Bassman 150 fuzzzzz

[drewl]

Early 2000's SS combo has a fizzy type fuzz or distortion, mainly when played softly or just barely hitting a note.
Static-y type scratchy sound as the note trails off.

Preamp fine, speaker fine, sounds like the biasing is off but signal looks great on a scope.

Was getting nowhere so I tried to see if changing the bias would help.

At idle there is no voltage across the .15 Ohm resistors.

No bias adjust, so I jumpered across R216 with a couple different value resistors and when lowering it to about 1.8K, signal was nice and clean with no more fuzz.

Tried the other side, R217 with no change in sound, so it was just the other side that bias seemed off.
Still no voltage across the .15's at idle.

Ran it for a while and the drivers aren't getting hot or overheating, so it looks like it's good to go.

[vintagekiki]

Fuzz or distortion, when play soft, indicates that the bias adjust does not work, and that idle current is low or absent, so there is no voltage across .15 Ohm resistors.
Check DC voltages on TP32 / TP33
Check Q209 / Q210; R215 - R218

[drewl]

There is no bias adjust in these.

That is why I lowered R216 to increase bias which corrected the problem.

[vintagekiki]

When you lowered R216, how much is voltage on .15 Ohm resistor?
1.5mV = 10mA at .15 Ohm
3.0mV = 20mA at .15 Ohm

[drewl]

There is 0 volts across all the .15 Ohm resistors at idle.

I'd say that's biased pretty low.

[vintagekiki]

What says check from the post

https://music-electronics-forum.com/showthread.php?t=50166&p=545176&viewfull=1#post545176

[drewl]

ummm, I think you're saying check everything listed in post #2
Which I did, and they're all okay and voltages in spec.

[drewl]

Of course the real question is
Do I mention the intermittent solder blob shorting Q210 base to emitter,
Or do I lie and say everything's fine so I don't look like (an even bigger) idiot ???


Fixed the intermittent short and amp sounds even better, no need to lower R216
No fuzz, no buzz.

Still no voltage across the .15's at idle.

Thanks.