Lab Series L5 problems...

[dkjar]

And looking at the schematic here http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html makes be believe the compressor is in the pre-amp circuit, which should be bypassed by plugging my guitar directly into the poweramp input....

[phatt]

Opps!! good point. xP
OK sounds more complex,, Then a fair chance the issue is in the powerstage or Pwr supply.
So you may have to wait until some of the better minds here read this.

Hum,, headscratch,,,Just out of interest?? You did say a speaker was replaced or something?

OK just check and see if both are actually MI speakers and not some kind of wobbly hifi woofer.
(MI speakers are very stiff and hardly move whereas hifi woofers are easy to to move.
They are often the ones with the big rubber ring around the edge.)

With a 100~200 Watt Amp driving long throw hifi speakers they will certainly pop and crackle on big notes as the coil hits the back of the magnet, especially if the Amp is open backed.
Phil.

[dkjar]

Yeah I made sure.  The replacement is a Eminence of some make and a guitar speaker, but mismatched.  8 ohm rather than 16.  I have it disconnected with a new 16 ohm coming today.  I bypassed both and plugged the celestion from seymour duncan into it (my gig amp, works like a champ) and still had the problem so I know it is not the speaker.  It is such an odd sound, the way it repeats and fades.  The reverb is out too and it's location is a bit strange to me in that schematic.  I am going to bring in to another tech tonight, hopefully he can spot the trouble.  It really does sound awesome so I want to get it fixed.

[phatt]

Yeah I made sure.  The replacement is a Eminence of some make and a guitar speaker, but mismatched.  8 ohm rather than 16.  I have it disconnected with a new 16 ohm coming today.  I bypassed both and plugged the celestion from seymour duncan into it (my gig amp, works like a champ) and still had the problem so I know it is not the speaker.  It is such an odd sound, the way it repeats and fades.  The reverb is out too and it's location is a bit strange to me in that schematic.  I am going to bring in to another tech tonight, hopefully he can spot the trouble.  It really does sound awesome so I want to get it fixed.

Cool just checking obvious stuff,,
OK, What do you mean about reverb?
Are you just referring to the odd driver or is it affecting the sound?   
You used the word *Repeating* which might indicate a reverb problem.

You could unplug the tank at both ends and see if things improve.

Some trivia,,,, That reverb circuit has the potential to be killer,,, I should know as I've built similar Reverb circuits with great success.

Ed; Oh forgot,, check the tank inside for broken springs they can wreak havoc
Phil.

[dkjar]

Now that you mention it I am going to disconnect the tank and give it a shot, the reverb pot makes no difference to the sound.  If you listen to the file I attached a few messages ago you can hear it yourself. 

Just tried.  No dice.  Still popping.  I let is sit and warm up without a guitar plugged in and it started popping on its own even. 

[phatt]

Oh well you tried and no dice

Sorry I can't be of more use,,, You will have to wait until someone who knows more can help.
I've never even been close enough to sniff the outside of this Amp let alone been inside one. lol
Phil.

[Roly]

Some thorts.

My first impression from the MP3 is that you may have a frayed speaker braid; these often seem to have their worst effect when the level decays away.

* Test the speaker with a different amp. If it still sounds dirty you can confirm by clipping a 1.5V battery across the suspect driver and gently moving the cone back and forth using spread fingers.  If it's a frayed braid you will get crackles, pops, and it may even act like a buzzer at certain positions.  My fix for this is to locate the break, almost always just behind the connection tag strip, where the solder ends, and make it good with a dab of solder. (but JM is the real speaker expert here and may have other ideas)

* Running a cab with mismatched drivers the lower impedance one will hog the power.

* Running a cab with one of two or more drivers will cause the other(s) to drive the disconnected one in opposition, and will reduce the air loading on the back of the driven units.

Try running the amp into a different cab. This should settle any question about which is giving the problem, speaker or amp.

{If you have already done any of these, please forgive me because I've been struggling with a crook internet connection for three weeks now and haven't been keeping up}

[dkjar]

Thanks for the suggestions.

Yeah, I plugged the l5 into another known good cabinet and the popping was still there.  Although the speakers were mismatched when I got it I just put in the a 16 ohm replacement (red coat private jack).  The speakers sound great until the amp warms up.  Then they still sound great except for the popping.

Last night I dropped it off with another tech.  I think he will be able to make some progress.

[phatt]

I'd go hunting cracked solder joins around Can type capacitors on the pcb.
Don't go in the jungle without very good light and a magnifying glass and that applies even if your eyes are 30 years younger than mine .

You are looking for hairline cracks either in the solder itself or in the tracks right at the join where  solder meets copper.
Start with the big ones first and work your way down to the smaller ones.

I had a Valve rig a while back with a cracked track which was hidden by overlapping solder and it worked for a few minutes then pop,, the Amp stopped dead. :duh

As luck would have it,, it was getting very late in the day and I noted a small flash of light from the Capacitor join as the PCB was gently tugged.
Phil.

[dkjar]

I am kind of suspecting something like this.  I noticed that the pcb, even on the preamp circuit, is a little brittle.  When I testing the led for the compressor I noticed the solder joints where starting to lift off of the board and looking a bit funky.