Acoustic 370 blows internal fuse when turned on

KeyWestBassPlayer · 11-17-2009, 02:24 AM

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 · 11-17-2009, 02:30 AM

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 · 11-17-2009, 02:35 AM

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

Phostenix · 11-17-2009, 03:08 AM

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 · 11-17-2009, 03:20 AM

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 · 11-17-2009, 03:24 AM

ok the capacitor is discharged

Phostenix · 11-17-2009, 03:54 AM

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 · 11-17-2009, 04:09 AM

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

Phostenix · 11-17-2009, 04:16 AM

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 · 11-17-2009, 04:19 AM

internal fuse is replaceable

KeyWestBassPlayer · 11-17-2009, 04:27 AM

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 · 11-17-2009, 04:37 AM

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 · 11-17-2009, 04:56 AM

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 · 11-17-2009, 05:16 AM

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 · 11-17-2009, 05:30 AM

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

Phostenix · 11-17-2009, 05:33 AM

I hope that's all that's wrong.

Working through the power amp & finding parts for these amps is a little more challenging.

Phostenix · 11-17-2009, 05:43 AM

You could do one thing for me. I'd love to see a picture of your internal fuse holder. I've got 2 of these amps that I'm going to start on in a couple of weeks. 1 is the older version (the preamp board is slightly different) & it looks untouched inside. It has a soldered in internal fuse (that's blown). The other has a replaceable internal, but it looks like somebody's add-on. I'm curious what yours looks like.

KeyWestBassPlayer · 11-17-2009, 06:08 AM

it is spring loaded , smart idea easy to change

KeyWestBassPlayer · 11-17-2009, 06:12 AM

hum not sure how to post picture here ?? do you have face book ?
Login | Facebook or send me your email and I can email them to you
Jack

KeyWestBassPlayer · 11-17-2009, 06:24 AM

its in my photo album here now

Phostenix · 11-17-2009, 02:14 PM

I've never seen a fuse holder like that before.

Enzo · 11-18-2009, 01:56 AM

Those are spring clips. You use them to use regular fuses as pigtails. Someone removed the original pigtail at some point and soldered in the spring clips. You can pull them off the fuse and stick a new fuse in them when needed.

Phostenix · 11-23-2009, 12:49 AM

Posting an email from Jack:

Quote:

Originally Posted by KeyWestBassPlayer

I installed the new bridge recitifier but the amp it still blowing fuses , would you have any sugestions ?
Thanks
Jack

Phostenix · 11-23-2009, 12:50 AM

It blows the fuse with just the rectifier attached or with everything re-assembled?

R.G. · 11-23-2009, 05:02 AM

Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.

Oh, and did I mention that a light bulb limiter is really a help with this kind of thing?

KeyWestBassPlayer · 11-23-2009, 05:26 PM

It blows with everything reattached....

R.G. · 11-23-2009, 05:33 PM

Quote:

Originally Posted by KeyWestBassPlayer

It blows with everything reattached....

Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.
Light bulb limiter.

Oh, and did I mention that a light bulb limiter is really a help with this kind of thing?

Phostenix · 11-23-2009, 08:06 PM

After you get done building your light bulb limiter, how willing are you to tear this apart, troubleshoot, & find parts? I will be going through 2 of these in the near future (I've decided to sell off some more gear, so ebay will consume much of the next week, I'm sure), so I could theoretically walk you through it as I do mine.

I'm going to make an assumption that some of the output transistors have failed, but we won't know for sure without testing. If they are, you will need to replace more than just those output xstrs, so it gets a little involved. 1 of the amps I have is from a guy who gave up trying to fix it.

KeyWestBassPlayer · 11-24-2009, 12:26 AM

I would much rather do it the proper way and troubleshoot it and find out why it went down. I would greatly enjoy the learning experiance...

Phostenix · 11-24-2009, 02:35 AM

Do you have a meter with a diode tester on it? Do you know how to test transistors with a diode tester?

KeyWestBassPlayer · 11-24-2009, 02:38 AM

no I do not but will aquire one tomorrow

Phostenix · 11-24-2009, 02:53 AM

Here's a good thread for testing diodes & transistors:

Bipolar Junction Transistor Testing Basics - AudioKarma.org Home Audio Stereo Discussion Forums

Phostenix · 11-24-2009, 03:01 AM

To access all the output transistors on the 370, you will need to either remove the preamp board or remove the screws holding the power amp board/heatsink assembly from the chassis. Removing the preamp board means removing all the front knobs & the nuts mounting the pots to the chassis. Removing the heatsink screws so that you can move that block around means having to replace the heatsink compound between it & the chassis when you're done. Not a bad idea anyway considering the age of these amps, but you will need to get some heatsink compound (I assume).

If you remove the preamp, you also will have to remove the input jacks & de-solder the bright switch.

Phostenix · 11-24-2009, 03:04 AM

And here's the info on building a light bulb limiter:
Light Bulb Current Limiter

KeyWestBassPlayer · 11-24-2009, 03:40 AM

ok thanks I will start there and read up

11-17-2009, 02:24 AM	#1
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	Acoustic 370 blows internal fuse when turned on 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

11-17-2009, 02:30 AM	#2
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	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 __________________ ST in Phoenix

11-17-2009, 02:35 AM	#3
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	370 amp yes I have tools not much experiance in repair but I have an understanding of the basics

11-17-2009, 03:08 AM	#4
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	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? __________________ ST in Phoenix

11-17-2009, 03:20 AM	#5
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	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 ????

