Acoustic B200

[caverat]

Need a schematic for Acoustic B 200 Amp. (Note this is NOT the current B 200 mkII) This one has an STK 404 output module for power amp.

Mine has a shutdown issue at high volume, speaker and wiring are not the problem, seems the audio itself is tripping the protection circuit. Output module appears OK.

[Enzo]

Link us a schematic, can't analyze the protection circuit without it.

[Jazz P Bass]

Here is the schematic.

Acoustic B200.zip

The Protect circuit will enable or disable the speaker relay.

So you need to figure out what is shutting down.

Is this a repair of an IC blowout or is this the only issue?

[caverat]

Thanks all! In case anyone has similar problem, mine was traced to a cold solder joint on one of the external resistors from the output IC. High volume caused the bias point to shift placing DC on the speaker output. This DC voltage was tripping the protector. Fixed now.

Once I got the schematic it took longer to rework the speaker wires which had to be cut to remove the amp from the cabinet. When they built the amp they filled the hole where the speaker leads come in with glue..... Probably figured it would never have to be worked on..... (Just goes to show, any idiot can get some letters after his name and call himself an engineer.)

[Enzo]

A lot of amps do that, especially bass amps. The hole into the speaker chamber where the wires go in, if left unfilled can whistle or make noises as the air blows in and out from the speaker action. There is tremendous air pressure inside. Some 4x12 cabs will actually blow the plug out of the jack on the rear panel.

[J M Fahey]

they filled the hole where the speaker leads come in with glue..... (Just goes to show, any idiot can get some letters after his name and call himself an engineer.)

Not arguing, just thinking aloud: doubt an Engineer (which would work probably designing the circuit and such), would be called to the production floor to solve such a minor problem, if any it might have been a foreman or similar guy decision, or the worker itself, or at most a QC guy, when/if somebody complained about whistling.

FWIW last week I repaired a Ross B60 combo, tried to pull the chassis out and could not because "something" was pulling it.

I pulled a dozen screws to take the metal grill out, 8 more to pull the front mounted speaker, and found its wires (soldered by the way, not crimped to pull out terminals) were too short.

It was an impossible situation: if neither the chassis nor the speaker could be pulled out far enough, and there was no back cover (it was solid glued by design) how the h*ck had they soldered it ?????????

Did they have some specially trained midgets with tiny soldering guns who could slip through the narrow openings?

Until I saw the Light.

No, I wasn't converted to any Religion or sect, just noyiced the wires were proper length for a speaker with the tabs oriented upwards, towards the chassis, and it must have been soldered that way ... but later it was rotated 180 degrees before bolting it down so the Logo printed on the speaker dust cap could be read the right way.

Being a 12" speaker, that accounted for "missing" 12" of speaker wire.

Doubt an Engineer was involved with that .

[caverat]

A lot of amps do that, especially bass amps. The hole into the speaker chamber where the wires go in, if left unfilled can whistle or make noises as the air blows in and out from the speaker action. There is tremendous air pressure inside. Some 4x12 cabs will actually blow the plug out of the jack on the rear panel.

I fully understand that. But why be so cheap that you can't seal the wires and put a plug on them which connects to the amp chassis? That way it could simply be unplugged from the amp and the amp removed for service when needed. (Might have added at least 50 cents to cost of the amp! Probably much less since parts are bought in quantity by manufacturers.)

[caverat]

I have run into similar situations and often wondered whether wire was so expensive that when a length was specified, it couldn't have been speced out about 2 inches longer? But there could also be another explanation for the speaker label orientation... Was this an after market or replacement speaker? Ordinarily manufacturers specify label orientation based on the amp it will be used in. Of course if someone later changed speakers that label could be positioned differently.

[Enzo]

It has to be long enough to be installable, of course. But you don't want too much slack, because wires can buzz. If they lay against the back of the speaker cone, they can make an awful racket. And sometimes just vibrating against the cab wall.

Sure, we can think of ways they could have made it more convenient, but most amps never come apart. I expect bass combos to have some sort of hole goop, I don't find it hard to deal with.