L212 Ultrabass Woes

[Dirk Morgan]

Help, please!

The amp built into the subwoofer of my beloved L212s is constantly blowing its power outputs.
My repair shop of choice here in Portland, OR has replaced them twice; the first set lasted about 5 hours, the second set three. The repair shop has now thrown in the towel and given me a full refund. Might anyone have a suggestion\scenario as to potential instabilities upstream in the amp's (circa 1977) circuitry that could cause it to keep blowing? At 90 pounds I'm hesitant to ship the unit to a more vintage-JBL-centric service center, but if thats what it takes, so be it. Any suggestions on the west coast?

Thanks!

Dirk

[4313B]

Pull the amp and take it to an Authorized JBL Service Center. They should have the service manual. It doesn't weigh a whole lot. I've taken a few apart over the years but I think Don C pulled one apart very recently and he might be able to offer a tip or two. It's a pretty simple amp. The four 2500uF 60V capacitors are a known weak spot.

[subwoof]

Also have them check the bias - on ANY solid state amplifier the voltage should be between .325 and .450 from base to emitter on *any* output device.

If it's too high, it will cook itself in a short time.

Just replacing them without doing this is like putting on new tires after after running over a curb and killing the old ones....... Just how many miles before they go too???

sub

[GordonW]

I assume you've already done this, but, just in case...

You also should check out the woofer itself, to make sure its flex-leads (terminal-to-cone) aren't displaced enough, to where they can touch each other. This would definitely cause the problem, should it happen.

If they are tendant to touch, you can bend the little metal tabs apart, on the backs of the terminals, to pull the wires further apart from each other. Also, just some hand-reshaping of the wire "arcs" can make them "take a set" facing outwards away from each other, too...

Regards,
Gordon.