Lab Series L11 intermittant "clipping" problem

[swamprock]

Hello everyone... new here. I searched around a bit on the net, as well as within these forums, and didn't find any similar posts describing the problem I'm having with my amp, so I decided to start this thread...

I have a Lab Series L11 200w head that I recently picked up. When I first tested it, everything sounded fine. Both channels and all inputs were working, as well as all of the pots and compressor, etc. I tested it with and without pedals, and had no issues for a couple of days.  Today, I fired her up and now I'm getting a bit of a clipping "thump" whenever I play anything on a lower string, and the signal seems really dirty and distorted, and not in a good way. It's almost as if there's too much power being pushed through the amp and it's horribly clipping and distorting; it's that kind of sound. This occurs with and without pedals, and in both channels. I opened the amp up, checked to make sure that the transistors weren't grounding to the heat sink (they weren't), checked for any cold solder joints (none that I could visually pick up), made sure all of the connectors are holding their wires and reseated them, double-checked some previous repairs and jumper wires that were added in that time (continuity checked out), cleaned it a bit with some compressed air, checked for loose jacks, and turned it on again. Everything was working fine again until I turned it off, then back on later, then the problem returned. I'm running the head through an 8ohm 4x12 cabinet, so that's covered.

All of the pots still function just fine, even with this problem. The volume level stays constant, but the amp just sounds horrible.

EDIT: Almost forgot... when this issue is taking place, and I turn the amp off, there's a quite humorous sound that sounds like someone screaming, like I just killed them, that comes through the speakers. This doesn't occur when the amp is functioning properly. Pretty funny.

I've done some board-level and component repair/replacement on old coin-op video games, pinball machines, skill cranes, and jukeboxes, and some monitor/TV repair in the past, but I'm not what you'd call a technician. I can solder, am aware of safety with working on electronics (and high voltage), can read and utilize schematics, and have some limited troubleshooting experience, so if I'm led in a specific direction of where to look in a specific circuit or component, I can usually do the repair.

I hope I gave enough information (or not too much). Any ideas of where to start looking? Any and all suggestions would be greatly appreciated. Thanks!

[Willabe]

Welcome!

What year is the amp? Might need a power rail filter cap job.

           
             Brad     

[smackoj]

I have the 4x10 L7 model that I have done some research and trouble shooting with.

My first suggestion would be separate the preamp and the amp. Use a preamp you know is good and plug it into the Pwr Amp In jack on the back. Test it to see if the power amp section sounds good with a stand alone preamp? If it sounds good move on to check the preamp.

Do the same with a known good working power amp. Take a patch cable from the Pre Amp Out jack and see if the problem persists? If it does you know that your problem is in the preamp section. If it sounds good also, then you might have a bad speaker, a bad patch cable or something intermittent?

Like Willabe suggested, I would be skeptical about old dried components. A lot of the Lab Series amps became closet queens for dozens of yrs which could mean trouble with weak, dead components that haven't been juiced up for a long time.

 

[swamprock]

The amp is a '76, based on the pot codes ('75 and '76). Incredibly, all of the pots are still original and in good working order.

Thanks guys. I'll follow your advice and post back my findings. Appreciate it!

[swamprock]

Well, I tested the preamp and amp per smackoj's suggestions, and everything sounded fine. I messed about a little more, and discovered that the amp would misbehave when I plugged my Turbo RAT through the front inputs. If I ran my guitar cable into the inputs right AFTER unplugging the pedal (without turning off the amp), it would still sound like crap. Turns out that the pedal is causing the problems I described in my earlier post. I shut the amp off, plugged the guitar in, turned it back on, and all was well.

So, the one thing I missed in describing my testing, because it completely slipped my mind, was that I didn't turn the amp off when I switched the signal from guitar->RAT->amp to just guitar->amp. I just pulled the cable and left the amp on.

Still, I may just rebuild the power supply and replace the insulators on the transistors for good measure. Thanks for the advice, guys!

[PRR]

The T-Rat or the Lab, and possibly *both*, have DC leakage issues.

Unscrew the shell on a cable and see if there is any DC when T-Rat is plugged to Lab.