Pioneer SX-1000 motorboats on the AM Band

[vortalexfan]

Hello everyone, recently my Pioneer SX-1000 TD has started randomly Motorboating (it sounds like a helicopter taking off or a Tommy Gun being fired) when its on the AM Band when the Bass and Treble controls maxed out, but when the controls are turned down to the "Flat" position (both tone controls set to the middle position) the motorboating stops and the receiver works fine then.

I've looked over the service information and I don't see anything obvious that could cause the issue (I was expecting to see an equalizer board like in my Pioneer SX-737, but all I see is a Control Amp board).

I was thinking perhaps maybe a bad electrolytic cap or two in the control amp board but I'm not sure which ones to check as there are over 10 of them on that board.

Also as a side note, when the receiver starts motorboating, if you switch modes to like the FM Band, Phono mode, tape mode, etc., the motorboating continues to go on in all of the different modes unless you turn down the bass and treble controls or if you turn off the receiver and switch it to one of the other modes besides AM and then turn it back on and then it won't do it anymore.
Also if you just have it on FM/FM Stereo or any other mode to start with and not AM and you have the tone controls maxed out then it won't motorboat at all, it only does it in AM mode.

Any ideas as to what might be causing my problem and where I might check to try and resolve the issue?

Thanks for your help.