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W A R N I N G !

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Thursday, April 14, 2011

Boston Acoustics HPS 10HO subwoofer repair - Day 2

Day 2 start - got my fuses, lets go.

The symptoms on day 1 got me feeling that it is the power amp section that is spoilt. So I have to disconnect just the power amp section. A good way is via removing the rectifier. But before I do that, lets look at a strange thing I saw on day 1:

Yes, one leg of the rectifier is missing. In replacement a wire is soldered on half that leg and the solder pad on the underside of the board. Why would anyone do that is out of my comprehension.

This does hint of something wrong with this portion of the circuit, or maybe the rectifier. Unfortunately the rectifier measures fine out of circuit and unpowered, plus the humming when turned on (after the capacitors get their power before the fuse blew) indicates something wrong further downstream. There is also a previously unnoticed burn mark on one of the connector pins for the wire from the transformer, which further hints that the power amp section is the problematic part.

The 20W of my Goot iron really shows its inadequacy here, I am unable to desolder the bridge. Thick leads and traces conducted heat away too fast. I had to cut the legs off to remove the rectifier then desolder the remaining sections of the legs.

The right side of the board, the blue capacitors and the parts below it, is the power supply for the pre section. This tested working fine, and nothing blew up with the pre board connected either. Now I just need a way to tap the pre-out to ensure it is working, then I'll stuff an amp inside and finish it up. But this is enough for the day.

Bugger has 50VAC x 2, wooorr. But that is also a problem because I don't remember any chipamp that can handle ±70VDC. (Erm yes those big capacitors are 6800uF 80V)


Anonymous said...

Did you have a schematic diagram or service manual for power amp? I have same subwoofer (HPS 10HO) that has a problem. The fuse keeps blowing everytime I plug it in the AC wall. I checked the power amp board and found out the 3 transistors have shorted pins. Locations are Q10, Q10B & Q11. I cant get the correct part number of the transistors. It was tampered. I would apprecaite if you can give me the correct part number of the transistors.

Many thanks

William Burton said...

Did you get the transistor numbers? I am looking for them too. I have schematics, but not for that section.

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gerardo alvarado said...

Me podría decir cuales son los números de los transistores de salida

Maheen Sulaiman said...

Transistor 1 is IRF 3315