Quad L-ite Sub (wharfedale sw-300) rustling

[JOHN BALI]

bulb test for safe test

The Sanken output transistors are just ordinary power transistors, so yeah, you can diode test them as you would any other transistor. You likely have some shorted C-E now. Oops

Also check the driver transistors Q13 and Q17.

A good tip for working on power amps - build a cable which has a 60-100w incandescent (filament) bulb in SERIES with the Live wire. This acts as a current limiting device. Normally you should see a brief flash as the capacitors charge, then a dull glow. If the bulb lights brightly, something is trying to draw a lot of current, and the bulb will drop most of the voltage, protecting the circuit.

Powering Up Your Radio Safely With a Dim-bulb Tester

Agreed ...
It is highly recommended to use bulb tester, my bulb tester used 100w incandescent lamp (for 2 x 300VA transformer) + on/off switch + fuse + direct switch to bypass the bulb when the bulb is not lights brightly

It is save me so many time & the fuse not need too blow again & again

 

[Graemedench]

Thanks I'll build something up. keeps getting better! got in the car this morning and pop amp in the car dies lol. just a cheap fli900s I was planing on replacing (not quite yet though !) Moved to my very first amp jbl gto2000 (built like a bomb) sounds better too lol.

Thanks for you help its much appreciated. probably be a few days before I get to test and then order Parts I guess. I'll keep the post upto date. I would probably just buy a power module to replace whats on but looks like there is an eq correction circuit in the corner of the board so not a straight swap + I like to fix

Again thanks ----- I'm off to do some tiling now!

PS got into work yesterday and a UPS reported a fault (APC SURT8000xli). thought I'll switch it off and swap it out, I kill the outlets then go to kill the ups on its on/off front panel BANG / FLASH it pops. lol I'd better get off this computer before something else happens!!
(switches to scribe tile cutter from diamond)

 

[Graemedench]

A quick update Or Lack off. I've not abandoned working on this but I had to put a hold on it as a swap of my wife's runabout meant a clutch change on the car. so now that's done I will get back to testing the transistors this week and report back asap.

Again thanks for the help

 

[Graemedench]

A long dead thread but for anyone searching I thought it would be worth a quick post on what the problem was now it is finally fixed ! So apart from the main output transistors and a small selection of other parts failed due to my own stupid mistake, it was one of the small transistors on the board that i guess without the schematic is part of an eq compensation circuit. If I'm honest and I was to come across this again I would probably just replace the lot of the small transistors and any electrolytics. Dirt cheap parts, Original caps are tat and probably past there best by now anyway, and for the time testing the transistors, you could have them all swapped out, defeatist I know but a quick cheap fix.
Me, I was an Idiot since it had been lying for 2 years I sent it to quad to be fixed, £48 + parts for Diagnostics and Repair £15 on parts (all the output transistors & various others) I should have done it myself but hey such is life.

 

[jlithen]

I had the same sub on my table now.

Changed all the 2N5551 and 2N5401, 9 of them.
Something changed, but bias went still out of hands...like 100...200mA in a few seconds, with trimpot set to minimum.

Then I changed the MPSW 42 & 92.
No change.

And last the middle TO-220 insulated body transistor. 2SC4883, this let me adjust bias down to 0. I used 2SC51??, it had high enough ratings 180V and 2A 20W and hfe in the same range.
Some slight hissing (always present) and humming (only at high input sensitivity) was there but it now seems to work.

Got close to 180W @ 8ohm.