11-17-2009, 03:24 AM	#6
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	ok the capacitor is discharged

11-17-2009, 03:54 AM	#7
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	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. __________________ ST in Phoenix

11-17-2009, 04:09 AM	#8
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	370 amp OK I did that and this time the external 7 amp fuse blew but not the internal fuse ??? power supply issues ?

11-17-2009, 04:16 AM	#9
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	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. __________________ ST in Phoenix

11-17-2009, 04:19 AM	#10
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	internal fuse is replaceable

11-17-2009, 04:27 AM	#11
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	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 ?

11-17-2009, 04:37 AM	#12
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	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? __________________ ST in Phoenix

11-17-2009, 04:56 AM	#13
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	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

11-17-2009, 05:16 AM	#14
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	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 __________________ ST in Phoenix

11-17-2009, 05:30 AM	#15
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	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

11-17-2009, 05:33 AM	#16
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	I hope that's all that's wrong. Working through the power amp & finding parts for these amps is a little more challenging. __________________ ST in Phoenix

11-17-2009, 05:43 AM	#17
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	You could do one thing for me. I'd love to see a picture of your internal fuse holder. I've got 2 of these amps that I'm going to start on in a couple of weeks. 1 is the older version (the preamp board is slightly different) & it looks untouched inside. It has a soldered in internal fuse (that's blown). The other has a replaceable internal, but it looks like somebody's add-on. I'm curious what yours looks like. __________________ ST in Phoenix

11-17-2009, 06:08 AM	#18
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	370 fuse it is spring loaded , smart idea easy to change

11-17-2009, 06:12 AM	#19
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	hum not sure how to post picture here ?? do you have face book ? Login \| Facebook or send me your email and I can email them to you Jack

11-17-2009, 02:14 PM	#21
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	I've never seen a fuse holder like that before. __________________ ST in Phoenix

11-18-2009, 01:56 AM	#22
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,366	Those are spring clips. You use them to use regular fuses as pigtails. Someone removed the original pigtail at some point and soldered in the spring clips. You can pull them off the fuse and stick a new fuse in them when needed. __________________ Education is what you're left with after you have forgotten what you have learned.

11-23-2009, 12:50 AM	#24
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	It blows the fuse with just the rectifier attached or with everything re-assembled? __________________ ST in Phoenix

11-23-2009, 05:02 AM	#25
R.G. Lifetime Member Join Date: May 2006 Posts: 1,050	Light bulb limiter. Light bulb limiter. Light bulb limiter. Light bulb limiter. Light bulb limiter. Light bulb limiter. Light bulb limiter. Light bulb limiter. Light bulb limiter. Oh, and did I mention that a light bulb limiter is really a help with this kind of thing?

11-23-2009, 05:26 PM	#26
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	370 It blows with everything reattached....

11-23-2009, 08:06 PM	#28
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	After you get done building your light bulb limiter, how willing are you to tear this apart, troubleshoot, & find parts? I will be going through 2 of these in the near future (I've decided to sell off some more gear, so ebay will consume much of the next week, I'm sure), so I could theoretically walk you through it as I do mine. I'm going to make an assumption that some of the output transistors have failed, but we won't know for sure without testing. If they are, you will need to replace more than just those output xstrs, so it gets a little involved. 1 of the amps I have is from a guy who gave up trying to fix it. __________________ ST in Phoenix

11-24-2009, 12:26 AM	#29
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	370 I would much rather do it the proper way and troubleshoot it and find out why it went down. I would greatly enjoy the learning experiance...

11-24-2009, 02:35 AM	#30
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	Do you have a meter with a diode tester on it? Do you know how to test transistors with a diode tester? __________________ ST in Phoenix

11-24-2009, 02:38 AM	#31
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	370 no I do not but will aquire one tomorrow

11-24-2009, 02:53 AM	#32
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	Here's a good thread for testing diodes & transistors: Bipolar Junction Transistor Testing Basics - AudioKarma.org Home Audio Stereo Discussion Forums __________________ ST in Phoenix

11-24-2009, 03:01 AM	#33
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	To access all the output transistors on the 370, you will need to either remove the preamp board or remove the screws holding the power amp board/heatsink assembly from the chassis. Removing the preamp board means removing all the front knobs & the nuts mounting the pots to the chassis. Removing the heatsink screws so that you can move that block around means having to replace the heatsink compound between it & the chassis when you're done. Not a bad idea anyway considering the age of these amps, but you will need to get some heatsink compound (I assume). If you remove the preamp, you also will have to remove the input jacks & de-solder the bright switch. __________________ ST in Phoenix

11-24-2009, 03:04 AM	#34
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 383	And here's the info on building a light bulb limiter: Light Bulb Current Limiter __________________ ST in Phoenix

11-24-2009, 03:40 AM	#35
KeyWestBassPlayer Junior Member Join Date: Nov 2009 Posts: 16 Member's Picture Albums	ok thanks I will start there and read up

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