Noel

Two usual suspects and how I would normally diagnose them are...

Power supply coupling (or at least the power supply lead of the EW
acting as an antenna). This is the most likely. Turn on stuff. Hear
the interference? Unplug the power cable from the MicroRecorder but
leave it it running on batteries. Ideally do this don't just switch
off the external power, I want the cable hanging out the EW removed.
Interference gone away? It was coupling via the power supply cables.
Install a choke/ferrite toroid as close to the power connector on the
EW Microrecorder as you can. See past discussions on r.a.s. for
different choke circuits for the Zaon MRX (if it does not go away with
a simple toroid you may want to build a better choke with capacitors
as well). If the interference did not go away when you disconnected
the EW then the problem is ....

RF leakage coupling (i.e. your radio picking up radio interference
leaking out of the EW). This could be via the radio antenna in the
tail or via faults in the radio coax cable near the device causing the
interference. Are there any other problems with the radio
transmissions/reception? Does the coax cable connector look and feel
firmly attached, coax in perfect condition? Does wiggling things back
there make any noises? Try undoing the coax connector from the radio
and reconnecting it. Homemade coax connections are often problems,
either breakage of the center wire or the braid coming loose from the
connector. Crimp type with a long ferrule that support the coax well
are best. If that does not fix things you can just try moving the EW
or wrapping parts in foil, but if that is needed then you should send
the unit back to be checked. I'd also check out in that case if it
interferes with other radios, to try to work out if it is a problem in
the EW or your radio. But the problem is much more likely to be just
plain everyday power supply coupling described above.

BTW shielded or unshielded wiring to the speaker is not near the top
things I would think about. The speaker circuit is low impedance (i.e.
takes a lot of interference to affect it) and likely won't pick up RF
energy in a way you can hear it. And many glider manufactures do just
fine with cheap zip/cheapo rubber speaker cable to the speakers, even
though they should use something better.

And since you already know the problem goes away when you disconnect
the external power this shows that the problem is likely power supply
coupling. So next step is to try a choke or ferrite toroid on the EW,
close as possible to the EW. Yes as others already mentioned, I just
wanted to emphasize that is the right next step, not looking for other
RF leakage. Tinfoil hats can come later if this does not work :-)

Darryl




On Mar 2, 12:23*am, "noel.wade" wrote:
Any electric gurus out there willing to help?

Ever since re-doing the wiring in my DG-300 last year, I get
intermittent loud static on my radio. *It seems to be connected to
whether or not I have ship's power going to my EW MicroRecorder. *The
odd thing is that I followed the Becker wiring instructions and made
sure to use a shielded wire for the connection from the Becker to the
remote speaker/PCB assembly. *I would love ideas on how to help stop
this! *Details below - any thoughts, see anything obvious that I'm
failing to do?

Powered Equipment in the panel:
-----
Becker Radio AR3201
Cambridge S-Nav (LCD and quick-response vario)
Tasman V1000 Vario
B400 (backup vario)
EW Avionics MicroRecorder
iPAQ 4700 (powered through the MicroRecorder via Goddard DB9f & I38C-1
connectors with ferrite rings at each end)

Wiring description:
-----
All connections are done with Anderson Powerpole connectors.
The battery (mounted behind the seat and appropriately fused) provides
power to a central block of connections under the instrument pedestal
- via an unshielded ~6-foot run of 18-gauge Tefzel wire. *The central
block consists of a common power bar and a common grounding bar, both
covered with plastic and with Powerpole connectors soldered to them
(done by a commercial outfit, not me).
From the central block, power is routed through 3 different Klixon
Circuit Breakers (one for the Radio, one for the Tasman & EW, one for
the B400 & SNAV).
From the circuit breakers, the wires route to a block of Powerpole
connectors that are joined together to make a "quick disconnect" block
so that the instrument pedestal can be easily removed.
(NOTE: Anderson powerpole connectors have plastic housings that slide
together to form these blocks, but all electrical connections are
still distinct and physically isolated from each other).
The power wire for the EW route from this quick-disconnect block
through a SPST switch (so I can force the EW & PDA to run off their
internal batteries if I have a concern about ship's power levels).
The B400 also has its power wire run through a 3-position switch to
control whether it is off, running on ship's power, or running on its
backup battery.
The Becker has a simple run of wire from the circuit breaker to the
main harness; and from the harness (attached at the back of the
transceiver), it runs as a big bundle back to an area behind the seat
where the remote speaker & circuit-board are located (this runs
through the same flex conduit as the ship's power line - my options
for routing are limited). *As stated before, the speaker wire in this
harness is a single-conductor shielded wire, with the shield being
used as the ground.

I thought that shielding the speaker wire was all that I needed to do,
to isolate it from noise generated by the other systems; but
apparently that's not enough.

A little help, anyone?

Thanks, take care,

--Noel