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Wednesday, 15 November 2017

Tannoy SFX5.1 Powered subwoofer repair.

Paul popped by.

"I've got this Tannoy subwoofer, it crackles and pops. Can you take a look?"

Yeah - why not......


It's a smallish cube, mounted on little cones, with a downward facing speaker...

I powered it up, and all was quiet. 

Just as I was about to connect the signal generator, I heard thunder in the distance .... and it got louder and louder! The speaker was popping and crackling alright. I quickly powered down.

I removed the screws around the outside of the rear panel, to reveal the speaker. The electronics and amplifier are mounted on the rear panel itself....

The main amplifier, a TDA7293 is heatsinked to the rear panel.










Removal of the board showed signs of distress....

Q151 and Q152 and there associated components had been running rather warm! The colour of the cases of the transistor should be black, you can see in the photo that they've changed to a sort of powdery brown.

Q151 is a 2SC1815, NPN, and Q152 is it's complementary PNP, a 2SC1015.











Static testing on the transistor tester showed both to be fine. But the fault seemed to exist only when the thing was drawing current, so I threw caution to the wind and replaced both transistors. The two diodes, the two resistors and the caps all tested within tolerance.







Switch on again, and it's cured!

In the above picture, you can see two wire links across F152 and F153, where there was place for a couple of fuses. The wire links are original, so the fuses were obviously deemed unnecessary, relying on the non-replaceable thermal fuse in the transformer...

Not sure I like that idea much....

2 comments:

  1. I found similar problems with the KEF PSx000 series subwoofers and Celestion which were identical inside. Lots of electrolytics next to resistor dropper resistors. Several small transistors not doing what they should. Fitting proper regulators on big heat-sinks as well as the trannies worked wonders. Many electrolytics were replaced with high temperature ones. So far after a few years no returns... :-) Andy 2.

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  2. I have the same issue. I'll pop mine open and have a look!

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