Old Connex 3300 that makes a popping noise when you key up

[Cityboy]

I have an older Connex 3300 that makes a popping noise through the internal as well as the external speaker during key-up. Anybody have a solution to my problem. Thanks

 

[74IN]

Change the mic.

Sometimes the key makes the noise.

 

[Cityboy]

Not a mic problem; the stock, RK56, and astatic 636l all do it. It makes a slow dissapating noise when I turn the power off. Keying up causes a kind of pop noise and turning the power off causes it to have a slow dissapating noise like a capacitor is leaking down. I suppose it probably a cap problem. Any help where to look?

 

[Cityboy]

To add to the information the radio is one of the original 3600 boards and has a single final. It also pops when echo is turned on and will pop when letting go of the key. It only does it on key-up with the echo off. My thoughts is that it is a capacitor near the transformer; one of them has been replaced with a 1000uf 16v instead of a 1000uf 25v cap. Any thoughts?