컴컴컴켸--   fiddly variable Wien Bridge deep
                                 notch type. So it is ideally suited
        0                        for setting up tape machines, where
     Function     Distort        the playback level & frequency will
     [][][][]    [][][][]        never be that steady.
 O                          o 
 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 & no return springs. One coil
is fed with rectified DC from the input level detector 1 & acts as the return
spring & 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 旼컴컴 旼컴컴    AC旼컴컴              旼컴컴컴컴컴
o컴퀽evel쳐퀹nput쳐쩡컴캑Level쳐컴컴컴컴컴컴캑LEVEL METER쳐컴컴
   쿌tten  Amp      쿏et 1 DC           읕컴컴컴컴컴     
   읕컴컴 읕컴컴          쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴커 
                       읕컴컴                         旼컴컨컨컴컴커
                      旼컴컴컴컴                       DISTORTION 
                    o캑400Hz HPF쳐o  旼컴컴  旼컴컴  2 COIL METER
                      쳐컴컴컴컴     Set   쿗evel  읕컴컫컫컴컴켸
                   >o캑 1kHz HPF쳐o<캑Gain 쳐캑Det 2쳐컴컴컴 
                       읕컴컴컴컴     Amp        쳐컴컴컴컴
                     o External 컴o  읓쩟쩟  읕컴컴 DC
                                        ^컴Dist Range

It just uses simple RC transistor Amps & 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.

                                      f1  f2
       f1                     0dB캑   |  憫컴컴컴       f1
Level     f2                        |   High             f2
             f3                    |   Pass                f3
                f4  f5   -60dB____|_/  Filter                 f4  f5
     읕컴좔컴좔컴좔컴좔컴         읕컴컴컴컴컴컴     읕컴컴컴좔컴좔컴좔컴
       Distorted Signal                 Filter            Filtered Signal

The high pass filter must completely remove the fundamental & all LF hums 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
& 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 훟-o컴10K컴쩡컴쩡컴컴커
 Osc 1   o                   
        __               Distortion
                      DVM    Meter
100mV                 __    __
2-20kHz 훟-o컴10k컴
 Osc 2   o

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 컴((((()컴쩡컴컴 Hi Z
 low Z                 Distortion
            0.28uF ===   Meter

Of course with a good quality PC sound card & 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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