Albrecht AE 5890 EU - Background Hum on Transmit

 This radio is also interesting to Ham Radio enthusiasts as it can also be used on the 10m band with 10W AM, 30 W FM and 30 W SSB. The EU at the end denotes that it meets all the European CB Radio standards.

This one came in with the owner complaining of an occasional background Hum on Transmit. The first thing to do was to  reproduce the problem being reported. At first, the radio seemed just fine but after turning on some nearby noise sources in the shack, I could hear them being picked up by the microphone cable. If I wrapped my hand around the microphone cable about half way down and then reached out towards some nasty LED lights, that I keep in my shack to reproduce ambient noise, I could hear the hum clearly being amplified.

At first, I suspected that the microphone shielding had perhaps come off, so I checked that first. However, shielding was intact but the noise was being amplified like crazy, as if no shielding were present at all.

A quick look at the circuit diagram, suggested the smoothing caps C419 and C425 might be toast:

Also, following the mic connector upward, there is another smoothing cap C20 at the microphone input to IC10:

The old caps were removed, and tested. Indeed all were leaky.

New caps were fitted and the radio retested. The hum was gone!

Looking at the other caps in this radio, I imagine it will not be long before a full recap is needed.