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December 02, 2023, 04:47:24 PM

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Lab series L11 - can anyone see where the distortion might be coming from?

Started by SwamiRob, January 10, 2021, 07:48:46 AM

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Hey guys, was just wondering if someone might be able to track down some OTT distortion I'm getting from the 2nd channel of my Lab Series?

The thing needed some attention after I'd got it and it was putting out a really inconsistent signal on both channels (although there was slot of noisy drive on the 2nd channel still) and I took it to a tech. I'd re socketed the reverb input/out from the board as it had all been disconnected. The tech managed to fix the power amp issues and there was now the ton of headroom you'd expect from a well made 70s 200w solid state amp. There was a weird feedback when you turned the master up, but so much volume that didn't matter.

Both channels now worked fine, but the connection to the reverb & distortion circuit had now been severed, reattaching it there was very very little volume from either the reverb or dry elements, and a really over the top clipping from the dry signal still present. Pretty much had to have the channel volume barely on to have lower gain clipping but nothing clean at all, although you could tell the signal sounded alot richer for going through that bit of circuit, as the sound is quite sterile without. Could live without it maybe as I can get round the reverb issue by splitting the signal and going through the footswitch, but that's predictibly noisy even with some signal boost from an EQ pedal, and also gets some nasty clipping when there's overdrive unless I really really full the signal (I know spring tanks get a LPF/HPFed signal to my knowledge, but it really doesn't sound right.

Got a new CA3094E for the distortion circuit, and the dry volume is better but not great, and there is now an increasing high/upper mid distortion signal that gets introduced as the master is turned up. The reverb however is now at a decent level with very little noise. I've messed with the distortion trim pot and that merely goes from very very distorted to extremely distorted. There is some breakup without the connection made to the circuit like it was when I got it back from the tech in a bit more of a controlled manner, just the overall signal seems like it'll be a tad warmer when it's working like it should, and the reverb sounds awesome.

Any ideas would be very appreciated! I can't find the listing from when I bought it, but they had mentioned that they had some unconfirmed reports it had belonged to some 80s metal band at some point, do not sure what might have been messed with!


Diagrams here, the disconnection to take the distortion circuit out is at O104 on the board layout. Bit confused by which chip it actually wants as there's reference to a CA3094, CA3094A AND CA3094AE in various bits of documentation, although from what could find it's just to do with voltage tolerance? Also confused about why that connection been severed takes the distortion out the circuit, as it seems to be split into 2 at that point and the distortion part is in the master section with the compressor (which still works regardless). I don't know a whole load about electronics however!

Also some pics here which I couldn't upload - https://photos.app.goo.gl/N4gUPZDWYLUg4khj9

*EDIT* Just had a mess with it again now, and the distortion circuit actually IS engaged when that O104 connection isn't made, seems to make a subtly quite nice difference to the tone when the trim pot is adjusted for maximum gain. Unless it's very low key I don't hear any breakup though, just makes the master more sensitive so don't know if the improvement in hearing is due to improve S/N ratio? It does seem to make a difference to how the 2nd channel clips when the disconnected wire is reconnected.

I've also tried disconnecting the reverb tank just in case I'd wired that incorrectly when I put the new connectors on that, however it makes no difference. So I'd be guessing the problem seems to be in the 2nd channel preamp somewhere but I'm a bit out of ideas now!

The only thing I can really think of now is the wire going from Ch2 has 2 connections, however the shielded part of it doesn't make much difference when it's connected. Says it goes straight to an earth as far as I can see on the schematic, but don't know if it's because something is broken and it's not having a proper earthing effect, or it's so noisy that the earthing effect is barely audible. Can't really imagine this has anything to do with it however!