Need help trouble shooting a Lab Series...Please...

Okie Dokie folks,

In the "Best SS Amp" thread, I mentioned that my faves were always Lab Series L5's, and that if I could ever FIND another one, I would grab it. Well, I did, last night, for $165.00. Great score, aside from some wear on the grill cloth, it's really clean, but it has one issue.

PLEASE HELP ME!!

When I turn on the power switch, the led's on the front of the amp flash on, then go out. When you play through it, no sound comes out, leading you to believe, that the amp has no power, but when you turn it back off, you hear the power go out of the speakers. THIS leads me to believe that the power amp is functioning, but the pre-amp is only flashing on, then turning off. Now it gets weirder. If I plug the cord in and out of the input jack a few times, the pre-amp kicks on, and stays on, for as long as I want to play. Wiggling the cord in the jack doesn't work, I have to unplug it, and then plug it back in, usually two or three times, and the pre-amp will kick on, along with the status leds. Are the jacks in these amps somehow connected to the pre-amps power supply? Is this somthing I can fix? It has to be a connection, cause once it turns on, it functions fine until I turn it off again. Can anybody help me?

Thanks,
Jake

Sounds like a bad or dirty jack to me. BTW check out the effects loop send switching jack too. They get dirty and short the signal.

Also that amp has 3 or 4 fuses mounted inside on the circuit board.

I haven't been inside one of these amps, but I did look at the schematic.

The power supply for the power amp/output stage (which is primarily discrete transistors) is separate from the power supply for the preamp (which is primarily op-amp based). Each is derived from a separate secondary winding on the output transformer.

What this means is that it is possible what you are hearing when you turn the amp on/off is the output stage supply. You can verify that the power amp stage is working fine when the preamp isn't by plugging a signal (just your guitar would be fine) into the jack on the back panel marked "power amp input".

The power pilot light does come off of the negative side of the +/-15 volt "preamp" or op-amp power supply. Unfortunately the schematic doesn't show exactly how the -15 supply is connected to the pilot light - just indicates a -15 connection which could be from various places.

Your input jack thing - there is no direct connection between the input jacks and the power supply circuitry. The input jacks are ordinary "open" jacks that do nothing more than supply the guitar signal to the input amp via a couple resistors. In other words, there is no switching going on with the input jacks.

If you can get the amp working by playing around with the input jacks, the only thing I can think of is you are causing enough vibration to close some loose connection connected to the preamp board.

The power supply connections from the power transformer are indicated as soldered. This amp does have a number of multi-wire connection sockets - of which any one could be loose.

With the pilot light symptom... you could open it up, locate the pilot light and follow that wiring... looking for some common intermittent or loose connection with the preamp power supply connection. That's where I would start anyway.

I haven't been inside one of these amps, but I did look at the schematic.

The power supply for the power amp/output stage (which is primarily discrete transistors) is separate from the power supply for the preamp (which is primarily op-amp based). Each is derived from a separate secondary winding on the output transformer.

What this means is that it is possible what you are hearing when you turn the amp on/off is the output stage supply. You can verify that the power amp stage is working fine when the preamp isn't by plugging a signal (just your guitar would be fine) into the jack on the back panel marked "power amp input".

The power pilot light does come off of the negative side of the +/-15 volt "preamp" or op-amp power supply. Unfortunately the schematic doesn't show exactly how the -15 supply is connected to the pilot light - just indicates a -15 connection which could be from various places.

Your input jack thing - there is no direct connection between the input jacks and the power supply circuitry. The input jacks are ordinary "open" jacks that do nothing more than supply the guitar signal to the input amp via a couple resistors. In other words, there is no switching going on with the input jacks.

If you can get the amp working by playing around with the input jacks, the only thing I can think of is you are causing enough vibration to close some loose connection connected to the preamp board.

The power supply connections from the power transformer are indicated as soldered. This amp does have a number of multi-wire connection sockets - of which any one could be loose.

With the pilot light symptom... you could open it up, locate the pilot light and follow that wiring... looking for some common intermittent or loose connection with the preamp power supply connection. That's where I would start anyway.

WOW. :shock:

I THINK, I understood almost all of what you just said. I am going to open this puppy up and see if I can find the bad connection, starting with the pilot light, as you suggested. Is there anything I should know about, IE, possibly painful/lethal votages I might encounter while doing this. I have never worked on an amp before, but I am fairly sure I can do this. If not, no biggie, I'll take it to the tech. I just don't want to get dead. I have heard that amps can blow your head off, even when unplugged. Should I be worried?

Jake

WOW. :shock:

I THINK, I understood almost all of what you just said. I am going to open this puppy up and see if I can find the bad connection, starting with the pilot light, as you suggested. Is there anything I should know about, IE, possibly painful/lethal votages I might encounter while doing this. I have never worked on an amp before, but I am fairly sure I can do this. If not, no biggie, I'll take it to the tech. I just don't want to get dead. I have heard that amps can blow your head off, even when unplugged. Should I be worried?

