Noob on a mission...Laney GC 50 problem

[hamerjunkie]

Hi, I`ve done some small electronic projects, I need a little direction to tackle the next one. I have a 1994 Laney GC 50 solid state amp that the preamp gain goes up and down after a few minutes. The way I explain it is that it goes from a roaring metal sound to a weak, barely overdriven one. I`m thinking it is in the preamp section of the amp itself and have gotten a schematic from Laney that covers it, but I need a little direction....... Thanks in advance.

[J M Fahey]

Fine, good start.
Now post or link to that schematic , so we all talk about the same.
Thanks.
PS: start by the classic suspect: plug your guitar or an MP3 or CD player into the return jack, ¿do you have sound?
Then plug said cable from Effect/preamp out to effect/preamp in. ¿Any better?

[hamerjunkie]

Didn`t get a chance to work on it last night or this morning yet, the schematics Laney sent me for it are :


http://s239.photobucket.com/albums/ff129/hersheybears2/?action=view&current=page0002.jpg

http://s239.photobucket.com/albums/ff129/hersheybears2/?action=view&current=page0004.jpg

http://s239.photobucket.com/albums/ff129/hersheybears2/?action=view&current=page0005.jpg

http://s239.photobucket.com/albums/ff129/hersheybears2/?action=view&current=page0006.jpg



Page 1 was a cover page and page 3 was the 220 volt one.....

[hamerjunkie]

Got to sit with it for an hour or so, I have sound when plugging into the return of the effects loop, not any better when plugging a cord from line out to line in, seemed to drop even when using the clean channel with a distortion stompbox........

[hamerjunkie]

Anybody......Anyone..........??

[J M Fahey]

Hi hamer.
Unfortunately, those schematiocs, as posted, are all but unreadable.
I guess you got the .PDF for that manual and you turned it into .jpg.
There is a lot of resolution loss in that conversion.
Please post the .pdf itself somewhere else (not on photobucket) or even better, attach it to your post.
If over the size limit (don't think so) I think you can post it in AMPIX (that's what Enzo usually does as a service to the community)
Meanwhile check that all TL072 have -15V on pin4 ; +15V on pin 8, and essentially 0V on pins 1 and 7.
Does the Led turn on?
When you plug your Distortion pedal into the Line In jack (as an experiment try it into all 3 shown at left in your "page006", labels unreadable).
It *should* be the middle one, but schematics and layouts do no match, so try all 3.
Your power amp should provide acceptable volume, not as if it were a mile away.
I *guess* the inverters (¿CD4049? or similar) should have +15V on pin 1, 0V on pin 8 and some intermediate value (neither 0 not 15) on pins 2/4/6/10/12/15 .
Not much more until having a cleaner schematic.
Good luck.
Meanwhile Google for and download some 440Hz or 1KHz sinewave test MP3 to use your MP3 player as a constant signal source, for tracing.
Also kludge a shielded cord with a plug on one end and a couple of crocodile clips on the other (solder a 10 or 15 inch regular wire to the shield so you can clip it to any convenient ground), you'll use the working power amp and speaker to hear signal along the way.
Unfortunately Laney *loves* to mix regular parts and surface mount ones (SMT) on the same board, which will complicate your work a little.