Acoustic 370 blows internal fuse when turned on

[KeyWestBassPlayer]

I have an Acoustic 370 that has treated me well for years, never had a problem till today. Internal fuse blows as soon as I put power to it. nearest repair shop is over 180 miles...Anyone walk me through checking it..I know a little about amps and electronics , not enough to solo on this one ..
Thanks in advance

[Phostenix]

Do you have electronics test tools?
How much experience do you have working on solid state amps?

The schematic is here:
http://acoustic.control.free.fr/file...chematics2.pdf

The service manual is here:
http://acoustic.control.free.fr/file...ice_Manual.pdf

[KeyWestBassPlayer]

370 amp

yes I have tools not much experiance in repair but I have an understanding of the basics

[Phostenix]

When you say that it blows the internal fuse, do you mean the one on the back panel or the one inside the amp that is usually hidden?

Do you have some 5 or 10 watt resistors & some test leads with alligator clips to discharge the big power caps?

[KeyWestBassPlayer]

370 amp

It is blowing the internal fuse inside the chassis, I have the test leads but no resistors, there is a radio shack on the island , I can pick them up in the morning. I have seen a socket and light bulb used in the past ????

[KeyWestBassPlayer]

ok the capacitor is discharged

[Phostenix]

I'm afraid to ask.

What value fuse is in the back panel fuse holder? Is the internal fuse soldered in or has it been modified from the original?

Locate the red & black wires going from the rectifier bridge to the biggest of the 3 caps (I'm assuming yours is stock at this point). There are 2 sets of red & black wires going to the screw terminals on top of that cap. One set goes to the rectifier bridge, the other to the power amp board. Disconnect the 2 wires that go to the power amp board & leave the 2 wires to the rectifier (tighten them back down).

The proper fuse is 7A, but for inital testing I like to use much smaller ones like 2 or 3 amp. Do you have any fuses?

With a fuse installed, turn the amp back on. If it doesn't blow the fuse, there are no problems with the power transformer, switch, pilot lamp, "death caps", or rectifier.

[KeyWestBassPlayer]

370 amp

OK I did that and this time the external 7 amp fuse blew but not the internal fuse ??? power supply issues ?

[Phostenix]

I hope you've got some extra fuses. Remeber, no cheating & bypassing fuses!

Is the internal fuse replaceable or is it soldered in?

Disconnect the red & black wires from the big cap - make sure those wires from the rectifier are not touching anything! - replace the fuse & try it again.

If it blows again, you'll need to de-solder the 2 red wires from the power transformer to the recifier bridge & try it again. As always, make sure the wires from the power tranformer aren't touching anything.

[KeyWestBassPlayer]

internal fuse is replaceable

[KeyWestBassPlayer]

370

ok when I took off the other two wires (red and Black) it didnt blow any fuses on the amp but poped the circuit breaker on my main disconnect for the house...no wires were touching anything, no sparks, no smoke ?

[Phostenix]

This is where variacs come in handy....

De-solder the red wires from the power transformer to the rectifier bridge & let's see if it still blows the fuse(s). If it does, then either the power transformer is shorted, the EMI cap is shorted or there is a wiring issue.

Try switching the GND REV switch on the front panel to its other position to see if it makes any difference here.

Oh, and do you have any 2 or 3 amp fuses?

[KeyWestBassPlayer]

ok after de sodering the rectifier bridge wires, the power stays on no fuses blow, am I to guess that i am in need of a rectifier bridge ? and I do not have any 3 amp fuses ...just 7 amp

[Phostenix]

Once you replace it, it'll be interesting to see if something downstream from the bridge took out the bridge. The bridge part# is BR401. Datasheet here:
http://www.datasheetarchive.com/pdf/...SA00240304.pdf

Looks like a 40A bridge with peak inverse of 100V.

I suggest you get some smaller value fuses for the initial power up after replacing the bridge.

Mouser has this in stock:
GBPC4001 Taiwan Semiconductor Bridge Rectifiers

[KeyWestBassPlayer]

thank you

Thank you very much for your help...you just taught me alot on checking through the power end of it. I have to learn more , it is very interesting to me. plus there is no one with in 180 miles of Key West, FL that can repair amps...at least I havent found any...I just ordered the rectifier bridge, I will let you know what happens from here.. Thank you so much for your help.
I hope I can return the favor to you or pass it forward to someone else .

Thanks
Jack in Key West