I have an old Cetrek P770 autohelm which operates perfectly. For a long time I
have experienced interference in my VHF radios (both DC and handheld) when the
unit is operating. Previously it was an annoyance, but since I installed a
better and more sensitive VHF the situation has become less tolerable.

I am about to write Cetrek, but groups like these are much more responsive at
times. Here's the info: The interference is a pulsed white noise (not
precisely like 'motorboating', a well known type of interference). I had a
radio/computer/antenna guru, ex-professor now working for HP, look at it. He
didn't have any answers but was most impressed that the interference 'broke
squelch'. Perhaps if I'd had an electronic schematic of the units he would have
had better luck.

Strong signals are able to be heard with no interference, weaker ones with some
interference heard, and some even weaker station signals are broken up into
packets by the interference. Simply listening to near signals isn't always an
option in two instances: A) When operating in remote areas (Desolation Sound)
where even the Coast Guard's signal may be weak, and 2) When operating in areas
where use of low power communications is either mandadory or prudent.

I would appreciate hearing from anyone who has had a similar problem with any
unit, or anyone with knowledge of conditions like this.

Chuck

Welcome to the painful world of RFI (Radio Frequency Interference).
This is almost certainly caused by the SCRs that control the steering
motor, pump motor, or solenoids, depending on what type of system you
have. We had it with the beta version of what is now the Nexus
autopilot (which is a great unit) on Swee****er.

First, Cetrek has probably had this problem before and may have a
solution. They (and you) are, in fact, required by FCC part 15 to
solve the problem, as it is a violation of Part 15 to interfere with
legal services (i.e. your VHF).
They "should" take on the problem and fix it, even though it's an old
unit. Whether they do or not is a good test of their customer service
attitude.

Second, the fact that it interferes with your handheld suggests that
it is radiating and not just propagating down the DC lines. This make
life harder.

My first try would be to put a DC power line filter on the DC supply
to the autopilot -- this could be Radio Shack 270-051 or 270-055 (one
is 10A, the other 20A). Put this right next to the AP electronics
box. You might also put one on the DC supply to the fixed VHF (right
at the VHF), which won't help the handheld, but may be an easy fix for
your main unit. (Both the Radio Shack units are outrageously priced,
but easy to do.)

After that, life gets more complicated, and I would find a new expert,
as your present guy doesn't sound very helpful. Hams are often quite
good at this. You could also buy The ARRL RFI Book for US$20 at
www.arrl.org which will tell you more than anyone needs to know about
this subject.

Knee jerk suggestions would be to box the AP electronics in a
conductive box with minimum hole sizes (this may cause problems with
heat, so you might have to think about a heat sink as part of the box
-- be careful, because the SCR heat sinks are almost certainly
electrically hot). I should note that it may already be in such a box.

Put ferrite beads or doughnuts on all wires in and out (this may
interfere with digital signals on those wires, so do it one at a time
-- do the power in and out first).

Move the AP electronics close to the power use -- the motor or
solenoids -- to minimize the length of the cable that is probably
radiating.

Shield the power wires between the AP electronics and the motor or
solenoids and ground the shield. (i.e. use coax, or shielded cable).


Jim Woodward
www.mvFintry.com

Chuck Bollinger wrote in message ...
I have an old Cetrek P770 autohelm which operates perfectly. For a long time I
have experienced interference in my VHF radios (both DC and handheld) when the
unit is operating. Previously it was an annoyance, but since I installed a
better and more sensitive VHF the situation has become less tolerable.

I am about to write Cetrek, but groups like these are much more responsive at
times. Here's the info: The interference is a pulsed white noise (not
precisely like 'motorboating', a well known type of interference). I had a
radio/computer/antenna guru, ex-professor now working for HP, look at it. He
didn't have any answers but was most impressed that the interference 'broke
squelch'. Perhaps if I'd had an electronic schematic of the units he would have
had better luck.

Strong signals are able to be heard with no interference, weaker ones with some
interference heard, and some even weaker station signals are broken up into
packets by the interference. Simply listening to near signals isn't always an
option in two instances: A) When operating in remote areas (Desolation Sound)
where even the Coast Guard's signal may be weak, and 2) When operating in areas
where use of low power communications is either mandadory or prudent.

I would appreciate hearing from anyone who has had a similar problem with any
unit, or anyone with knowledge of conditions like this.

Chuck