SWR Workman's 15 Bass Combo (buzzing / Fuzz noise)

[plastic cloud]

My name is Brian. I have a SWR Workman's 15 Bass Combo. The problem is a buzzing (fuzz) noise coming from the speaker at any volume especially in the low bass tones. The date written on the Amp is 27/9/04 (made in Mexico). Printed on the board is 170021, Workman's 12. I have read the manual to make sure I'm not doing something wrong with the settings. I have tried both a Fender passive and Fender active bass in the appropriate inputs and tried 3 different guitar cords. My guitars and cords work fine with another amp so have eliminated that as the problem. I have relaced the speaker with one that I know works and the same noise persists. I have pulled out the amp and tried to make sure there were no loose connections. Still not solved. Any suggestions would be greatly appreciated.

[52 Bill]

Welcome to the place!
It sounds like you've eliminated the most obvious causes.

One thing that I always check for on the SWR boards are broken transistor leads. Like a lot of amps that get bounced and vibrated there is the possibility for metal fatigue on the legs of the power and driver transistors. Carefully inspect the legs of each of the larger transistors on the board and on the heatsink.

Other than that, it could be a bias problem.

[plastic cloud]

Thanks, the board looks solid.. I don't see any obvious open connections. I didn't remove the board, just a visual inspection with bright light and a magniyfing glass. I'm trying to do what I can before taking it to a technician. Can you explain more about the possible bias problem..

[52 Bill]

Removing the board on those is difficult due to the mounting arrangement of the output transistors.

The bias is set so that the top and bottom half of the signal are amplified in the correct amounts to eliminate crossover distortion. Sometimes a mis-adjusted bias setting will show up as a slight distortion or buzzing in the signal. It would be unusual for the amp to suddenly just become mis-adjusted without some cause like a bad transistor, etc.

[olddawg]

Removing the board on those is difficult due to the mounting arrangement of the output transistors.

The bias is set so that the top and bottom half of the signal are amplified in the correct amounts to eliminate crossover distortion. Sometimes a mis-adjusted bias setting will show up as a slight distortion or buzzing in the signal. It would be unusual for the amp to suddenly just become mis-adjusted without some cause like a bad transistor, etc.

IME if something can be adjusted, someone is going to do it. So you never know if something has been diddled. Also, bias pots are notorious for being cheap, dirty, and failure prone. I replace them every time I do an output repair if possible or at least clean/lube/check them. Anyway, if you have a scope and a 1KHZ tone crossover distortion is obvious.

[Enzo]

DOubtful it has gotten out of adjustment. Adjustment is usually only required when parts are replaced.

My first thought is weak filter cap in a power supply.

[plastic cloud]

Everyone - thank you for your feedback. I'm taking it in to a tech armed this your information. I'll let you all know what happens. Brian