Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: smackoj on November 06, 2013, 08:39:15 am
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Hi; Looking for suggestions (help) on my lab series L7. The volume is low on both channels 'Hi' gain input. Also, the Lead channel vol is way low (low or hi input) unless the 'Filter' pot is turned almost all the way off (right=off).
schematic is found here:
http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html (http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html)
hopefully someone has a thought or 2? :icon_biggrin:
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This may sound dumb, but did you check the compressor? if it is turned down low (and on) it can affect the output.
Also try feeding the preamp-out into another amp and see if it too is as quiet. Do the same with another preamp into the power amp in.
The way the input jacks are setup, both hi and low tie together before the first op-amp, so shouldn't be working different unless a jack is messed up. There should be another knob next to the filter knob that will determine whether it cuts or boosts, and is a relatively narrow bandwidth over the guitar audio spectrum. I'd check the opamps too, looking for any that are overheated. If they are, replace them. They normally do not run very hot. (IIRC, I had one go out on the normal channel and had to replace it to get any volume from that channel.)
It's a starting point that will help us focus, if not cure it.
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thanks bill; I do have a preamp I can plug in, and another pwr amp for same. I'll try it out tonight when I get to the church where the amp resides (too heavy to lug around for me). I'll give a report about what a diff. pre and pwr amps make. I don't think there is a "cut" or "boost" switch on it? I'll look closer and see about that also.
good suggestions bill. thanks
:icon_biggrin:
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thanks bill; I do have a preamp I can plug in, and another pwr amp for same. I'll try it out tonight when I get to the church where the amp resides (too heavy to lug around for me). I'll give a report about what a diff. pre and pwr amps make. I don't think there is a "cut" or "boost" switch on it? I'll look closer and see about that also.
good suggestions bill. thanks
:icon_biggrin:
That L7 I had (410 Combo) was definitely heavy... the speaker magnets and that huge heat-sink added some weight.
As you already know that first channel is fairly simple: Vol, Bass, Mid, Treble, and a bright switch with the hi/low inputs (hi/low in reference to signal strength, UNLIKE Fender's Hi/Low meaning) That second channel has the same inputs, the volume, bright switch, bass and treble controls, plus a parametric EQ. (You dial in the center frequency, and the knob just to the right of that controlled the level of cut or boost.) And finally... a Moog multi-filter and Reverb control. To the right of that (and the model ID) is an optical compressor (knob, LED, and switch) and the master volume control.
(http://medias.audiofanzine.com/images/normal/gibson-lab-series-l7-15122.jpg)
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I got the chance to do some 'perimenting with the L7 tonight. bypassing the preamp with a preamp from another SS amp, it sounded great with lots of power. next I connected the L7 preamp out jack to a hefty slave power amp. results were not good. The 'hi' input on the normal ch. and both the 'hi' and 'low' inputs on the lead channel sound weak, lifeless and way too quiet. The only input that sounds close to normal is the 'low' gain input on the normal channel.
thanks again, jack :icon_biggrin:
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Nice clean looking amp. I remember when the Lab came out in the 70's, was a big deal. Platrefire
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I got the chance to do some 'perimenting with the L7 tonight. bypassing the preamp with a preamp from another SS amp, it sounded great with lots of power. next I connected the L7 preamp out jack to a hefty slave power amp. results were not good. The 'hi' input on the normal ch. and both the 'hi' and 'low' inputs on the lead channel sound weak, lifeless and way too quiet. The only input that sounds close to normal is the 'low' gain input on the normal channel.
thanks again, jack :icon_biggrin:
I'd check those op-amps. They're the most likely culprit I think.
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thanks for the help Shrapnel
:icon_biggrin:
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If your real lucky it could just be very dirty EQ control pots or preamp jacks. I have worked on a few of these. There not bad to work on. I have a stash of all the Lab series transistors just in case they go N/A some day
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thanks for the input Plexi
:icon_biggrin:
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I found the service manual documents if they will help. I am picking up a L5 today as well.
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I just picked up a Lab L5 2 days ago. Got it for free because the person said the volume and preamp were real weak. I got it home and took it apart cleaning all the pots and checking the power amp jack and other jacks. All cleaned up i realized that the master volume next to the compressor was turned all the way CCW (off). Turned up master and big bold sound! Was your master turned up? It's the funniest things that we sometimes over look. It happens to the best of us. Let me know if you found your problem.
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well Plexi, yes and no. yes I make sure the MV is turned up and NO I didn't find the problem. I have been just using a different preamp section from a Peavey Renown plugged into the power amp connector on the back panel. I would like to "check the first opamps" but I'm not xactly sure how to do that? I think I sprayed all the controls when I first got the amp but I'm getting too forgetful and can't member fo sho. I guess I'll have to do that again. I want to make sure I know how to test the opamps before I pull the chassis again.
thanks to any and all
:icon_biggrin:
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There's signal and DC voltages on the schematic. Go through the DC points. (Most opamps will be zero DC +/=100mV, so you can run those fast.) Then get a signal generator and a good AC voltmeter, read the notes, and check the signal voltages. You don't care 300mV or 330mV, you are looking for a place where should-be 330mV is really 20mV or something way out.
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message rec'd PRR
gracias :icon_biggrin: