Kenwood TK-790 Distorted and Low Audio

[Bw1332]

Hello,
The Fire Department has a TK-790HP that is connected in one of the engines. The speaker sounded almost as if is was blown, so I replaced the speaker with one that I knew was working, however the audio still sounds crackly. Almost as if someone was speaking through a tin can. The only thing that I can think of is that the audio amplifier is starting to go bad. Does anyone have any advice on where to start, or any similar issues? Thanks in advanced!

 

[buddrousa]

What does it sound like if you use a service monitor and send a 1k tone on the receive frequency.
This will tell you if it has drifted in frequency or has other problems.
Backyard trouble shooting will only go so far and then you need shop tools.

 

[mmckenna]

I concur. A TK-790 is going to be a well used radio, and if it hasn't had a recent trip to the service monitor, it likely needs one. I'd start there, it needs preventative maintenance anyway, and that will likely resolve the issue. If not, you'll have at least narrowed the issue down.

 

[ramal121]

Not super easy but a test can be made. The PA amp is a stereo type. You could plug a speaker to the "B" side of the amp via the OS2 output on the connector at the back of the brick. Then program a button for External Speaker. Pressing the button will shift audio to the other side of the amp and out the OS2 pins. If that sounds OK, then the "A" side of the amp is hosed. TK-X90 audio amps can be zapped fairly easily.

 

[Bw1332]

We discovered the issue! There is a headset system set up through the RS232 port off the rear off the radio that was causing the issue. As soon as the plug was removed, the speaker was crystal clear as it should be. Thank you for all of the input everyone.

 

[ramal121]

Aha! That'll do it. Receive audio (AFO) out of that connector is DC coupled. Depending on what you hook that to you can load down the bias voltage and cause that issue. Adding a series capacitor (0.1 or 1.0 uF) to the AFO line should clear things up.

 

[buddrousa]

Thankyou for posting the answer as it will help others down the road.