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Jonas83 30th December 2012 09:11 AM

Jmlab SW-33A repair
Hi all!

Searched for some time, hope this is no dupe!

Got this Jmlab sw-33a (12" focal sw) that needs alot of love!
One transistor blown (visualy), 1 15000uf cap vented and possibly more!

I have not been able to find any schematic or specs on sw element such as impedence, it measuers 6.7ohms and is a real pain to drive as my only amp around tripps when i raise vol, so i wounder is it really this heavy to drive, or is it busted? it is a impressive peice of element, so would not be supprised if it is heavy to drive.

The amp has been tempered with, previous owner probably done a cooling paste replacement and overlooked the plastic washers between heatsink and transistors causing a shortout trough the heatsink, leaving one transistor visualy blown and vented one of the big caps (elna lp5 15000uf/63v).

Starting out like this, pictures of the thing will popup as i work with it.

Jonas83 2nd February 2013 07:52 AM

2 Attachment(s)
Very much later....

This is a "china board" and maybe crappy from start, anyhow i want to fix it since the sub is kind of nice.
Changed the output transistors, and have a coulpe of new 15mF caps waiting.
Checked all power/signal transistors and diodes (even zeners on my bench) but i still have a BIG problem.

It works whitout the filtercaps of course loads of hum). Now to my real problem, as to this everything looks good and measure well but when i try it with the caps on, nothing happens (bulb as ballast) except a nasty heat development making the heat sink varm. i am very sure of polarity of caps so thats not the problem. Could this be and effect of a halfly bad bridge rectifier giving halfwave rectification on negative and fullwave on positive or am i just stupid here?


Jonas83 2nd February 2013 08:55 PM

Nope, rectifier would not be the problem and my previous theory of that was probably mumbo.

Did find the issue, owner before me (who killed it) abused the pcb so bad that the trace trough/over collector pin was left open in one direction. this did the trick of dumping a massive current from positive supply side and left a vented cap.

anyways it works now, doubt it, but hope this helps someone else.

dragon64 9th April 2014 08:38 PM

33A sub
Hi Jonas83

I own olso a 33A buy it last year.
My board look not simulair mayb later version.

My project starts in nov-2013.
Starting th sound check! the low bass end was not very good.

First making a plan.
-Recap power circuit. 2 powercabs yes 15000uf 63v
-Extra wood braces inside cabinet
-New damping material
-Replace the cabinet.wood finisch

Greatings from Holland

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