Cobra 142GTL pin 10 of audio chip not connected

[Lkaskel]

Good Evening Everyone,

So, I have a Cobra 142GTL on the bench for an alignment. The radio does TX and RX normally but could use some minor adjustments. I was looking through it and noticed that C118 had been upgraded from a 470uF to a huge (in size) 1000uF 50V cap that was blocking VR6. I decided to look through this section and noticed that pin 10 of IC4 (audio IC) had been lifted and is not in circuit. The schematic shows pin 10 connected to a 47uF cap and then to C118. Has anyone seen this before and if so, what was someone trying to accomplish?

Thanks as always!!!

 

[brandon7861]

The 7222 datasheet says pin 10 is the bootstrap. Bootstrapping an audio amplifier increases the input impedance and gain. I imagine that cap failed in some way, caused an issue, taken out of circuit, and because of the reduced gain, someone thought a bigger output cap would make up for it.

Unless someone had a louder mic and had to reduce the gain on purpose, I would test the cap and replace it. As for changing 470uF to 1000uF, at 1kHz, thats going from .339Ω to .159Ω reactance. Not much of a difference.

 

[brandon7861]

And I doubt gain reduction was the desired result, otherwise they wouldn't have tried a bigger cap there. So that shoots down my loud mic theory.

 

[Lkaskel]

And I doubt gain reduction was the desired result, otherwise they wouldn't have tried a bigger cap there. So that shoots down my loud mic theory.

As always, thanks for your thoughts!!! I'm going to put pin 10 back into circuit and see what happens.

 

[nomadradio]

Rather than reconnect the old capacitor, I'd just replace C114 the 47 uf cap that has the positive side on pin 10 and the negative side on pin 9. Use a 25-Volt or higher-rated part. The original is probably marked 10 Volts.

If you have one, of course. And maybe someone unhooked it pursuing someone's "secret sauce" mod found on the internet. Unhooking C114 won't make the radio's transmit signal any meaner than it was.

73