Kenwood ka 3500 loud hum after warmed up

[Kastanza]

Hello. My amp suddenly has a loud hum (through the speakers) after 15 to 20 minutes running. After it gets warm. I removed a 6 plug wall connector and plugged directly into the wall receptacle but the hum came back after 10 minutes or so. I just unplugged the Yamaha wxc50 from the receptacle and from the AUX rca input to see if the hum starts again without any component hooked up.

Any ideas what the problem is? So far after 10 minutes there is no hum (120 hz hum I guess) with nothing hooked up to the amp. It seems to me it starts after it gets warm. Any other troubleshooting I could do (I don't have any testing equipment.

Bad caps? Transformer?

Thanks in advance for any info!!

 

[Ian Finch]

The amplifier is at least 40 years old and not many caps, good or bad originally, can last that long even at a reduced volume level. Unless the main power supply caps have been replaced in the last 20 years, I'd consider all electrolytic caps to be dried out and needing replacement, just as much as the most likely culprits. No doubt, more caps are on their last legs but replace those main electrolytic caps as a priority. IMV, there is little point in spending big dollars on boutique audio grades - modern standard grade caps in recognized brands will be fine here. Replace with like for like ratings or slightly higher voltage or capacitance ratings if necessary to suit what's available.

 

[Kastanza]

The amplifier is at least 40 years old and not many caps, good or bad originally, can last that long even at a reduced volume level. Unless the main power supply caps have been replaced in the last 20 years, I'd consider all electrolytic caps to be dried out and needing replacement, just as much as the most likely culprits. No doubt, more caps are on their last legs but replace those main electrolytic caps as a priority. IMV, there is little point in spending big dollars on boutique audio grades - modern standard grade caps in recognized brands will be fine here. Replace with like for like ratings or slightly higher voltage or capacitance ratings if necessary to suit what's available.

Thanks Ian, good advice. I am sure this needs a good check up. I think my original problem was a ground loop issue. Plugged all electronics into a power bar. Not sure why it took 15 to 30 minutes to start the hum but it seems to be resolved at the moment. I'll try to find someone to check the amp and replace caps.

 

[Ian Finch]

Usually, when you have hum induced by a mains ground loop, it is constant from power up to power down and only varies when other devices are plugged in and connected to the amplifier inputs. If the hum takes minutes to appear, it is most likely to be heat related though perhaps via secondary effects inside an ailing cap, deteriorated wiring, or even the power transformer windings, for example. It could even be due to a fault in another device connected to your amplifier or simply connected to the mains. However, those problems should be easy enough to identify by removing signal leads to the various input devices, one device at a time. Otherwise, just pull the power plug of unrelated appliances.

FWIW, a power bar, strip, board etc. is a good way of eliminating or reducing the size of mains earth loops, if the individual power leads are also short. Problems arise when you use different wall sockets, long leads and possibly different ground circuits for your system.

 

[Kastanza]

The again Ian, great info much appreciated!

 

[mbz]

Would be helpful to know if hum is 50/60Hz mains hum or rectified 100/120Hz possibly
cause by psu issues, granted most causes linked to mains loops.