Tube Amp Archive

I'm looking at a JBL MR822. The owner says there's no sound from the horn. It actually does put out some sound. I'm not sure what a blown diaphram would sound like -- arctic wasteland, raspy, low output? The crossover works fine and woofer is good.

Then no problem to just throw a 1' (guessing) replacement at it.

Pull the diaphram and look at it. It probably has a dimple. If it looks good throw 1.5v at it to see if it cares, then replace it.

Notorious failures. Weak link. Physics rules.

Everything you said is possible and proves one failure or another. They break in a few ways. Most overdriven ones get a big dimple and they sound like you are holding them from excursion with your fingertips. Barely any output and rattly if anything. The coils burn too. Nadda noise then.

Reeltarded wrote:Then no problem to just throw a 1' (guessing) replacement at it.

Pull the diaphram and look at it. It probably has a dimple. If it looks good throw 1.5v at it to see if it cares, then replace it.

No dimple. It reads about 3.6 ohms. The + lug is good and tight. The - lug is loose.

Are you saying to put a 1.5V rms sine wave across it?

Aha.. See if you can make that reading fail by tampering with the lug then. If that fails just goo on a little 5min epoxy and make sure it is working when that sets.

I just use a battery and tap it 2-3 times. Morse code.

Reeltarded wrote:I just use a battery and tap it 2-3 times. Morse code.

No sound by itself. Reattached to the horn, it does make some high pitched clicks. AA battery.

Only high quality compression horn diaphragm I've ever replaced you couldn't tell it was stuffed. Measured OK, looked OK, sounded terrible. The owner of the speakers actually had a full set of replacement spares he'd bought when he made the speakers. These things were 7 foot towers with double 12" drivers, plus a mid and the compression horn. Pair of them in his dining room. It was an understatement to say they dominated the room decor. But they still are the best speakers I've ever heard. Anyway, I put the new diaphragm in and all was back to normal.

All I can suggest is that the sound of it is the only way you'll tell. Isolate the signal to the other drivers in the cab so you can just hear the horn by itself. If it's blown the output won't just be low, but it'll be really harsh too.

I'm going to put it back together and connect it to my PA and see how it sounds. [Edit -- no sound from the horn...oh well] It may have been the loose connection. Otherwise, it looks like there's an aftermarket part for $40. The JBL part is around $185...makes no sense.

Aftermarket replacements might be just fine though. I'd take that chance on a pair.

Check the crossover. I have seen more than a few JBL Crossovers fail or just a open connection, but usually in the crossover.

billyz wrote:Check the crossover. I have seen more than a few JBL Crossovers fail or just a open connection, but usually in the crossover.

The crossover looks good. I measured all the caps and checked the inductors. I ran a sine wave through all the connections.

I've ordered a diaphram.

Now I'm irritated...the new diaphram didn't help at all.

I'll have to troubleshoot the crossover...which seems pretty simple, but I thought it was fine.

I don't have white noise generator, though I'm sure I can find a circuit on the internet somewhere. I've got a decent enough signal generator. I think it's 50 ohms out...have to look.

The crossover looks good. I measured all the caps and checked the inductors. I ran a sine wave through all the connections.

Did you see an output signal on the high and low side on the output of the crossover when you checked it?

vibratoking wrote:

The crossover looks good. I measured all the caps and checked the inductors. I ran a sine wave through all the connections.

Did you see an output signal on the high and low side on the output of the crossover when you checked it?

Yes...I have to do it again.

[edit]moron...it works fine now. I really needed to drive it from a PA, not a function generator. aargh!