broke my kenwood 120s radio no rx now

[nfsus]

heres what happened. i had the kenwood all hooked up and it was working fine. i was jacking around with my power supply setup and believe i sent a 120v spike to the dc side of the multitap bus i use. it didnt damage anything else on the power supply however the kenwood has lost most of its receive capability. vol all the way up and rf gain all the way up i can barely hear a station. so i spiked the front end. tx still works like it should.

so where would i begin to look on this rig? its in the front end i figure but where

 

[buickid]

What is your power supply setup and what did you do to get 120V on the DC side? Something sounds messed up if that's possible without purposely doing that.

 

[nomadradio]

i spiked the front end

If so, the S-meter will not show much if any movement. And if the S-meter is still showing channel chatter, you have an audio problem not a RF problem. Since the DC power input branches off to the speaker-amp chip before the voltage gets dropped down for sensitive receiver circuits, the audio amp would be my first suspect for surge damage.

Unless the S-meter is not moving. If that's true the next suspect would be the 8-Volt regulator that feeds the receiver's sensitive circuits.

73

 

[357]

are all bands deaf?

 

[nfsus]

Yeah.

 

[nomadradio]

About that S-meter. Really does provide a dividing line to begin troubleshooting.

73

 

[brandon7861]

While we wait for the S-meter report, I would just like to add that the schematic for the TS-130s is a slightly better quality scan. Other than power output and a few minor features, they are nearly identical radios.

The power regulation is done on the AF board too, but I would also like the answer to nomads question and see what he thinks before I throw ideas out there. I have a 130s, but I have spent very little time inside it.

I do have a parts 130s in pieces as well, so chances are, if you find the culprit, I may have parts. It all worked besides the band switch when it was gutted.

Edit: The PLL board is gone though, sold that to a guy in Alaska.

 

[nfsus]

I’m going to check that meter today.

I was fiddling around with a ground jumper that was hooked to the radio and was trying to ground to the power supply for some noise reduction and hit the wrong damn terminal and that’s how the spike occurred.

 

[brandon7861]

I am heading to bed soon so I figured I'd share my best guess now.

If the power supply transformer is the isolation type and AC mains was briefly applied to the chassis, the reverse protection diode would make the DC supply follow the chassis potential, but only for the + going part of the waveform. As the chassis is pulled back negative, the + DC supply would stay lifted to the higher voltage as the reverse protection diode is no longer conducting when the chassis drops back down.

For the brief moment the chassis goes negative, I would think that the filter network T1/C10/C11 would protect the 9v regulator and everything after it, whereas the 5v regulator that lacks such filtering would take the high voltage spike. Since the radio was probably floating, I assume this high voltage spike was a reasonably high impedance, which may explain why the whole thing didn't turn to smoke. I would start by checking Q2, the 5v regulator.