What could be causing a Mains (50Hz) Hum in Right Channel only of my AKAI AM-35 Amplifier?

[NicholasD]

Strong 50Hz (mains) Hum with Harmonics coming from Right Channel Only

• NicholasD
• Jul 19, 2018

So I have this old(ish) AKAI AM-35 AMP that I am quite fond of & there is a solid 50Hz HUM coming from the Right Channel of it. This occurs with no inputs connected. So it is nothing to do with an Audio Ground Loop or the like. The Left Side is practically silent even with the volume knob at full.

*As the picture spectrogram displays it is clearly a mains hum with harmonics (We are on 50Hz 240V here in Australia) I recorded the audio directly from the speaker and analysed it.

*The noise increase as the volume is increased.

*The audio tone / quality does not seem to be affected... Just the HUM

*As mentioned NO INPUTS connected

*AMP plugged into UPS to rule out dirty power

*AMP has CHANNEL A and CHANNEL B. The noise occurs on both Channels on the Right Side.
No noise on either channel on the Left Side.

How could this occur only on one side? Is the power circuit such that it is independent to both sides? So it could be a single bad capacitor in the Power Circuit? What areas would you be inclined to suspect for this problem? What CAPS should I replace / check? I'd rather hear from someone before going ahead and replacing all of the caps. Could it be also a bad transistor or is this unlikely?

I have inspected the circuit board on the Voltage amp and the Driver on the right side and re-soldered any slightly suspicious joints. So I doubt it is a cold solder joint.

Any thoughts from an expert would be much appreciated... Thanks in advance!

(Ps in the photo of the right side I have currently removed transistor TR23, as it looked a bit suspicious.)

 

[elnaldo]

50 Hz usually points to some ground issue. Is the noise present with something connected to the input?

I'd suspect from some soldering or ground connection at the preamp section.

Also, proximity to the AC wiring can cause that, so check inside the amp, the AC wires are twisted and move them far from the signal wiring.

 

[Hunted11]

In your 3rd pic of the amp there is a wire that looks like it has a ball of electrical tape on it. Is that a splice, or was it burnt. Check the other wires in that area to be sure they are not affected by whatever that is.

 

[Scott75s1974]

See if moving/rerouting any of those wires that cross the transformer affects the sound? Are those grey wires signal carriers?
Check local DC filtering caps on the Right side amp circuit?

 

[NicholasD]

In your 3rd pic of the amp there is a wire that looks like it has a ball of electrical tape on it. Is that a splice, or was it burnt. Check the other wires in that area to be sure they are not affected by whatever that is.

There was just a tiny knick /cut in the insulation of one of the wires so I covered it liberally with electrical tape to be safe Thanks though.

 

[NicholasD]

See if moving/rerouting any of those wires that cross the transformer affects the sound? Are those grey wires signal carriers?
Check local DC filtering caps on the Right side amp circuit?

Thanks for this - sounds promising... So, the wires are all not in their usual positions currently due to the fact that I had to remove the board. Of course I will tie them all out of the way... I am much more inclined to think that it is a component because the wiring was cable tied in its factory position before I started work... Sorry, can you point out to me exactly which capacitors are the "local DC filtering caps on the Right side amp circuit".... (colour, picute, location, number??) that would be a great help, thanks

 

[elnaldo]

I'd start routing the wires as far as possible from the transformer. Transformer Hum gets into the wiring, even if the factory did it that way. I've fixed at some amps with buzzing caused by the wires too close to the transformer.

 

[Scott75s1974]

Thanks for this - sounds promising... So, the wires are all not in their usual positions currently due to the fact that I had to remove the board. Of course I will tie them all out of the way... I am much more inclined to think that it is a component because the wiring was cable tied in its factory position before I started work... Sorry, can you point out to me exactly which capacitors are the "local DC filtering caps on the Right side amp circuit".... (colour, picute, location, number??) that would be a great help, thanks

I don't see any obvious local DC filter caps on the boards. Might be easier to see on the schematics. I'm not really expert enough to say. And I know nothing about this circuit. Maybe C19 on the drivers?

