Sugden Audition T light hum

I have a Sugden Audition T, a model without tone control, the board is written on Optima 80. There is a light hum after I play music, hum occurred when there is not a signal. But if I turn on and I don't listen to music there is no hum.
In which direction I should direct my inquiry?

Regards,

Jean
 

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The first step is to isolate it to the amplifier itself. Play some music, get it to hum, then disconnect all inputs to the amplifier while it is still on and see if the hum continues. If it stops the problem is elsewhere.

What pitch is the hum?
(Note: you may need heaphones for the links below)
Power supply hum usually happens at about 120hz Interconnect or ground hum usually happens at 60hz.

If it is ground hum, it could be a bad patch cord or something new in your system.

If it is power supply hum you need to either take it in for service or you can post a schematic and specs here and see if anyone can help.
 
It sounds like the hum might increase with temperature, so after using the amp for a while there is noticable hum, but not from cold?

Hello Mark...

Are you suspecting Bias problems?

If so there are a couple of trimmers on the board that appear to be turned all the way up. Most engineers design these circuits so the adjustments should most appropriately fall somewhere near 12 oclock.

I circled them in the attached image. But without a schematic I can't be sure of their functions.
 

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Hello Jean,

I know I'm late to the party but better late than never.
My reply could serve others having the same problems.

I just had an Audition on my bench and found the following.
The capacitors near the voltage regulators and the sink resistors are all goners (from memory, 10uF and 22uF).
So much heat comes from the combination of the regulators and the sink resistors that if you put capacitors near them, they won't last.
I replaced them all with the same value but with higher voltage, as space permits.
Make sure you use 105°C and longest life possible.

I removed the 2 W sink resistors and replaced them with 3 W (from memory they are 470 Ohm).
I installed them under the PCB and kept their lead as long as possible to keep them away from the PCB as it probably is getting dark.
Be gentle when soldering as the PCB is not very high quality and damages easily.

You'll soon realize that to work on the PCB there is no way around dismantling the whole thing.
The back is the easy part but the front is another thing.
You have to remove the front panel to have access to the nuts holding the volume potentiometer and selectors.
Once this is done you can flip the PCB and have access to the solder side.
Be extremely careful as the heatsinks are heavy and if you move them too much, the leads from the output transistors will break.

Once the PCB is accessible, make sure you replace the relay as they are most of the time defective and are a pain to get to.
I usually replace them with a sealed Omron G2R-24-DC24.
The best way to determine if a relay is due to be replaced is to power the amplifier and try it with a 5 kHz, at low volume.
The easiest is when you have an oscilloscope but a multimeter can do.
While playing the 5 kHz, gently tap on the relay with a hard piece of plastic.
If the signal varies significantly, its time for it to be replaced.
Another way to know if the relay is to be replaced is when you listen to music at low volume and the sound cuts, but comes back as soon as you raise the volume.
What you're actually doing when you raise the volume is bring the voltage high enough that it breaks the oxidation layer developing on the relay contacts.

Good luck to all!

I'm available if you have questions.

Michel