B&W subwoofer amp repaired
Just a quick repair tip some of you may or may not have seen.
I received this B&W subwoofer amp for repair. The owner already removed the amplifier from the sub box. He is having the woofer driver repaired with a speaker repair company.
The minute he mentioned this to me I immediately thought: DC across the speaker, meaning that the amplifier is probably outputting a large DC voltage., that damaged the speaker voice coil.
So I decided to slip a voltmeter across the speaker terminals. Strange thing is, I didn’t see a steady DC voltage, but a pulsating (very low freq) DC signal of around 25Volt max, dropping down to 0V periodically.
I then continued by removing the Screened signal cable, linking the preamp/crossover board to the main amplifier board, to see if the pulsating signal was coming from the preamp board. Slipped my analog meter on the preamp out pins this time so I could see the needle move easier. The same pulsating DC was still there.
So, I knew the problem was on the preamp board.
I saw a large capacitor on the board surrounded by some type of glue. I could see the glue had hardened, and was spread over some other resistors and wire links.
From experience I’m always skeptic of this type of glue.
I put my digital multi-meter , on ohms range, across this blob of glue. Guess what? It measured around 570Kohms. This is low enough to cause some problems, especially around some high gain, high impedance circuits.
I removed the capacitor for easier access and cleaned of the glue with a hard brush and some acetone/ alcohol mix. Replaced the capacitor and the fault was gone.
The unit, now works great.
Next time you see a circuit without an obvious fault, check for glue around the larger components that might have gone conductive.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs.
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