Lab series L3 Schematic help

[shun]

Hi,

I stumbled across this site whilst looking for circuits - and i am new here so hello everyone.

I have decided to try to get my old Lab series L3 amp working again - there seems to be very low output from the pre amp section. I have spent all day on it with no luck so far.

Does any one have any idea where i could find a schematic for this amp? I have looked all over the web and can only find circuits for the rest of the series which seem quite different from mine.

Cheers all

Paul

[J M Fahey]

Hi Shun. Haven´t found it either, but I´m sure it´s a stripped down version of its big brothers, so having it´s brothers schematic in front of you should be help enough. Start by checking that TRS single-jack effects loop. You might have a problem there.
Bye.
J M Fahey

[J M Fahey]

Hi Shun.
Nice little amp you have. I have not been able to find much about your beautiful L3; definitely no schematic, but  I know usually designers have a certain way to do things and apply those ideas to most of their products. So all Fender/Marshall/Acoustic/Peavey/MusicMan/Gallien ... anything,  from a certain era are very similar and the schematic from one model helps a lot servicing a "brother". Yours certainly is a stripped down version of the L5 up series. I  guess that the preamp is probably like the Channel I , which is simpler, Fenderish, passive controls, and not Channel II which is more complex, active, and definitely you have no multifilter or parametric. Of course I may be wrong !! The reverb circuit must be very similar. The power amp is probably basically the same, but with only two power transistors and a +-B supply voltage of around +-40 Volts. I guess the distortion/master with the CA3094 *should* have been used there too, because it´s the "signature sound" of the Labs, but of course, they may have used just a couple of diodes to keep the cost down. Sigh! What you don´t have, and it´s a pity, is the compressor, because almost everybody at Harmony Central praises its clean sound up to "4", but complain it breaks up from "5" up. If possible post a couple of clean pictures of the preamp and output board, and a list of IC and transistors codes (the ones you can read), that will help.
Bye.
J M Fahey.

[shun]

Hi,

Thanks for the replies and advice - really appreciated, thanks.

First problem is the two input jacks high and low - the switchcraft jacks were not making contact with the jack plug and seem to be wired strange - not what i was expecting at all. Even after removing them and temporarily wiring another standard socket to the hi input on the PCB the volume is very low with both master and volume up full. If i have any luck getting it fixed i will have to work out how to get new inputs as i doubt if i will ever find replacements. It seems to have complicated switching (to me!!) on the inputs that i need to work out - as i have now removed the sockets there are extra PCB lands that are not being used - running into the preamp.

I tried running out the preamp direct into another amp - and the volume seemed ok - although i still had the master volume up full. If i touch the input leads to the power amp there is the usual loud buzz.

Oh and the reverb doesn't work!!

Still, at least i am learning!!

Paul

[Boprikov]

Maybe the switch in the power amp in jack is oxidized. Sometimes it helps to connect preamp out to poweramp in with a guitar cord. It bypasses the jack switch.

[J M Fahey]

Hi Shun.
Everybody seems to say that a copy of the schenatic is glued somewhere inside the amplifier, maybe on a back panel or the "roof" of the cabinet.
Ahem!! Did you check that? Thanks.

[Enzo]

If we can post files directly here, I don;t spot how, so I posted this schematic in the Enzo Gallery at www.ampix.org.  Right now they are in the "most recent" area as well, but look under member galleries for mine and they should be there.

Sorry it is in three pieces, it is a long foldout drawing.

The center segment has some resistor numbers in gray circles which obscures them on the scan.  From highest to lowest position, they are:

R4  3k  (above C4)
R3  15k  (below C4)
R7  470  (base of Q3)
R6  15k  (base of Q2)
R1  33k  (base of Q1)
R5  680  (series C3)
R2  1.8k  (collector Q1)

[shun]

Enzo,

You are an absolute star!! Thanks for taking the time to do that, much appreciated.

Thanks to everyone else with your suggestions as well

Cheers

Paul

[J M Fahey]

Hi Enzo , thanks. It´s great that you are so kind as to take your time to search, scan, and post schematics, which are *very* valuable to us all. In this particular case, I guessed wrong, assuming it was somewhat related to its brothers. Not at all, it clearly was designed by someone else, and even more, probably from the synthesizer team and not the audio team.
Not many in our field would have used all those precision servo biasers.
On the contrary, thanks God, the power amplifier is fairly conventional, which is good because it´s something more easily overworked and damaged.
The unspecified output transistors can be safely replaced by 2N3055.
But the Preamp!!: fully hung from the -15 supply, with servos everywhere to keep everybody exactly tied where they belong, even feeding the first FET from a stabilized negative current supply ... I should at least that it´s original, no doubt.
Enzo: thanks again.

[shun]

Hi all

Found the problem - it was he second LM358N - replaced that and back came the volume!!

Thanks again

Paul

[J M Fahey]

Shun/Paul, congratulations !! That bad 2nd LM358 would have killed the biasing of both CD4007, namely active tone control, reverb send and receive and Master driver, almost nothing !!!
Did you measure something wrong there or just changed it out of suspicion? Because the unusual thing about them is that they don´t handle audio at all, only bias different Fets and Mosfets.
Bye.

[shun]

Hi

Thanks  - i am chuffed because it's also my birthday as well!!

I have some basic electronics knowledge - but after seeing the circuit diagram i realised it was a bit different. I started checking component by component from the input starting forwards. After that it was a choice of FET (which i could not source) or changing the two chips. Luckily i had a couple spare, and after changing the second one back to life!! (wish i had done that first!!)

I had difficulty with the circuit - i can follow normal amps to a degree but this one had me stumped what was doing what.

I have been playing guitar and bass on and off for 25 years and for guitar have always used tube amps (still do) but after hearing the sound quality of this i have to say that if i was to restart gigging again, i would go SS. I was really surprised at the quality this amp gives.

All i need to do now is find a couple of input jacks for this - they are knackered - even cleaning them is no use.

Cheers

[teemuk]

The attachment option can be found under "Additional Options" menu (you can't see it in quick reply mode, though). I put together a single schematic combined from the three pdf files and included the component values given by Enzo. Thanks for sharing this one.  Interesting design.

[Enzo]

Ah, always good to have additional options.  Thanks for cleaning up the scan for me.  I didn;t want to gray-scale it and make the file huge.