Lab Series enigma

[phydauex]

Howdy, I haven't posted before, but I don't know where else to go with figuring this one out. I'm trying to repair an L5 amp that initally had the symptom of playing for about a minute then fading out(dying) to very quiet and distorted. After messing with it a bit, pulled out preamp board to clean pots etc., I got a hideous scrall, which I've discovered is related to grounding of input jacks. Now no signal at all, unless chan vol is cranked and master cranked and you can just barely make out a distorted sound, but barely audible. Here's what I know/have done:

1) If I plug into the 'power amp in' with guitar I get clean guitar sound although not very loud, cd thru walkman gives more volume, but it seems that the power section is ok.

2) If I jump the preamp out to another amp, I get very quiet distorted signal. Again, barely audible and distorted.

3) I figure that since it does the same thing on both channels that the ic's at the master section were probably suspect, so I've replaced the CA3080, CA3094, and the 2N4303, with no change in symptoms. I also found the 2 diodes in that same area leaky and replaced them, no change.

4) Both +-15 v rails seem ok, and hv rails ok.

Any thoughts? Other areas to look at?
Thanks for any assistance or experiences with this beast.
Matt

[g1]

Sounds to me like you had an intermittent connection (or pot) on the preamp board. Moving the board (or the burst of feedback) may have made the intermittent connection go completely open. Possibly a bad solder connection or bad pot.

[phydauex]

Well, I'm checking all the solder joints and wire jumpers between the boards, but they all seel to be solid and functions. As for the pots, both channels are dead and they only thing in the circuit after the two channels is the master volume pot. It seems to be working as at ccw there's no hiss or sound, as it's rotated the hiss gets louder and I start to hear the distorted signal. That's the same for both channels.

Thanks for the suggestions,
Matt

[g1]

How about that 4558 in the loop of the 3094?

[phydauex]

That was going to be my next part to check. I've been busy with rehearsals and haven't had time to look at it for a few days. Thanks! I'll update when I've checked it out. I need to get a signal generator, and a better handle on using a scope. I've got an old Tektoronic scope that is a bit beat up, but still works sort of.
Matt

[Enzo]

This isn't subtle distortion, this is a gross symptom. Basically you have sound or no sound, more or less. You don't need a signal generator, ANY signal is enough, even just loud hum. I mean if yuo pulled the cord out of the guitar and touched its tip, could you tell by listening to the resulting hum if the amp was screwing up or not?

A signal from your stereo or an MP3 player or a tape deck, really just about anything

ABout all you need is a signal that you have some idea what it opught to sound like. Stick it to the input jack, and trace it through the stages. Scope stage to stage. If you can get a trace on your scope at the input of something, then expect it to look similar to that at later stages, though maybe larger.

[phydauex]

Drum Roll ............

Got it!! The 4558 after the mix of the two channels was the main culprit. Popped a new one in tonight and got signal to actually make it all the way thru to the speakers, Amazing. After replacing the leaky diodes on the +/- rail also knocked down some of the noise too. The amp is dead quiet and all else works as it should. So I ended up replacing the two ic's (ca3080, ca3094) and the 2n4303, and one 4558. Adjusted the bias and offset. Sounds like a new amp.
These are actually very nice amps I think, I should try and score one for myself.

Thanks for all the reply's and suggestions!
Matt