The Kendrick Dew Remover Heater System.

The Kendick Dew Removal System relies on a pulse width modulation system feeding a pass transistor, the output of which feeds four phono sockets arranged in a parallel format.

This design allows for maximum transfer of power to the output independent on load, therefore efficiency, during the device's on period. The period being settable from 1Sec to 7Secs from the front panel control, the on-off duty cycle changing accordingly.

The Kendick site has further information regarding sizing and typical Ampere/Hour ratings for each of it's heater strips.

Modifications:-

1. My unit was a Mk4 design which required anti input polarity protection. Later models had inbuilt diode bridges to give that protection, but adding further difficulties of inbalance to the 0V grounding system.

A single 5Amp schottky diode was added in series with the +ve input supply, resulting in the protection of the unit if the input polarity is reversed. This has proved a worthwhile mod, as on a number of occasions when assembling the telescope in the dark and getting the polarity the wrong way round, has resulted in failure of a number of elecronic component failures.

2. The Phono connection system employed by Kendrick relies on a compression fixture between the socket barrel and the PCB mounting hardware. This compression joint had become loose over a relatively short period of time and created a resistive connection into the Dew Heater strip, producing localised heating instead of transferring the heat into the heater strip.

Solder the phono socket barrels to their respective mounting frame as soon as possible to secure a permanent fix. Also resolder the component PCB mounting pads and allow the solder to flow properly. A number of cases have reported breaks between the component and the PCB, resulting in failure.

3. The pass transistor became very hot when used with the larger heating strips. These strips can consume up to 2Amps continuous at max heater setting.

Increasing the component's dissipation by adding a heatsink, along with a smear of heatsink compound, remedied this problem.