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ZL1ANM > OLDSET 22.07.09 23:57l 56 Lines 2570 Bytes #999 (0) @ WW
BID : ZL1ANM-571
Read: GUEST
Subj: Re: Interest in vintage radios
Path: IZ3LSV<IK2XDE<DB0RES<DK0WUE<7M3TJZ<ZL2BAU<ZL1AB
Sent: 090722/2245Z @:ZL1AB.#06.NZL.OC #:31764 [AUCKLAND] FBB7.00i $:ZL1ANM-571
From: ZL1ANM@ZL1AB.#06.NZL.OC
To : OLDSET@WW
The early mains-powered broadcast band radios did not have loudspeakers
with permanent magnets (the type that we are familiar with today).
Their loudspeakers had a non-magnetized iron core, on which was wound a
multi-layer winding. The B+ supply (250 vdc. or more) that supplied the
RF, IF, and audio stages was fed through this winding and the current
flow magnetized the iron core, creating an "electro-magnet".
The winding's other function, in conjunction with the power supply's
electrolytic capacitors, was to smoothe the B+ supply, performing in the
same way as a power supply filter choke.
All of the radios I saw used a length of cable and a plug to connect all
four loudspeaker connections to a socket on the radio chassis. Thus,
the loudspeaker could be quickly connected to an adjacent radio for
fault-finding or comparison purposes.
If the electrolytics dried out a little and started to fail, the resulting
increase in hum was countered somewhat due to the hum induced in the iron
core being out of phase with the hum in the voice coil audio leads, but
only if the phasing had been carefully observed! If a careless serviceman
happened to reverse the connections to the electro-magnet, or any of the
audio wiring including the output transformer primary or secondary leads,
the hum level would be adversely affected.
This type of loudspeaker became obsolete in the early 1930's (I think)
when permanent magnet speakers began to be fitted.
The hum-bucking coil is an interesting adjunct to the electro-magnet
loudspeaker. I don't recall ever seeing one fitted to a domestic radio
here. Langford-Smith says this:
"Hum-bucking coil
Some electro-magnetic fields are fitted with a hum-bucking coil connected
in series with the voice coil such that the field coil induces equal hum
voltages in the voice coil and hum-bucking coil. By connecting the hum-
bucking coil and the voice coil in series opposition, the hum due to
ripple in the field is minimized."
Radiotron Designer's Handbook,
Fourth edition 1952.
page 840.
It may be that the hum-bucking coil was never applied to domestic radios
but it may have been an important hum-reducing technique in the electro-
magnet era for situations that demanded very low hum levels, such as movie
theatre sound systems, public address sound systems, and recording studio
sound systems.
73 de Neil ZL1ANM
T4 1.5à24
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