Solid State Bass Amp hums

[gerryt]

Everyone helped me so much to solve the overheating problem I had with my QSC Amp, so I thought I would ask for help again with another amplifier problem. I have a small Ampeg BA - 108 (first generation) solid state bass amp that hums continually and also presents a very small amount of distortion even at low levels. I have replaced the filter capacitors C18, C19, C20 and C21 with no improvement. I have attached a schematic of the amp. I am an amateur in electronics but my hunch about the hum is that it is a grounding problem.

 

[bertus]

Hello,

Hum can also be pickup.
Are the used cables shielded?
Is the hum there with CD input?
And how is it with the J1 and P1 input, wich get amplified much more?

Bertus

 

[gerryt]

Hum is present even when no instruments or CD are plugged in. Input cables are shielded. I am using a bass that does not hum when played through my other bass amplifiers. I have noticed after listening more carefully that the humming sound comes mostly rom the amplifier itself more than through the speaker.

 

[WHONOES]

What is it like with the level controls turned to minimum?

 

[gerryt]

The hum is present even with all control levels turned to minimum. It does not change if the controls are turned up.

 

[Bluejets]

maybe transformer lamination buzzing....

 

[Harald Kapp]

Apart from possible transformer hum (you should be able to identify/locate whether it's the transformer just by listening closely with the cover off), mains frequency hum is typically a sign of a bad power supply.
Since you already swapped the main filter capacitors, I see the rectifier as a possible source of the issue.
Check voltages Vcc and Vee in the power supply section (across C18 and across C21). Check with your meter in DC range to verify the Dc voltage. Check with your meter in AC range (hoping you have a classic AC meter, not a fancy true RMS one ) to see whether there is undue ripple on top of the DC voltage.
Check also R21 and R22. Are they 620 Ω as they should (you may need to de-solder one side of each resistor for correct measurement).

 

[WHONOES]

Sounds like transformer laminations then assuming it is not a toroidal. Does the hum persist with no speaker connected?