Speaker pulsating with or without any input signal. What causes that?

[Kenrod]

I'm

I'll have a looong look at the schematic but I suspect some negative impedance or motion feedback trick , a regular high damping SS amp does not care much about what it drives, as long as over dissipation does not kill it.
Sorry again

Is this some kind of Recording/Studio monitor?

I guess some kind of tremolo effect must have been quite evident.

In any case, I'll study the circuit, such speaker wizardy usually has an adjustment trimmer somewhere.
Some reverb circuits which feed back a little of delayed signal to increase its depth also have such a trimmer pot.
See Champ 12 for one example, some Gibson amps too.

Hmmm. "Active Servo Technology". A clue perhaps

[Jazz P Bass]

I have worked on a number of these little Goomba's.

I will say, taking that amplifier apart is a real treat.

[Kenrod]

I have worked on a number of these little Goomba's.

I will say, taking that amplifier apart is a real treat.

[ATTACH=CONFIG]37808[/ATTACH]

yep. I've been deep into the preamp (head) portion of this one in the past. It was tedious to say the least.

[nickb]

Just FYI, this amp uses speaker current sensing ( JMF was right, again!) The sense resistor is R230 on the MAIN 2/2 assy. The current does have to go through a couple of connectors to get there so do make sure they are clean and tight. So, there is feedback and the speaker is part of that feedback. That in turn means that all components of the speaker contribute in some part and that includes the surround. Still, I'm surprised that they designed it to be so marginally stable at low frequencies.

[J M Fahey]

[ATTACH=CONFIG]37804[/ATTACH]I'm

Hmmm. "Active Servo Technology". A clue perhaps

You bet.
Back home after an unbearable very hot and humid Sunday, without the slightest breeze, pavement and buildings radiating stored heat as very efficient long wave infrared, and oraying for some rain, but will start fresh tomorrow (hopefully ) and check that "servo".

Studied them *long* ago but remember that you learnt to make them track properly in 1 Month ... and spent the rest of the Year learning how to make them stable, not to overshoot, not to ring, worst case not to oscillate ... and it was a nightmare.

You can do almost anything with electronics, but mechanical parts do not cooperate, have weird resonance modes, a mess.

You have recurring nightmares about submerging the full mechanism in molasses or something similar and calling it a day.

[Kenrod]

UPDATE: I ordered the foam surround kit on Monday afternoon and had it Wednesday when I got home from work.
Simplyspeakers.com
I followed the superb directions and put it back together today.
Sounds fantastic.
Even though it didn't require an electronic repair, I still appreciate the help.
I did learn something several things along the way.
Thanks again.

[Tom Phillips]

What was the final status of the speaker pulsing problem?