Batboard

Hi folks,
I was called into another mess made by a previous shop (I seem to get the really, really fun ones lately). This one has me a little stumped. I have a CPI tone remote adapter TTP1 attached to an Icom aviation mobile. The rx audio from the radio to the remote is fine (can adjust with internal pots and adjustments on the console cards). The tx audio from the remote to the mobile is fine EXCEPT for a howling feedback/intermod-like sound when the PTT is pressed and no one speaks. This sound also appears during pauses in normal speech. This symptom has been duplicated by both the console and a separate single-channel CPI deskset I use to troubleshoot these issues. The problem does not appear when the TX is locked via the DIP switch setting on the bottom of the CPI deskset (there is no tx audio, just the TX function tone is sent). The problem does not exist when I force the console to send the tx tone with no voice. This problem does not exhibit with the Vega tone remote adapter that was replaced by this equipment. The problem does not exist when talking quickly into the mic at a normal volume. The TTP1 is new out of the box (minus me adjusting the rx and tx audio volume. For full disclosure, I am not very experienced in adjusting levels (due to lack of training and equipment), but I seem to do well with the dB setting on the Fluke and a basic o-scope. I have triple checked the pinout. The only common items in this setup between the shop and customer's location are the power supply and the remote adapter. Maybe a tone filter is jacked? Thanks, FatBoy.

By any chance is this "howling feedback" sound like the 2175 PTT holding tone? In a tone controlled system
there is a 2175 Hz tone that is used to hold the TX keyed. A couple of points come to mind here. One, you need to measure the audio level from the console and determine what level the holding tone (low level 2175
Hz tone) is at. This should be some place between a -27db and a -33 db. The other problem is that you may
have the TX audio level set too high and have caused the TX audio to go way into compression and as such
it has brought up the 2175 tone level. There is a notch filter on the CPI board that notches out this tone.
Hopefully you didn't play with this tone notch filter adjustment.

The instructions that come with the CPI remote adapters are very well wrote. They explain just how to adjust
the audio levels and clearly point out which trip pots to adjust.

Jim

FatBoy wrote:Hi folks,
I was called into another mess made by a previous shop (I seem to get the really, really fun ones lately). This one has me a little stumped. I have a CPI tone remote adapter TTP1 attached to an Icom aviation mobile. The rx audio from the radio to the remote is fine (can adjust with internal pots and adjustments on the console cards). The tx audio from the remote to the mobile is fine EXCEPT for a howling feedback/intermod-like sound when the PTT is pressed and no one speaks. This sound also appears during pauses in normal speech. This symptom has been duplicated by both the console and a separate single-channel CPI deskset I use to troubleshoot these issues. The problem does not appear when the TX is locked via the DIP switch setting on the bottom of the CPI deskset (there is no tx audio, just the TX function tone is sent). The problem does not exist when I force the console to send the tx tone with no voice. This problem does not exhibit with the Vega tone remote adapter that was replaced by this equipment. The problem does not exist when talking quickly into the mic at a normal volume. The TTP1 is new out of the box (minus me adjusting the rx and tx audio volume. For full disclosure, I am not very experienced in adjusting levels (due to lack of training and equipment), but I seem to do well with the dB setting on the Fluke and a basic o-scope. I have triple checked the pinout. The only common items in this setup between the shop and customer's location are the power supply and the remote adapter. Maybe a tone filter is jacked? Thanks, FatBoy.

I agree with Jim202 and add this: take a look at the programming for the ICOM mobile and see if AGC can be defeated. A telltale of an AGC problem is if the errant noise is suppressed by voice but then ramps up during non-voice times.

Thank you guys! I will look at the system again. My main issue is that I do not have the experience or equipment to measure or generate an audio signal of a certain dbm (ie-what equipment should I be using to generate and measure a signal in dbm?). Several VOM meters will report in DBM, but because I do not understand what/how I am doing it, I feel that my efforts are not repeatable. FatBoy.