Need help diagnosing plate amp.

nathank · 2nd November 2005, 02:38 PM

Hello all, I found the site last night, and I have been enjoying hours of reading. Looks to be many very knowledgeable people here.

I was wondering if you all could offer me some assistance with an Infinity sub problem. It is a BU80 sub that has a 8" driver and 75wRMS plate amp. Nothing big but it fills the bottom in everyday situations.

When turned on and connected it has output through the speaker, somewhat of a crunching sound. No noticeable real sound, just the static crunching. It was connected low level, I tried new low level cables, new low level source from a different receiver, and high level connections with the same result, just the same crunching. I pulled the driver and connected it to another amp and it worked fine.

The board on the plate amp shows no visible burn areas, everything looks normal. I have been looking everywhere online for a schematic and can't find anything.

Could anyone offer me a starting point with this repair? This has been something I really wanted to learn, but I really don't know where to begin.

richie00boy · 2nd November 2005, 02:55 PM

My first guess would be a dodgy output or driver transistor, or dry joint around those devices.

nathank · 2nd November 2005, 04:53 PM

One more additional spec of info. The sub actually belongs to my parents. When I went to their house to help them out with it, I noticed it was plugged directly into a wall outlet, with no surge protection.

anatech · 2nd November 2005, 04:59 PM

Hi nathank,
Also check that the woofer moves smoothly in & out. Bad joints are a good start. Pay attention to heavy components, and where parts are attached to things like a heatsink.

-Chris

nathank · 2nd November 2005, 06:26 PM

Quote:

Originally posted by anatech
Hi nathank,
Also check that the woofer moves smoothly in & out. Bad joints are a good start. Pay attention to heavy components, and where parts are attached to things like a heatsink.

-Chris

I am 99% sure it doesn't have anything to do with the driver. I tested it on another amp and it worked perfectly.

anatech · 2nd November 2005, 06:37 PM

Good, so it sounds like cracked connections.

-Chris

nathank · 2nd November 2005, 07:29 PM

Quote:

Originally posted by richie00boy
My first guess would be a dodgy output or driver transistor, or dry joint around those devices.

Is there anyway to check the transistors on the board? or do they have to be removed?

I found this:

http://sound.westhost.com/troubleshooting.htm#tests

with a transistor quick check.

anatech · 2nd November 2005, 07:47 PM

First try to resolder the leads carefully. You don't have shorted transistors or the fuses would blow. In circuit transistor tests in this instance are not going to help you.

In circuit tests are okay for checking a transistor junction, depending on the circuit. They can detect shorts, depending on the circuit.

-Chris

2nd November 2005, 02:38 PM	#1
nathank diyAudio Member Join Date: Nov 2005	Need help diagnosing plate amp. Hello all, I found the site last night, and I have been enjoying hours of reading. Looks to be many very knowledgeable people here. I was wondering if you all could offer me some assistance with an Infinity sub problem. It is a BU80 sub that has a 8" driver and 75wRMS plate amp. Nothing big but it fills the bottom in everyday situations. When turned on and connected it has output through the speaker, somewhat of a crunching sound. No noticeable real sound, just the static crunching. It was connected low level, I tried new low level cables, new low level source from a different receiver, and high level connections with the same result, just the same crunching. I pulled the driver and connected it to another amp and it worked fine. The board on the plate amp shows no visible burn areas, everything looks normal. I have been looking everywhere online for a schematic and can't find anything. Could anyone offer me a starting point with this repair? This has been something I really wanted to learn, but I really don't know where to begin.

2nd November 2005, 02:55 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	My first guess would be a dodgy output or driver transistor, or dry joint around those devices. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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2nd November 2005, 04:53 PM	#3
nathank diyAudio Member Join Date: Nov 2005	One more additional spec of info. The sub actually belongs to my parents. When I went to their house to help them out with it, I noticed it was plugged directly into a wall outlet, with no surge protection.

2nd November 2005, 04:59 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi nathank, Also check that the woofer moves smoothly in & out. Bad joints are a good start. Pay attention to heavy components, and where parts are attached to things like a heatsink. -Chris

2nd November 2005, 06:37 PM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Good, so it sounds like cracked connections. -Chris

2nd November 2005, 07:47 PM	#8
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	First try to resolder the leads carefully. You don't have shorted transistors or the fuses would blow. In circuit transistor tests in this instance are not going to help you. In circuit tests are okay for checking a transistor junction, depending on the circuit. They can detect shorts, depending on the circuit. -Chris

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