I have this wonderful Lab Series L7 that I bought cheap.
The only thing not working is the compressor. So I opened it up, detached the front board hoping to find a quick solution like corroded leads etc. Couldn't find anything so I fired it up and got a loud noise, probably due to lack of grounding from the front preamp board. One of the two fuses most closely to the main switch was blown but after replacing it, the main switch wouldn't lock in and prevented the amp from starting up. The main switch sometime works again, but the fuse most closely to the main switch keeps blowing out. The amp itself works fine if the main switch works, but the compressor doesn't.
All in all very confusing.
Any ideas are very welcome.
http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html
http://www.rru.com/~meo/Guitar/Amps/Lab ... cheme.jpeg
Lab Series L7 compressor problem. SOLVED
Moderators: pompeiisneaks, Colossal
Lab Series L7 compressor problem. SOLVED
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Last edited by guile on Fri Jun 25, 2021 8:27 am, edited 1 time in total.
Re: Lab Series L7 compressor problem. Please advise
Switches/breakers CB1 and CB2 are suspect and should be replaced? I don't know. I'm guessing.
Can you use an ordinary double pole mechanical switch and fuse both legs? (Fuse or breaker probably required for 240V operation.) I'd place the fuse between the switch and the plug, insuring the fuse is in line and under load any time the amp is plugged in. Would that be an acceptable permanent substitute for the existing CB1 and CB2?
Otherwise, there is a short downstream from CB1 and CB2. I hate to say it, but maybe it is a bad power transformer?
There is an auxiliary outlet shown on the schematic. Though I am doubtful it is a problem, you might disconnect it to eliminate it from the circuit and verify it is OK.
Can you use an ordinary double pole mechanical switch and fuse both legs? (Fuse or breaker probably required for 240V operation.) I'd place the fuse between the switch and the plug, insuring the fuse is in line and under load any time the amp is plugged in. Would that be an acceptable permanent substitute for the existing CB1 and CB2?
Otherwise, there is a short downstream from CB1 and CB2. I hate to say it, but maybe it is a bad power transformer?
There is an auxiliary outlet shown on the schematic. Though I am doubtful it is a problem, you might disconnect it to eliminate it from the circuit and verify it is OK.
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Stevem
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Re: Lab Series L7 compressor problem. Please advise
That lack of a ground is a big issue!
That fuse blowing is on one side of the power supply rail so either the negative or positive side. This in tern means one side of the output stage now has at least a bad output Transistor or two if it's over a 50 watt amp and maybe that section of the output stage has bad drivers Transistors now also.
A Quasi complementary output stage like this amp has can not be powered up with out a ground reference for the circuit!
That fuse blowing is on one side of the power supply rail so either the negative or positive side. This in tern means one side of the output stage now has at least a bad output Transistor or two if it's over a 50 watt amp and maybe that section of the output stage has bad drivers Transistors now also.
A Quasi complementary output stage like this amp has can not be powered up with out a ground reference for the circuit!
Last edited by Stevem on Thu Jun 24, 2021 3:04 pm, edited 1 time in total.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Lab Series L7 compressor problem. Please advise
I just had a similar issue in my old Audiolab hifi amp, -ve rail measured short circuit to ground. I suspected an output transistor but it turned out to be the bridge rectifier.
Craig
Thinking about my second build.
Re: Lab Series L7 compressor problem. Please advise
Thanks for the replies.Stevem wrote: ↑Thu Jun 24, 2021 2:53 pm That lack of a ground is a big issue!
That fuse blowing is on one side of the power supply rail so either the negative or positive side. This in tern means one side of the output stage now has at least a bad output Transistor or two if it's over a 50 watt amp and maybe that section of the output stage has bad drivers Transistors now also.
A Quasi complementary output stage like this amp has can not be powered up with out a ground reference for the circuit!
Could you please point out which transistors (please see above layout or scheme).
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Stevem
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Re: Lab Series L7 compressor problem. Please advise
If you have to ask this question about the output transistor locations ( which are not on that power supply board anyway) then you should be attempting to work on this amp in my opinion.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Lab Series L7 compressor problem. Please advise
I think you mean NOT working? Thanks dad I'll be fine, I'm just not used to reading schematics every day.
I'll figure it out and learn somewhere else.
