I just purchased an L5 and it came pretty messed up. I have now just noticed that they replaced one of the original speakers with an 8 ohm speaker (so it has a 16 and an 8).
When I turned it on, the compressor light briefly lights up and then goes out regardless if the comp switch is on or not.
The volume is very low. To get any kind of sound out of it you have to turn both pre and pwr amp volumes up to 8 or so.
The compressor does not work or light (just that initial light).
The power LED does not light at all.
When I disconnect the 'replacement' speaker none of this changes.
Getting ready to return it. Any suggestions from you guys? Did they fry it with the oddball speaker configuration?
and it had super gummy pots but I fixed that with electrical component cleaner.
The first thing I'd do is measure the DC voltage across the speaker. This should be pretty close to zero, certainly less than 100mV.
Next is connecting the speaker output to a known good cab, and feeding a signal in to the Power Amp In on the rear panel. My guess is that it will come to full life, meaning the output stage is okay, but if it doesn't you could have dead devices in the output stage. If it does come good then you need to test its speakers with a known good amp. If they also test okay then we are looking back towards the preamp area.
Since you are considering returning it rather than attempting a repair I won't go more deeply into it at this stage, but another test you should do is patch Preamp Out into Main In with a known good lead, and see if that clears your problem.
From what you say about the LED's my feeling is that you have a problem with the low voltage supply for the preamp.
Find circuit attached.
HTH
Wow, thanks for the heads up. I just dropped it off at the shop and sent them your notes and schematic. Fingers crossed. I don't want to send this back to Florida....
Do you think it will be repairable? Extremely expensive? Not worth it?
I think the LED's not lighting are the clue; the lower speaker impedance isn't good, but your description is not really consistent with a power amp problem, and that wouldn't explain the LED behaviour.
When faultfinding we normally try to find a single cause of all the symptoms we see, and in this case the LED's suggest the problem is in the preamp, and specifically it's power supply (the "power' LED L101, (upper-centre) comes off the -15V supply, so it's Sydney to a brick that the -15V supply has failed - this would also be consistent with the compressor not working and the weak distorted output).
Everything is repairable. My *guess* is the zener diode CR204 (bottom-centre) has failed. If so it should be easy to find and inexpensive to repair.
Wonderful! I sent this to my tech... Hell I will give it a try if he gets stuck.
It's a guitar amp, not the space shuttle. Anyone who calls themselves a "tech" should be able to fix this without a circuit, no matter what is wrong with it. If it's what I suspect it should be a doddle. :tu:
QuoteDo you think it will be repairable?
Yes.
QuoteExtremely expensive?
Define extremely expensive.
Some people find 1$ way too much.
QuoteNot worth it?
Define "worth".
And the compression control on the Lab Series amps is a threshold control, not an amount control. SO it sorta works backwards of what many might think. SOme folks wind up trying to "fix" it when it is working properly.
Apparently it is not that cr204. He found only 5v heading to the preamp but now he suspects it is something in the preamp itself. . He replaced one 741 opamp down by the reverb connection but that was the only piece he could get anomalous readings out Of on that board. Everything on the power amp came out roses. He said something about checking the opamps located on preamp.
He also said the compressor is working, just the low voltage is keeping the light off.
I hope you realize that this indirect communication does not exactly help.
Besides that, I would be very alarmed if I found a 15V rail now is 5V, would try so find why, and correct it.
And then continue repairing.
Just sayin'
What JM said. :dbtu:
Lack of a power LED indicates a lack of minus 15V. The plus 15v rail at 5V could be either lack of supply or excessive load. If it were me I'd pull connector P22 and see what the supply did, but assuming that your tech is a real one I'm just going to stand back over here out of the way, content with picking 8.5/10 - not even having seen the amp in question. If he's any good he should have it fixed by now anyway. 8|
{Frankly I'd be inclined to replace a 741 in an audio path with something better, simply because it's a 741 (but then I'm funny like that).}
I agree with the above advice about communication. Why not invite your tech guy to visit here and even join. After all it is free, and we don;t bite. We have had various Lab Series discussions on this board over time.
