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New 8.33 radios sound rubbish

I’ve have now put plenty of hours in 6 aircraft that have had new 8.33 radios fitted.

The 3 that have had simple trig radios fitted all sound worse often with an echo on transmit.

The other 3 have had Garmin nav/com’s fitted but have kept box 2 which all have king nav/com’s

Again the new 8.33 box sounds poor. Simply not as clear and again with a touch of echo. Yet if you switch to the 25 box 2 on the same frequency it instantly sounds way better.

Has anyone else noticed this?

I should all radios were fitted by a well respected avionics guy and for a considerable sum.

What model of radios and what type of aircraft? Are intercoms (audo panels) installed?

Administrator
Shoreham EGKA, United Kingdom

I have not with Garmin 225, but I put a high end audio panel in at the same time (GMA 345) which did wonders for sound interference related to the KNS80 and use of its DME. I couldn’t turn on the KNS without hearing a huge amount of interference to the point it was hard to understand ATC with it on. With the audio panel, no problem.

Last Edited by WhiskeyPapa at 06 Aug 07:55
Tököl LHTL

The audio is excellent (much better than the Narco COM850 it replaced) on my Garmin GTR225. I don’t have an audio panel, I use the GTR225’s built-in intercom.

Andreas IOM

Peter

Aircraft 1: Simple removal of a King 96 and replaced with a Trig TY96. Sigtronics SPA400 intercom retained. This is by far the worse with the echo at times making it totally unusable. However at other times even with the same headset its OK

Aircraft 2: Narco 810 replaced by a TrigTY96. Sigtronics SPA400 intercom retained. This unit works OK but the transmissions are not as clear as they were with the old Narco but its certainly usable.

Aircraft 3: Narco 12D replaced with a TrigTY96. Inbuilt intercom used. Works pretty well but transmit and receive are still not as clear as they were but certainly usable.

Aircraft 4:

Aircraft 5: King 175 replaced with a Garmin 255. SPA400 intercom retained. Ok but the transmissions are a little echo and receiving isn’t brilliant. Yet change to box 2 Garmin SL30 and everything is crystal clear

Aircraft 6: 2 king KX155 replaced with a pair of garmin 255’s. Bit of an echo at times and when that isn’t a problem the receive and transmit are simply not a clear.

I did wonder if the SPA400 intercoms were at fault but turning the intercom off doesn’t make any difference.

As for the fact that the receive and transmit are simply not as clear is this a case that the reduced ‘bandwidth’ means that the case with 8.33?

You need to get the installer back and get him to fix it. He may be minting it due to the amount of 8.33 work but this is not acceptable. The bigger problem I see is:

  • why do it badly in the first place (a lack of knowledge on how to run shielded cables, perhaps)
  • why release the aircraft to service in that condition

8.33 has more than ample audio bandwidth. The transmitter is the same. The receiver and only when set to an 8.33 channel has a reduced bandwidth but it is still wider than normal aviation audio. I have a KX165A/8.33 which is superb and sounds exactly the same as the KX155A above it.

BTW it is not necessary for the work to be done by an engineer qualified to work on certified aircraft. You just need one of these to sign the job off. The avionics scene is full of people with no formal qualifications (some good, some bad, some in between, some are very good wiremen but with no understanding of the principles – just like much of the “145” avionics scene really ) and they work under the approval of the hangar owner in whose hangar they are working.

I don’t know much about specific radio installations (been doing electronics since the mid 1960s though) but I do know that the wiring needs to be done very carefully, with the shields grounded in the right place, and not connected to the airframe all over the place because that feeds back airframe currents. Also one has to use good quality coax, like RG400, which most don’t like using because it is expensive and since most customers don’t know this, it makes an installer using it appear to be uncompetitive.

Administrator
Shoreham EGKA, United Kingdom

We had the same replacement as Aircraft 1 above
The Trig is quieter, but has better quality audio, and there is more interference (from the strobe light) but this is caused by the intercom having unshielded cables so I switch it off when flying alone. Generally the radio is an improvement, and the extra functionality from the Trig is a bonus. If all else fails try fitting ferrites?

EGHO-LFQF-KCLW, United Kingdom

I can tell you for sure it must be a duff installation if you’re getting poor audio/deaf radio with a Garmin GTR225. I have one and it is WAY better than the Narco it replaces – better audio quality, much better Tx and Rx performance with the same antenna. No echo. The receiver is most definitely not deaf (from Cumbria I can hear aircraft calling London Info when crossing the French/UK FIR boundary)

Peter wrote:

Also one has to use good quality coax, like RG400, which most don’t like using because it is expensive

RG400 is not really necessary for VHF, especially the short runs that light aircraft have. RG58 is perfectly adequate, unless you’ve got a very long run. A 3m run of RG58 will have a loss of less than 0.5dB on VHF (unless you have a badly matched antenna, but then the solution is to fix the antenna). Also your installer probably doesn’t have a crimp tool for anything other than RG58 :-)

Last Edited by alioth at 07 Aug 09:08
Andreas IOM

It isn’t so much the loss (yes RG400 is a GHz cable) but the leakage through the shield of a crappy coax which feeds back all over the place. See e.g. here for one result of using a crappy coax (by Socata) and if this wipes out VHF comms it isn’t going to be helpful anywhere else either The leaked RF is probably gonna get rectified by various nonlinear semiconductors and mess up the audio. This is why I never believe statements like “wifi cannot interfere with avionics because it is 2.4GHz while ILS is only 1xx / 3xx MHz and any harmonics will be above 2.4GHz”.

I’ve just stripped a few bits I had lying around:

Unshielded intercom wiring is utter negligence. All RF, composite NAV, analog NAV signals (e.g. +GS -GS) and all audio must be shielded cable.

Administrator
Shoreham EGKA, United Kingdom

RG58 (with a PVC sheath) has been banned for years in certified installations because it doesn’t meet the flammability requirements, not just that the screening capability is poor. I don’t recall seeing any installations in recent years where an installer has been cheap and used it.

Avionics geek.
Somewhere remote in Devon, UK.
13 Posts
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