Laney GC50 Problem

[iexton]

Hello,

I'm Ian from Surrey. I've acquired several amplifiers over the last 30 years and nearly every one has a problem of some sort. I live in a damp old cottage and wonder whether that has anything to do with or if they just give up after a short period of having to amplify my torrid guitar playing!

My electronics skillset is limited to building and soldering circuitry (I've made a lot of pedals) with only a rudimentary understanding of how the circuit actually works. I can solder, have basic test equipment (multimeter, signal generator, etc) and know how to follow a schematic.

The first amp I'm trying to tackle is a 1994 Laney GC50 that I bought new. A few years ago, it developed an annoying crackle (like someone screwing up newspaper - crackle, gap, crackle, gap, long crackle, short gap, etc)) that made me sideline the amp and buy another amp. I'd like to get the Laney up and running again as it takes up a lot of space for something that's currently useless!

There are two boards - the combined power supply and power amp board and the preamp board. If I disconnect the ribbon cable between the PSU/power amp board and the preamp board, the problem persists and I can still play a guitar through the Effects Receive jack so it's clear that problem exists somewhere in the PSU/power amp board. This board has lots of hole-through big components on its upperside and as many surface mount devices below.

So far I have:
1. Cleaned the contacts on all the jacks (speaker, headphone, DI out, Effects send and return)
2. Tested with a cable between the send and return jacks
3. Replaced the two 2200uF PSU electrolytics (a cheap and simple fix if it worked - it didn't!)
4. Replaced all the upper side (hole through) capacitors, both polyester and electrolytic (ditto on the cheapness and ease....and the not working bit)

And the crackle is still there!

But there are still 50% of the components to replace and my policy of blindly replacing stuff until I eliminate the issue was fine for cheap capacitors but BD909s are £4/$5 each. So I'd appreciate some help with a more nuanced approach!

I've attached a photo of the board and the schematics associated with it. Note this board is part number 9074 but the schematic for that board shows TIP35 power transistors. My amp has BD909 transistors and a pair of 2200uF power supply capacitors, but everything else on the 9074 diagram seems to be the same. There's a rectifier module they don't give a part number for but it's a KBL403.

Thanks for any help you can give.

Ian

[phatt]

Ahh yes nothing worse than intermittent crackles.
Yes a component dying or a cracked or cold solder joint,, my guess you have a cracked track hiding in the maze.
Get yourself a wooden skewer stick and while amp is on gently poke and move components on the PCB. you can get lucky and find the part that is causing the drama. The bottom side will not be easy to access so disconnect an pull the board out then with a magnifying glass in bright sunlight go looking for hair line cracks, especially around the solder pads.

I actually own a SS Laney amp, even earlier model that yours and it developed a crackle last year which drove me mad but finally found a hairline crack which was actually hidden by a solder lump so Even with magnification you could not see it.
Keep us posted, Phil.

[iexton]

Ahh yes nothing worse than intermittent crackles.
Yes a component dying or a cracked or cold solder joint,, my guess you have a cracked track hiding in the maze.
Get yourself a wooden skewer stick and while amp is on gently poke and move components on the PCB. you can get lucky and find the part that is causing the drama. The bottom side will not be easy to access so disconnect an pull the board out then with a magnifying glass in bright sunlight go looking for hair line cracks, especially around the solder pads.

I actually own a SS Laney amp, even earlier model that yours and it developed a crackle last year which drove me mad but finally found a hairline crack which was actually hidden by a solder lump so Even with magnification you could not see it.
Keep us posted, Phil.

Thanks for the response and suggestion. I'll start poking it with a stick (!) and see what happens.

I bought a much later LC15 when this one went wrong and that's popping and crackling too. I owned several minis when they leaked or died in wet weather. I'm clearly a sucker for brand loyalty to brands whose products are a bit.....interesting!

[phatt]

Also with power on gently pressing on the board in different places
(with an insulated probe) can help track down an offending component.
Phil.