Lab Series L5 problems...

[dkjar]

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 . 
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.

[Roly]

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

[dkjar]

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....

[dkjar]

Do you think it will be repairable?  Extremely expensive?  Not worth it?

[Roly]

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.

[dkjar]

Wonderful!  I sent this to my tech...  Hell I will give it a try if he gets stuck.

[Roly]

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:

[J M Fahey]

Do you think it will be repairable?

Yes.

Extremely expensive?

Define extremely expensive.
Some people find 1$ way too much.

Not worth it?

Define "worth".

[Enzo]

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.

[dkjar]

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.

[J M Fahey]

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'

[Roly]

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).}

[Enzo]

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.

[dkjar]

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.

[Roly]

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: