P1225 race car accessory building question

[Carl]

i'm having a little issues building my race car harness. I'm trying to copy the racing electronics harness and helmet wiring kit but i'm having a problem when i need to RX. The steering PTTand tx audio is working fine, but when i RX i'm earing the squelch breaking but only for 1 second. Here are links to what i'm talking to visualise :

car harness: http://www.racingelectronics.net/index. ... tem=RE4302

helmet kit : http://www.racingelectronics.net/index. ... item=RE705

i'm using a 3 pin stereo 1/4 plug exacly like they do. from my radio to the "black interconnection box" i'm using a original brand new speaker mic cord so i got my 4 wire. Again ptt and mic audio are working fine but i can't RX. I'm sharing the same ground for both mic and earphones, have any tips ?

[Bill_G]

If you unplug from the radio, does the radio work fine?

There's a monitor button on the front. Do you hear squelch noise in the headset after you press and hold long enough for it lock open?

Does this problem happen all the time whether the car is running or not, or only while racing?

If it happens all the time, off the top of my head I'd say you need to go back through your workmanship just in case you missed something. If it only happens when you're racing, does it only happen as you pass the pit on the straight, or all over the course?

[Carl]

right now i'm building this kit, so it happens on my bench. the radio is working fine alone and with other accys. what i'm thinking is i can't share the same ground for both speaker and mic audio or i need to build a little circuit to prevent this.

[Bill_G]

As far as I know they are common in the radio. You should only need ptt, mic hi, spkr, and grd. Sounds like it's time to revisit a manual.

[Carl]

can you get a schematic of all this or explain what you discover in the manual ? thanks !

[Will]

The audio amp is a bridge audio output circuit. The internal and external speaker audio is NOT grounded anywhere.

So the earphone/headphone circuit must NOT be grounded, both 'sides' must return to the speaker/mic jack on the radio.

So you need a FOUR wire/connector interface.

[Carl]

so how they do that with the 3 pin helmet connector ? because thats what they're using.

[Bill_G]

Probably with a transformer buried in a box someplace, or an isolation cap in line with one side of the spkr audio.

[AEC]

Probably with a transformer buried in a box someplace, or an isolation cap in line with one side of the spkr audio.

I don't remember the type of audio amp is used in the P1225, but if it is a single ended amp, then they should be able to get away with 3 wires(leads) and tying the grounds together.
I only have tattered remains of my old P1225 service manual left...it suffered from too many relocation efforts.

I think this radio uses a LM741 Op Amp as the audio PA, but I do not remember.

But using a transformer and Iso-Cap does appear to be workable to keep the audio outputs above ground reference, and still maintaining a balanced output to drive the speaker.

3 leads should be workable....

1. PTT
2. Audio Output
3. Ground/Return

[Carl]

the 3 pin is only for the helmet part, to be able in the helmet to get rx audio and mic audio

[Will]

the 3 pin is only for the helmet part, to be able in the helmet to get rx audio and mic audio

It HAS to be a four wire interface to the P1225 or the bridge audio amp will be overloaded and shut down hopefully before serious damage results. Same for GP300/350 series.

To get the most audio transfer to the helmet spears/earphones you need the full output from the radio audio stage.

Remember the audio output impedance from the radio IS a balanced 20 ohms.