Thanks
I'll figure it out and learn somewhere else.
Thanks
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Stevem
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- Location: 1/3rd the way out one of the arms of the Milkyway.
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Re: Lab Series L7 compressor problem. SOLVED
I don’t say this to be mean or come off high and mighty by any means, but Reading schematics and having enough electronics knowledge ( especially with SS gear! ) to know what can be unplugged/ disconnected while having the amp powered on are two different things, and you have proved this yourself with your actions !
It’s not that I go not want to help you out, I don’t want you to blow more of the amp up then what you already have, but I guess you can’t see that, or I did not get across in my first post to you here.
It’s not that I go not want to help you out, I don’t want you to blow more of the amp up then what you already have, but I guess you can’t see that, or I did not get across in my first post to you here.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Leo_Gnardo
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Re: Lab Series L7 compressor problem. Please advise
Right up there with "Okayyyy, Boomer!"
Good luck to that "somewhere else" our fearless fosdick brings his amp fixit queries. He sounds like a hopeless case to me, with an amp to match.
down technical blind alleys . . .
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thetragichero
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Re: Lab Series L7 compressor problem. SOLVED
with a bit of googling one can find the schematics and a three page document that gives troubleshooting info and tells how to bias the output stage (involves powering up with one of the fuses removed and adjusting a trimmer pot until the current draw between the two fuse contacts is a certain measurement... i think 200ma but my memory could be wrong). the 'compressor' is really a limiter and there's a few components that could go bad. hopefully not the ca3094 as these are quite a bit harder to find that the ca2080 or 4558s/jellybean transistors
all THAT being said, working on a sick solid state amp is a bit different than working on a sick tube amp (lightbulb limiter being your best friend until it's operating fine both without and with a load attached). you may have learned the same hard lesson i had to by fixing it more broken. if unfamiliarity with schematics is an issue, one really should try crawling before they attempt to hit the ground running full speed. effects pedals are a good place to start as they are orders of magnitude less complicated. a couple sites like electrosmash are really good at breaking down schematics into circuit blocks so one can understand what's going on while looking at an unfamiliar schematic
please do not think that folks are unnecessarily gatekeeping. there are many here that go above and beyond trying to help others (frankly with loads more patience than i'd ever have) but trying to diagnose an amp that is not in front of you is difficult to begin with. if someone seems to be beyond their depths, there may be more harm done than good
just as an aside, i LOVE these old lab series amps. even the little brother L3 (which never seems to get mentioned in the same breath as the big boys) uses cmos switching chips (cd4001?) in an interesting way. those moog engineers did some neat stuff while working for norlin (the rd series gibsons being another example)
all THAT being said, working on a sick solid state amp is a bit different than working on a sick tube amp (lightbulb limiter being your best friend until it's operating fine both without and with a load attached). you may have learned the same hard lesson i had to by fixing it more broken. if unfamiliarity with schematics is an issue, one really should try crawling before they attempt to hit the ground running full speed. effects pedals are a good place to start as they are orders of magnitude less complicated. a couple sites like electrosmash are really good at breaking down schematics into circuit blocks so one can understand what's going on while looking at an unfamiliar schematic
please do not think that folks are unnecessarily gatekeeping. there are many here that go above and beyond trying to help others (frankly with loads more patience than i'd ever have) but trying to diagnose an amp that is not in front of you is difficult to begin with. if someone seems to be beyond their depths, there may be more harm done than good
just as an aside, i LOVE these old lab series amps. even the little brother L3 (which never seems to get mentioned in the same breath as the big boys) uses cmos switching chips (cd4001?) in an interesting way. those moog engineers did some neat stuff while working for norlin (the rd series gibsons being another example)
PRR wrote: Plotting loadlines is only for the truly desperate, or terminally bored.
Re: Lab Series L7 compressor problem. SOLVED
This forum is known to me having very patient and often helpful members. I indeed lack active knowledge, but I managed to build several tube amps so far.
As I love this amp also very much, I'm looking for solutions; a point from where to start, to make it work again, but I think this problem is over my head. I'll have expert look at it.
Thanks and I'll report back!
As I love this amp also very much, I'm looking for solutions; a point from where to start, to make it work again, but I think this problem is over my head. I'll have expert look at it.
Thanks and I'll report back!