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AREC Modification to Increase Modulation % of the ZC1 Mk-2


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. 

The Modifications

  • 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 resistor.

  • Connect a 1KΏ resistor across R19A.  This will reduce R19A's value from 500Ώ to 333Ώ

  • Connect 25mfd 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 modification to 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.)


top                                                                                         Septemberl 2010