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Dr Fopp,
Talked with engineering and they have some ideas and comments which I am including here. They stated that the bosses' Glasair 1 FT which runs the VM1000 / EC100 is not affected at any COM frequency, BUT the headsets, intercom, marker beacon receiver and Century 2000 autopilot IS affected. The boss shuts off the altitude hold function if he has to transmit a lot. The intercom and noise canceling headsets will squeal unbearably at frequencies above 130 MHz. He stated that he did a VSWR check using a high quality 'BYRD' meter and found a very high VSWR above 130 MHz. VSWR is a measure of the antenna & coax systems' ability to deliver the power to the airwaves instead of reflecting it backwards on the coax and hence into everything near it. An interesting note is that if the VSWR gets too high (lower is better) the transmitter can be damaged.
The BYRD Meter is a sophisticated instrument for measuring VSWR/power of a transmitter. Note that using the cheap meters that are found in Radio Shack etc may not give reliable accurate results in VHF ranges. Check with avionics / ham radio people about VSWR and its negative effects.
Being specific, we believe the problem IS NOT THE DPU having a sensitivity problem internally. We would be more inclined to think that the IO Board itself is coupling high RF fields into its 'on board' logic either directly (through the air) and/or via cabling attached to it (annunciator light wires/ribbon cable/etc.) If the ribbon were being bombarded, then we might expect to see some disruption of the other indicators on the ribbon cable. Close proximity of high RF sources may be coupling to the board and may require some shielding etc.
Lets start with the following:
1) Are indicators disrupted in coincidence with the annunciator light problem ?______________________________________.
2) Have you connected a 'Mute switch' to the IO Board'?________________________________________________________.
3) If no mute switch, have you installed a jumper in its place accd to manual?______________________________________.
4) Are any other devices attached, such as the Jim Franz annunciator system etc.?_________________________________.
5) Is the IO Board close to transmitter cables/junctions?________________________________________________________.
6) If yes, is the board enclosed in a sealed metal housing and then ground to the aircraft ground plane?_________________.
Let me know on these points and we can see if we can identify and eliminate the culprit.
Best Regards,
Dave
-----Original Message-----
From: Michael Fopp [SMTP:70734.101@compuserve.com]
Sent: Wednesday, September 27, 2000 10:27 AM
To: Vision Micro Systems Tech Support
Subject: Vision Microsystems Problems
Hi,
This was my contribution to the thread on the Lancair Maillist. Is mine a
common problem and do you have any advice on a solution? All my avionics
anttenae are coupled to male/female connectors mounted on the avionics
stack (for ease of disconnection) about 2" forward of the VM DPU. Should I
shield the DPU in some way?
Appreciate your help,
Michael
** This CISMail msg forwarded by Michael Fopp 70734,101 **
----------------------------------------------------------------------
>> Date: 24-Sep-00 09:48:23 MsgID: OUTBOX
MgTo: Lancair Mail List INTERNET:lancair.list@olsusa.com
Subj: Vision Microsystems Problems
Jim Scales problems with the VM1000 system are similar to mine. I have
only 10 hours on my 320 but since the 5th flight I have had all the
annunciator lights come on when I press the PTT. I believe this is due to
radio interference with the VM1000 DPU. For the following reasons. The
first five flights were testing using only two radio frequencies (122.85
and 129.55). These frequencies were for the airport we operated out of and
for along range radio test. The first time I flew cross country I had to
call on a frequency higher up the band (I cannot remember the exact freq
but it was around 132). This was the first occasion all the annunciators
came on since then it has happened quite few times and only when the PTT is
pressed. However, if I let go of the PTT, and transmit again it's okay.
I believe the VM1000 DPU is sensitive to certain radio frequencies and that
it is "spiked" by these on occasion. All my installation is Vision stuff
and so far I have not had fuel indication problems.
I was 'putting up with it' but would be very interested in the experience
of others.
Michael
LNC2 G-FOPP (Cranfield UK)
Dr Michael A Fopp
michael_fopp@compuserve.com
Signed at 17:27 on 24-Sep-00
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