I wish he would join but he does not appear particularly comfortable with computers. He is quite good but old school. If he hits a wall with this I will definitely ask him to post what he has done (even if I have to type it for him) and see what you guys think.
I think he is taking his time and enjoying it so I don't want to rush him. He is pretty methodical (and careful) about it.
He did pull p22 and I can ask what happened. I plan on bugging him again this afternoon.
Yeah, well go easy; if he's old school and knows what he's doing he won't appreciate having a mob looking over his shoulder - I know I wouldn't.
Gotta say 'tho, as a 60+yo tech I can't live without the net these days, Google is a great help. :dbtu:
If he joins and asks about something, he'll be treated with the utmost respect, like we all here.
FWIW *I* joined here looking for advice , which I got by the truckload with excellent results. :dbtu:
Yeah, and look at the mess you're in now! :lmao: ;)
I too am a crusty old mid 60s guy - on medicare and social security - and if I am anything it certainly is old school. But I couldn;t imagine doing all this without the internet these days. Imagine coming up with a Lab Series schematic without the internet today.
The last thing we would want to do would be get in his way if he feels he is making progress. We won;t talk down to him. What you have here is some old pros with serious experience and happy to share.
No InterWeb? Well fust we wouldn't be trying to fix some amp on the other side of the planet by remote control, and if it was under our noses we'd be doing what we always did - seat of the pants.
(http://www.ww2aircraft.net/forum/attachments/aircraft-pictures/50180d1299775587t-flying-seat-your-pants-those-were-days.jpg)
"Yes, I know the engine has stopped ... "
But as a commercial repair tech, the internet helps save me from ever having to say "Gee, I dunno, beats me, I have no schematics." AT least the TV repair guys had Sams Photofacts.
I mainly did industrial electronics BI (Before Internet) and if there were circuits available at all they were generally in a language I didn't speak. TV? Half the machines I worked on I first had to figure out what they were supposed to do, then how they were supposed to do it, and finally why they weren't.
"What's it do?" I asked.
He looked astonished and said "How can you fix it if you don't know what it DOES?"
"If it works on known physical principles, you didn't import an alien technology from Alpha-Proxima, I can fix it."
I considered documentation of any sort a luxury.
QuoteI considered documentation of any sort a luxury.
Er...., you don't live in Argentina, do you? :lmao:
Real technicians don't need circuits, right JM?.
Many times had to think so and go ahead, but the sure do help if available :tu:
Well.... after 4 months the amp is back. It works now but now it has other issues. The compressor is still out, and if I turn the volume up much it starts to pop almost like a blown speaker. I will do some more trouble shooting tomorrow to figure out if its pre or power amp issues. Seems to be affected the most when I turn the frequency pot down all the way and the mid range up all the way.
I plugged the guitar straight into the power amp and still got the sound so I assume that means it is post pre amp. I made a recording. Sounds pretty awful in mp3 (was amr) but you will definitely hear the issue. The first part is just straight through on the second channel. you can hear the repeated crackle/pop. Then there is a pause as I plug directly into the poweramp and you can again hear the same problem. Compressor on or off does not matter.
Anyone know where I should start? I think it sounds like some others problems that have been on here so I am going back to read those old posts again.
And.... it takes a little bit before it starts. So something is warming up and then causing the issue I would assume.
By a little bit I mean 1 minute or less.
Assuming all supply voltages are reading normal first,,,I'd be thinking of bypassing that compressor part (if possible) as that part seems to be an obvious suspect.
Phil.
Bypass the compressor meaning more than just turning it off?
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....
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.
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.
Quote from: dkjar on February 20, 2013, 08:21:39 AM
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.
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.
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.
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}
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.
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 8).
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.
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.