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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....


  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.

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

  3. Great post. Mine shows exactly the same heat problem. I'm more of a software guy these days, so what would you replace the diodes D151 and D152 with as I intend to replace ALL of the components around the scorched bit of PCB? I'm guessing something like 1N914.

    1. Mine checked out OK. I think the scorching is caused t=by the transistors being faulty, rather than the diodes being poorly rated. They're nothing special, a 1N4007 or something would probably suffice...

  4. Thanks for this post. I've just replaced my transistors, and my random distant thunder type noises have disappeared.

  5. My Transformer also hums, and probably needs replacing. Is there a compatible transformer available?

    1. Measure it's output voltage underload , and get something sutably sized from your local RS/Farnell etc ... I doubt Tannoy supply parts, but maybe worth a go. Note if you are fitting a replacement transformer, you must provide some protection fuses, as a replacement is unlikely to have the same protection built in as the original.

  6. Hi Andy

    In regards to your comment about the wire links across F152 and 153, what fuse amp ratings would you replace the wire links with. I have the same fault that I'm fixing and think that protecting with 2 fuses is the way to go.

    Thanks for your super-helpful post, I eagerly await your reply!

    1. A wise move. I'd measure the current under load, double the measured value, and use a time-lag fuse to cope with the inrush to the caps.... As a guess 1.5A?

  7. My mate switched voltage on back of sub now won't turn on can it be fixed?

  8. I suspect that will have killed the transformer, and probably some other damage. Check the output of the transformer.