Sunn 300T loses volume control

[Rhodesplyr]

A client of mine brought in a Sunn 300T I worked on about 18 months ago. It had been working fine ever since, but he told me that at a show this last weekend, it "went berserk." Specifically, he said that it got very loud and that *none* of the front panel controls or EQ settings had any effect whatsoever on the volume.

Of course, when he brought it to the workshop yesterday, it worked perfectly.

The thing that's baffling me is how the signal could bypass ALL the many volume controls on this amp. The only thing that I recall from the first time I worked on it is that it has a Line Out feature, connected to the Tuner output, that completely bypasses all the preamp circuitry, depending on the position of a switch on the front panel. I'm a little suspicious of user error, but I suppose the switch could have an intermittent short.

Any other ideas?

[52 Bill]

The first thing that comes to mind is a bad ground connection. If the ground on the volume and or tone circuit opens up the amp would do something like the owner described.

[Rhodesplyr]

The first thing that comes to mind is a bad ground connection. If the ground on the volume and or tone circuit opens up the amp would do something like the owner described.

Bill -- Good idea. I'll check that. The only thing that might make me suspicious of that theory is the owner's insistence that NONE of the front panel controls had any effect on volume, including the Master Volume Control, which works via NFB around an IC, not via a pot with a ground connection. The amp has several gain controls in the signal path

If the Line Out switch--or the wiring to it--shorted, it would take the IC-amplified output of the bass guitar and shunt it directly to the output of the Master Volume Control.

[Rhodesplyr]

As a follow-up, despite checking voltages, wiggling everything, and literally poking the amp with a stick while running a constant signal through it. I was unable to make this amp malfunction in the shop. All I found was a tiny piece of the foil shielding material from the amp's case floating around the circuit board. The owner retrieved it today, and we're going to see if this odd problem pops up again.