Reviving 1978 Lab series L3

[Vinny]

Yes, the LED does light, I know its weird, but I will keep testing maybe I will de-solder the transformer from the board and then test again

[J M Fahey]

I'm trying to make you think.

Yes, the LED does light, I know its weird,

Weird?
Wouldn't you rather think it's getting power fom *somewhere*?
Wouldn't it mean an actual working transformer woud have something to do about *some* voltage/current reaching said Led?

but I will keep testing maybe I will de-solder the transformer from the board and then test again

Well, desoldering a transformer from the board will certainly turn it into a non working transformer, at least from that board's point of view.

Now on to somewhat more positive ideas:
1) Your transformer *is* working so far
2) Your only symptom, so far, is that the amp turns on, leds light, but it's silent.
*DID* you

Insert a bare stereo plug in that processor jack and touch either of its hot legs, do you get some hum in the speaker?

I guess not.

[teemuk]

For some basic starters
- Did you limit current (e.g with a light bulb in series with the amp on the main circuit)? If not, then a good chance is that former repairs you did have now gone down the drain. Always limit the current before powering on equipment most likely having faults.
- Now that the current limit issue is out of the way... Is the current limiter indicating that the amp pulls excessive current. For example, a blown output transistor could sag a power supply rail. Is the transformer making noises?
- You suspect a blown transformer? Easy test: pull the secondary circuit off and test the transformer unloaded? Is it working or not? Don't see what we benefit from your suspicions of what could be wrong when you can actually measure something and have verified information. So far it seems the power transformer is actually at least powering the LED, which should further on mean that the -19V regulator works as well, so.... Don't guess: measure.
- Other suspectible problems that can mute the thing: failed inverters, failed OpAmps DC servos throwing the output to plain DC, failed solder joints, dirty contacts, etc. Have you ruled these out yet?
- You have the schematic, the amp and a multimeter in front of you. I don't see how we can actually help much further.

[km6xz]

I have a better idea. Don't unsolder the transformer or anything else unless you can prove that it is defective. Ignoring that will make it hard to get the unit back into proper shape with the real problem is finally found.
Everything you need to know is available to you with the meter but since you are getting nothing, yet the speaker is responding to touching transistor legs, you can just about rule out the power amp section. Since you have the speaker still connected, and the speaker is not smoking or the amp fuse blowing, you can assume there is no DC offset at the output.
Before assuming that the laws of physics have been overthrown in this one amp, it is a better odds bet to think in terms of what you are doing wrong in your measurements is the power amp develops the scratchy sound and the LED lights.
You have not even begun to get data so guessing what problems there are is just a waste of time. Figure out why your meter is not registering, probably because what you think is a ground return is probably not. In DC mode measure directly across one of the two large filter capacitors to see if the meter is working.
If you turn any of the pots, does any scratchy sounds come from the speaker...even if very weak? Did you try the experiment with the 3 circuit plug in the Ext Processor jack? Until you figure out the meter reading problem, and have performed the simple stereo plug test, no one can help you.

[Vinny]

update

Ok little update,

did some voltage testing, secondaries have near 40v,( I say near because I am still using a dime store multimeter. There is voltage getting through both 2200uf caps and there is voltage reaching the output transistors,no voltage going to the speaker that i can detect but I will check that again with a better meter. There is zero voltage on any of the other power section caps or resistors. The rail into the pre-amp around 20v, there is no voltage on the rail coming out of the pre-amp back to the power section and when testing that lead it creates a fairly loud harsh buzz.

I tried the 1/4 jack test on the proc ext nothing, no sound from the plug either there or at the input of the amp, but if the master is cranked there is a buzz similiar to grounding problem noise , and also notice at the proc ext jack if I go near it with my hand it acts like a pickup and the buzz amplifiers drastically.

[km6xz]

With those simple tests you have proven the whole power amp section is working. That is a long ways towards tracking down the problem. The 20volts is a good sign also, it is actually a -20VDC which is very close to the -19 that is shown on the schematic. The noise you are picking up when touching the ext proc jack shows that the signal dies before it gets to that point. The entire preamplifier pc board runs on a single voltage of -15volts referenced to ground. The -19v is dropped down to -15v by going through a voltage follower configured opamp IC labeled A1. Check the voltage reading on pin #1 of that IC. If it is not -15V the preamp will not work.
If it is OK, you can try injecting signal(or noise from your meter probe) at different points in the signal path to determine if the problem is before or after that point.
The first place to try is on the terminals of the volume control. Touching your probe to pins 6, 10,11,3,2 or 1 of each of the two rectangular IC1 and EC2 both marked 4007 or CD4007. If the preamp is working anywhere, and you have -15volts on pin 1 of A1 opamp you should get noise from the speaker on some or most of those pins 6,10,11,3,2 or 1. Check those and report back

[MiloCroton]

Hello everybody,

I got here via google looking for potential fixes for my amp. I also have a L3!

