In ZC1's the depth of modulation was
originally designed to be around 30%. Today the reason for this is
not clear although it has been speculated that it was to prevent over
excited signalers yelling into the microphone under battle conditions
causing over modulation and splatter affecting the readability of their
transmissions. Another opinion is that at the time it was believed
that a "strong carrier" was essential to carry the modulation for best
readibility. ie the exact opposite of today's SSB transmissions where
little or no carrier is transmitted.
Regardless of the original reasons, most operators would
agree that in today's world the ZC1 benefits from more modulation.
Over the years many ideas have been tried but those devised by AREC in
the late 1950's, at the time the ZC1 was extensively used by this
service, are considered the simplest and generally effective
enough means of improving the set's modulation.
Take out 100KΏ
resistor R8D and replace it with a 200KΏ
or 250KΏ resistor.
Take out 500KΏ
resistor R4D and replace it with a 1 Meg.Ώ
Connect a 1KΏ
resistor across R19A. This will reduce R19A's value from 500Ώ to
25 or 50 Volt electrolytic capacitor across C6D connecting the negative
end to earth. Note:- There are two similar capacitors mounted on
the back of the chassis where C6D is shown on the parts layout.
C6D is the lower one of the two, when viewed from below the chassis.
Note:- If this
modification is made, a
No.7 Microphones to be used with the set is recommended.
(Reason, ZC1's in original configuration suffer from background vibrator
hum on AM transmissions caused by a problem with wiring in the No.7
microphone. With the increased modulation resulting from the above
modification this vibrator hum becomes much more noticeable. Even
if if you keep your set in original configuration modifying the No. 7
Microphone is worth doing.)