HUM

[MotorT]

I have a President Madison 1st generation base, with the correct Madison speaker. I get a 60 cycle hum coming through the speaker. It's not affected by the volume at all (except of course if I turn up the volume it masks the hum). It's not always present, sometimes goes away completely, but I'd say about 80% of the time it is present.
This radio is new to me, so I'm really not sure of it's previous history. Is this hum normal? If not, how do I go about isolating and eliminating it?

[MotorT]

Oh, and it doesn't matter what speaker I use.

[Red Warrior]

If running base with a power supply, the hum is coming from the power supply.

[Foxhunter]

Could have a problem with the phone connector jack on the back of the radio for plugging in an external speaker, it is possible. Good chance with an older radio that you have one or more filter capacitors gone bad, allowing the passage of AC into the system where it normally shouldn't be (common cause of "AC hum"). I am assuming because of the 60Hz hum that you have the radio pluged directly into a wall outlet.

Isolate that possibility by pluging the Madison into a 13.8VDC power supply or battery and see if the problem clears up while hooked to straight DC. Although I don't own that model I believe it has dual power hookups both AC and DC, try the DC and see if the noise continues. Hope it helps

[Night Crawler]

Good chance with an older radio that you have one or more filter capacitors gone bad

Isolate that possibility by pluging the Madison into a 13.8VDC power supply or battery and see if the problem clears up while hooked to straight DC. Although I don't own that model I believe it has dual power hookups both AC and DC, try the DC and see if the noise continues. Hope it helps

That's what the problem most likely is and that's what you have to do to check. Also check the rectifiers replace what's needed.

[Century21]

If that wire in the microphone is shorted it will also hum. Do the 13 volt supply test first. If that cures it, it should be a no brainer for a tech to rebuild the powersupply cheap.

[MotorT]

Thanks guys. I just connected it to a DC power supply, and the noise was worse! A LOT worse in fact. This was just a cheapie power supply though that I have never used before. I have yet to connect it to a battery since they're all in vehicles at the moment but I will try that.
It's not the mic, I tried a few different ones and no change.

[Ryder505]

The best bet to get rid of the hum in your madison is replace the filter capacitor. That cap is a big one that is in the power supply. It it about 2 1/2 to 3" long and about as big around as a quarter, maybe a little bigger. Most likely it has a value 3300 microfarads at 16-25 volts. Replace with 4700 microfarad at 35 volts and should take out the hum and also not have any lights dim when you key the mike.

[Ryder505]

Oh MOST IMPORTANTLY make sure to observe the polarity of the cap before you remove it and make sure the new one is put back in hooked the same way the old one was taken out.

[MotorT]

Thanks Ryder505, it does dim when you key the mic that's for sure! Kill 2 birds with one stone, that would be cool. I'll let y'all know what happens!

[TheCBDoctor]

Hi MotorT

Since, you have tried an external power supply I would check and make sure that all the chassis screws are tight. When you say an older Madison I assume you are talking about the 858 chassis with the 4 pin microphone. Did you try a different Mic? If that does not cure the Hum I suggest that you go to the Mic gain potentiometer and the volume control. Scrape some of the coating off of the pot on the inside of the radio controls and solder a ground wire to the pot casings to a good chassis ground.

I just had the same problem with a Cobra 142 GTL and grounding the mic gain pot casing to the closest chassis ground took care of the Hum. Also, make sure the mic jack has all of the coils and caps still in place. They can be found inside on the mic jack board. This is more critical with a 5 pin jack than a 4 pin jack, but is important. Those coils and caps are there for a reason.

The fact that it is intermittent and goes away at times may be due to loose chassis board screws or an electrical devices close by the radio, such as a computer, dehumidifier, or any number of devices. Another thing to try is to take a wire and ground the base station cover to a water pipe or the earth ground of a standard 3 pin AC plug. That would be the screw that holds the faceplate cover to the AC plug. The cover and the board chassis is not at the same ground potential. The cover is actually part of the antenna system ground.

I hope this has been of some help.

Good luck,

[MotorT]

Well hooked up with WR954 who worked my radio over. Wow, what a difference. He ended up taking one of the relays apart and cleaning it, replaced two of the capacitors on the power supply that were way out of spec, cleaned up a few of the potentiometers, and that's just the start of it. Turned the radio down, 14 watts dead key would have surely killed the radio in no time!! Radio works awesome now, I am so happy with it!