Mine is working but the sound cuts out intermittently, nudging/prodding/pulling the preamp volume knob used to wake it back up whenever this would happen (which was not often enough to bother me).

But know it seems I cannot play through it for 10minutes without it dying. And rather agressive knob twiddling has no predicable effect, it still doesnt work most of the time. After a while of random knob turning, waiting and turning the power on and off, it eventually comes back, but there is no predictability in it. Very frustrating, and I would say the amp is finally becoming unusable.

This is sad, because I LOVE the amp. Had it since I was 16 (first real amp after a little 8w park fuzzbox). It sounds brilliant when working.

Anyhow, I have no experience of electronics at all. But I do own a multimeter and soldering iron (which I got to rewire a guitar which I have not done yet). It seems the fault with my amp is simply the preamp volume pot, either it has become detached from the board or is simply worn out in itself.

Would you guys think I could attempt fixing this ? I can take detailed pictures and follow all your instructions to the letter, can you help me?

If a couple of you knowledgeable folk agree to help, I'll start dissassembly/performing tests upon your recommendation.

[Jazz P Bass]

The very first item I would address is the goofy potentiometer.
Either the soldering cracked or the pot is bad.
Try resoldering the three pads that the legs stick out of.
You may very well have other issues, but this is where I would recommend starting.

[52 Bill]

Welcome to the place. We all will be glad to help you with your amp, but please start a new thread so that we can get a clean start.

I agree with JPB check the pot and the soldering first. These amps use a reverse mounted pc pot that will be difficult to source. Also be sure to check the break jack as well.

[MiloCroton]

I dismantled the amp this evening to get some pictures for you guys. I thought I had actually fucked it up real good by removing PCB and thought I would crack it trying to get it back on. I also realised how impossible it would be for me to remove or solder any components onto the board. But guess what, I actually found a simple fault.

It seems someone had done a bodged repair in the past of by attaching a non-factory input jack in the "lo" section where the standard one had snapped off the PCB. It was not secured so was moving around shorting and stuff.

I just moved it out of the way (I didnt cut it off for fear of killing something), and I am socked and pleased to find my amp is working perfectly again.

I have some pics on my blog:

Dreadful Furious Playing: My Lab Series L3

[jmaf]

That is a great sounding solid state amplifier, glad you got it working.

If you can't find the stock jack, you can always resort to some shielded cable and a switchcraft 12A jack, then run the shielded cable to where the PCB pins would be. I've fixed a couple of solid state amps that way, have one here in front of me matter of fact.

[ifys]

Welcome to the place. We all will be glad to help you with your amp, but please start a new thread so that we can get a clean start.

I agree with JPB check the pot and the soldering first. These amps use a reverse mounted pc pot that will be difficult to source. Also be sure to check the break jack as well.

Hi everyone,
i just created a post with another faulty LABSERIES L3 ,figured that some guys might be able to help
preamp section not working on LABSERIES L3

[Speakernut85]

[ATTACH=CONFIRMED G]36141[/ATTACH]I also have an L3 that I came across this summer I've got it looking pretty nice (I think) but there is a LOUD hum (Constant) when power is on
, thought it may of had a bad ground but all those seem to be good as well speaker (original) sounds amazing when connected to other amp, all controls solid and seem to be working correctly..... Fingers crossed this is an easy fix help please !!!!

[nsubulysses]

Welcome to the place. We all will be glad to help you with your amp, but please start a new thread so that we can get a clean start.

............

[52 Bill]

Welcome to the place. Why not start a new thread for your amp, this one is quite a few years old.

Look at the speaker in your amp when you turn it on. Does the cone move in or out and stay there when it is turned on? If it does, then there is dc on the output of the power amp, and you should disconnect the speaker from the amp, as the dc can cause damage to the speaker voice coil.

If the cone does not do this, then there may be a problem with the amp's power supply, like a bad or loose filter cap.

What sorts of test equipment do you have and what are your skill sets regarding electronics repairs?