StarGeezers 2/21/2010 6:18 AM
Laney becomes intermittent after two hours...
Guys , I can play my Laney LC50 just fine for about two hours , then it starts cutting out/getting Thin ... I'd guess it was a thermal thing , just have no idea where... seems to happen most when I have the overdrive channel engaged.. but not always ... Perplexed... Amp is about 16 years old... never played much ...
 
R.G. 2/21/2010 8:53 AM
The first rule of fixing intermittents is to find out what, exactly, makes it intermit. After that, the fix is pretty much an afterthought.

But techs properly *hate* intermittents, especially ones which take hours and heating to get to happen. They will often get out the shotgun...

... approach.

If I were pressed for time in finding this one, I'd open it up and run it while applying a hair dryer (actually heat gun, but this is an awfully high powered thing to use) to various places to see if I could get it to happen. If no luck there, I'd record pin voltages on tubes for later, then let it get hot, start thinning and QUICKLY measure pin voltages again.

Possibilities include tubes (have you tried new, known good tubes?), sockets, drifty resistors, coupling capacitors leaking when hot, solder joints, filter/bypass caps leaking when hot, cathode capacitor leaking when hot, OT developing a short when hot, rectifiers leaking when hot, PT developing a soft short when hot... that it, pretty much everything in the amp which sets a gain or bias point, which is pretty much everything.

You gotta make it intermit. Don't even try unless you have the skills to do this without electrocuting yourself or others with it out in the open.
 
Jazz P Bass 2/21/2010 9:49 AM
Intermittant
I had a Fender Super in for an intermittant.
On the bench the chassis performed flawlessly.
In the cab it failed.
Turns out it was the right angle speaker plug.
The wire inside at the bend, where it goes over the rear cover, was broken.
 
StarGeezers 2/21/2010 3:07 PM
Thanks for all the suggestions/possibilities ... Although I've built many amps ,this one is a puzzle ... for sure... Worse it only usually happens at a gig ...Grrrr!!! I'll start by changing all the tubes , probably time for a re-tube anyway.. Thanks !!!
 
Danila-master 6/11/2018 5:50 AM
Hope it will be useful.

I had a Laney LC50 MK2 on my desk.
It had an intermittent issue with heater circuit.
When Power switch is ON - heaters are not glowing at all and of course no sound on speaker.
In 2 or 5 minutes heaters are back and sound appears.
Root cause was in ATX style socket between Power transformer and PCB.
One of 6.3 V connectors was black and oxidized.
Fixed this issue by removing grey wires (6.3V) from ATX socket.
I added one more socket inside the Amp and soldered heater wires directly to PCB.
Schematic shows 3 wires: 2 Grey and 1 Black. In Fact - Black wire (center tap) is not connected.
Near the 1-st preamp tube is a trimmer "Hum Adjust" which connects heater line to the ground.