Jake[/quote]

You should always be worried about poking around the insides of a live amp. A solid state amp doesn't have the same high-voltage potential as a tube amp, but you always respect electricity.

If you've never done this before, I suggest first trying to find a problem with the amp unplugged - and unplugged for at least an hour to give the electrolytics in the power supply time to bleed off. You don't need to be trying to discharge them your first time inside an amp.

Follow these rules, at all times, even with an unplugged amp (especially recently unplugged).

Rule #1: Never ground yourself. This means to always work wearing good rubber sole shoes - on some sort of rubber throw rug if possible. Never directly on concrete as the moisture content of concrete can make it real conductive.

Rule #2: Never put more than one hand inside the amp. Sit on the other hand or stick it in your pocket and don't move it. Never rest your arm on the amp chassis while your hand is inside.

Rule #3: Poke around with some insulated probe. Wooden or plastic drumstick... chopstick, length of dowel, something like that. Do not ever use your fingers directly - and never a screwdriver or other conductive tool even with a plastic handle. No pencils or pens either.

When you take the chassis out of the amp, before you do anything else make certain that the chassis is solidly supported. Short pieces of 2x4s or even telephone books can be used to make sure it is level and secure. This is important - because it is an instinctive reaction to reach out and grab it if it starts to fall or tilt - and you don't want to be in this situation. It is very easy to grab the wrong thing.

If you feel you have to plug it in to find the problem, make absolutely sure you understand these rules and follow them to the letter. AND... be honest with yourself - if you have even the slightest doubt as to your ability to do so, pack it back up and take it to a tech. It is very easy to forget and make a mistake when the excitement of finding a problem takes over.

I don't want you dead either.

It could be as simple as a loose plug, or a bad solder joint. Just take some time to look at stuff and see if the solders look good. Use a nice, bright light, the cracks or telltale solder melts you're looking for are very small.

everything is connected with connectors and these get loose and/or dirty. I generally open the amp up and blow the 20+ years worth of dirt and shiznet out of the amp and connectors and that seems to take care of the majority of the problems
tom

Thanks for all of the help and advice. I appreciate you guys. (Especially you Joe! Great rules to live by!)

The amp has been unplugged since about 10pm last night, so I think I'll be alright. I have absolutely no intention of working on it while it's plugged in. I have found that when you are not entirely sure of what you're doing, there is a fine line between cofidence and stupidity, and I always do my best to stay on the right side of it. I will look for everything you guys told me to look for, and fix anything I can readily identify. If, after that, it still ain't working right, it'll go to the tech.


Thanks,
Jake

Do you have some Deoxit spray? Canned (compressed) air?

If not, get some before you "go in"

Blow out the goo, and spray the jacks. Scrub with a swab. Also the Moldex connectors are alwat suspect...

There's some good pics of the chassis in this thread,

http://www.tdpri.com/viewtopic.php?t=38177

Do you have some Deoxit spray? Canned (compressed) air?

If not, get some before you "go in"

Blow out the goo, and spray the jacks. Scrub with a swab. Also the Moldex connectors are alwat suspect...

There's some good pics of the chassis in this thread,

http://www.tdpri.com/viewtopic.php?t=38177

Wow, that chassis is a bit different from mine. Mine must be either a couple of years older, or a couple of years newer, basically looks like the same circuit, but with a coulpe of different components, and some minor layout differences.

I went in last night, fixed the problem. First of all, I would like to say that this amp was SPOTLESS inside. I mean pristine. Like you could eat off it. Not a speck of dirt, dust, corrosion, or oxidization anywhere.

So I open her up, and instantly, no more problem. This makes me suspicious, because I just know that as soon as I put it back together it won't work right. So first I put the control panel back in, plug in, and test, still works. As a final test, I leave the amp on, and start tapping on stuff with a chop stick, just to see if I can get anything to cut out. Cut out it did. Seems one of the fuses wasn't seating tightly enough in one of it's fuse holder-downer-thingys. When it vibrates loose, the pre-amp turns off, when it vibrates back, it comes back on. I have it stuck in place with a little piece of tape for now, I will start hunting down a new fuse holder today, and get it replaced asap. Amp works fine though, and sounds mighty nice. Thanks for all the help!


Jake

Don't you love it when it's something simple and easy! Play the heck out of that amp!

Don't you love it when it's something simple and easy! Play the heck out of that amp!

Yes I do, and thanks, I intend to!! :D

Jake

Congrats Jakedog! I think that was how my first amp repair went, but it sounds like you did all the right things, so you should be proud.

Now think of this: Classic tube amps are a lot simpler inside than your Lab Series. Sure, you have to take a few more precautions, and it's not quite as simple to discharge those electrolytics, but there's a whole lot fewer pieces inside to break.