Cobra 148 GTL (1994 Malaysia) Audio hum in transmit

[mdniterder]

So I've done my best to get this radio back to specs, it was my fathers and I do wish to keep it.


The unit transmits just fine"ish" and receives amazing but has a hum during transmission (no hum on the PA output). I've tried multiple power sources, battery, and different power supplies. 1 Linear Rigol DP832A and one switchmode Tenma 72-7655. I was thinking something in my garage/bench could be causing the issue but I tried turning everything off in the garage. I ran it on a dummy load and on battery power and a scanner on battery and still a hum. I also confirmed the hum on a 2nd CB radio.

I am at a loss of what could be happening.

Things I've done but had hum before and after

I resolded R131. I checked the alignment and it was off and I realighned it using anti static alignment tools. Set AM to 4 watts and Sideband at 14 watts

Tools ued Siglent SDG1032X, Keysight DSOX1204G Fluke 87V and Diamond SX-200

 

[nomadradio]

Change C18 to 470uf or more. Original value is 330, but this radio has reached the awkward age. Maybe old enough for electrolytic caps to begin failing if the mileage is high. If you only hear this on AM transmit, this cap tends to be the culprit. And if this fixes the problem, consider when you'll be ready to replace the rest of the aluminum electrolytics in this radio. After the first domino falls, the next one won't be that far behind.

73

 

[mdniterder]

I replaced cap 18 with no luck, and yes this only happens on AM not SSB

 

[gcook]

A 30 year-old radio should have all of its electrolytic capacitors replaced, anyway.

 

[Gregory Garner]

Recap it, then go from there. You will need to realign it after the recap also. You should have recapped it first, then aligned it. Just food for thought. All.of these radios are due for recap at this point. Either way, if they sat or had a well used life.

 

[mdniterder]

I will order a kit from Klondike Mike's,

I will then realign it and see if it is still humming and work from there.

 

[loosecannon]

This video explains a change they made in the malaysian 148's.

the video pretty much explains what they did and how to change it to be the same as the older 148's.

this could be the reason for your audio hum.

LC

 

[Gregory Garner]

That is a very VERY good piece of info! Yes, this is very possible. If the radio has been returned to stock (power levels included) this would (should) make a difference in this.

 

[mdniterder]

This video explains a change they made in the malaysian 148's.

the video pretty much explains what they did and how to change it to be the same as the older 148's.

this could be the reason for your audio hum.

LC

While this mod did not eliminate hum, it is a good bit of knowledge.

Thank You