Problem with Kendrick Digifire 7 & Dew Bands

A few years ago I bought a Kendrick Digifire 7 dew heater controller.
images-a.hayneedle.com/mgen/digimarc.ms?...D002.jpg&h=368&w=368
I also bought a 12V DC 10A power supply and wired them up. This worked fine for a few years.

Lately, my power supply stopped working so I bought a new one of similar spec and wired it up. The controller appears to work fine, LED lights come on and I have my Robofocus controller wired up to it with no problems - all working fine.

However, I am having difficulty with my dew heater bands - they don't seem to work. As they were all from the same manufacturer (Dew-Not), I thought it may be a problem with them. As it happens, I needed to buy a new dew band for my 16" SCT, so I got one made by AstroZap as the Kendrick ones are VERY expensive in the larger sizes. I connected this up to the dew heater controller a few nights ago and it worked fine - plenty heat from it. However, I tried it again last night and not a thing from it.

So, I got out my multimeter and checked the voltage of each output. There are 4 outputs on one side which can have their output adjusted by the user. These 4 outputs registered at 12V DC with centre negative.

On the other side of the controller are 2 constant non-adjustable outputs intended for powering focus controllers, cameras, etc. I checked these and they registered at 12V DC centre positive.

One thing I do note is that if I have the pin of the dew heater band plugged in for a few minutes and then unplug it, the metal on the pin feels a little warm.

Based on all this info, does anyone have any idea why none of dew heater bands will heat up?

Regards & Thanks,

Michael.

Michael,

Couple of things which may help.

First you should check that the dewband has not broken inside. i.e. the nichrome wire or resistive tape inside might have broken. If your multimeter tester with a 'continuity' function or at least the capability of measuring very small resistances then measure for a very small resitance just a couple of ohms across the tape while it is unplugged. If there is no continuity i.e. infinity resitance, (som testers display O/L for open loop) then your tape is broken.

The other thing I have found is the phono type plugs break very easily, check the connections on the plug coming from the tape. If there is definitely a voltage coming from the controller (check for arounf 12V at full power scaling down to 0V at 0% then that should be fine.

FYI The continuity function of a tester is often symbolised by a sound icon and the tester should beep when you touch the two probes together. It will display very small resistances up to about 200 Ohms.

Dave,

Many thanks for the reply.

I have three dew heater bands: one for a 4” scope, one for a 12” scope and a new one I got for the 16” scope. I tested the resistance in them with a multimeter to see if there was a break. The values I got were:
4”: 18.1 ohms
12” 6.5 ohms
16”: 4.5 ohms

If I remember correctly going back 15 years to my Leaving Cert Physics days, V = R . I
V= 12V
For 4” Heater band: => I = 12 / 18.1 = 0.66 amps
For 12” Heater band: => I = 12 / 6.5 = 1.85 amps
For 16” dew heater band: => I = 12 / 4.5 = 2.66 amps
IIRC for reading about dew heaters, these values would seem about right, yea?.
So, it would seem that all three are in correct working order. Perhaps there is a fault in the dew heater controller?

Michael.