Signal wires should not run next to/parallel with power wires. If they have to cross, should be as close to right angle as poss.
I'd imagine there is some predictably shielded and unshielded territory around a standard rectangular transformer. I don't know what it is though.

I also see that R44 is bent a bit over close to the inductor L1 on the R driver circuit as well. It also seems mounted a lot higher than its twin on the left driver. Probably nothing but if its easy to bend farther away, worth a try.

 

[Hunted11]

If the hum increases with the volume, I wound check every thing ahead of the volume PC board. Also check the jumpers at the processor connector on the back of the amp. I would clean all switches and controls and check all grounds connected to the RCA jacks on the back of the amp.

 

[NicholasD]

If the hum increases with the volume, I wound check every thing ahead of the volume PC board. Also check the jumpers at the processor connector on the back of the amp. I would clean all switches and controls and check all grounds connected to the RCA jacks on the back of the amp.

Thanks - I'm about to get into this and check everything over... I wasn't aware of any Jumpers... Can you please point out the processor connector for me if possible? Cheers

 

[Hunted11]

OOps, my mistake, I just looked at the schematic again and the jumper is only on the 55/65. not the 35.

 

[NicholasD]

OOps, my mistake, I just looked at the schematic again and the jumper is only on the 55/65. not the 35.

No worries, Say do you think a volume potentiometer alone could cause this? I have just cleaned all of the switches & Pots and made the effect less now by spraying more contact cleaner on the volume pot and turning about 100 times... Now the problem is gone at full volume and at zero volume! So I am getting somewhere.... What are your thoughts on this?.... Thanks

 

[Hunted11]

It shouldn't hurt anything, just make sure you are using the correct cleaner on the pots, you do not want to damage therm. I'd do the switches over also. Years of corrosion and dirt to remove.

 

[elnaldo]

Did you move the wires far from the transformer?

Sometimes hum is canceled at a certain volume levels and doubled at some other level. It depends of the source of the hum.

 

[NicholasD]

To all who have helped and chimed in, thanks so much... It was a great first experience on Audiokarma. I am pleased to say that the Amp is fixed... I cleaned everything and re-ran the line cabling as far away from the transformer as possible. I think the actual cause was gunk on the volume potentiometer as a mild noise sometimes comes back and I have to turn the volume knob profusely to get it to quieten down again.. Possibly a combination of things but surprisingly no components were stuffed. Anyway, I learnt a lot and am satisfied... Here is the result

 

[elnaldo]

That's a much better wiring layout

 

[ginetto61]

To all who have helped and chimed in, thanks so much... It was a great first experience on Audiokarma. I am pleased to say that the Amp is fixed...

Hi i am about to receive one myself. Could you provide me please your opinion on its sound ? lately i have rediscovered the Akai brand ... great products ... Thanks !

 

[ajaj]

Hello.

I also have a Humming issue on the Right Chanel or an Onkyo M5090 Amp.

I've grounded it every which way. checked the inside and didn't see any bad caps or anything else.

Once I turn the Amp on and the VU meters (large ones) change from Orange to Green, the hum comes on. Sometimes it is better than other times.

I've replaced speakers, ...etc. but nothing improved.

Right speaker only. Present even when nothing else in On or connected.

Any advise please ?

Thank you.

 

[mbz]

I also have a Humming issue on the Right Chanel or an Onkyo M5090 Amp.

Probably best to open up a new thread, to avoid confusion on model numbers...
First step would be to identify if it's 50/60Hz mains hum or 100/120Hz rectified ac hum, apparently there
is a mobile phone app that can detect it, otherwise make a recording and load it into audacity or similiar... I assume
you don't have an oscilloscope...

 

[ginetto61]

To all who have helped and chimed in, thanks so much... It was a great first experience on Audiokarma. I am pleased to say that the Amp is fixed...

Hi ! may i have your opinion about the sound from this unit ? even if it is a little late because i have already bought one myself and waiting to receive it
Thanks and regards, gino