Laney LC15 crackling noise - which components to replace first?

[markyb]

Hi all, it's been 14 years since my last post here, which I can't quite believe. But hey.

I have a little 1x12 Laney LC15 which I bought spares/repairs on eBay a few months back - it is working but has a loud intermittent crackling/hissing noise. Sometimes it won't make any noise at all, and functions perfectly. It's a great little amp, 3 x 12AX7 and 2 x EL84. I have the schematic Laney-LC15-Schematic.pdf

I'm fairly happy with doing my own mods and tube amp maintenance, I've been maintaining my Carvin Nomad and Bel-Air amps for 20 years.
I'm located in Yorkshire, UK (the sticky suggests to put your location), (240VAC 50Hz)

Following troubleshooting guides in Tom Mitchell's excellent book, I reckon I've traced the issue to something around V2, when I pull V2 the noise stops, and pulling V1 makes no difference to the noise. I thought initially it was bad contacts on the tube sockets but I've tightened these all up and cleaned thoroughly and although initially I though this had worked, after a bit of playing the noise came back. I wonder if it's a bad cap or resistor around V2.

I've attached an audio file of the kind of noise, because I think someone will recognise the sound and will be able to have a good guess what is causing it.
laney-lc15-hiss-128.mp3

So my question is: does anyone have any good advice about which components to start replacing first? Or any other thoughts about cause.

[markyb]

Looking at my post the schematic doesn't seem to have uploaded correctly, so here is an image showing just the relevant bit of the schematic. I've had to reduce the quality quite a bit to get under the upload size limit though

PDF also obtainable from here https://el34world.com/charts/Schemat...Laney_lc15.pdf

[Enzo]

it is a long story, but your schematic file already exists here, but it is from before teh big server crash some years back. COnsequently we can't open your file. Can you zip your file and post it? Or alter the existing file and rename it?

I don't start out thinking what to replace. I think in terms of isolating the problem.

I don't have sound on my computer, but if we are talking noise rather than hum, plate resistors are more likely than other things, mostly.

[markyb]

Thanks Enzo. Probably easiest to get the schematic from https://el34world.com/charts/Schemat...Laney_lc15.pdf
It's definitely a noise, there is some hum in the background but that is only because I turned the amp up loud to highlight the noise issue. The 50Hz hum is entirely within tolerable limits.

I agree it's best to make an accurate diagnosis before starting to replace parts randomly. (In my work life, I am a doctor!) So, in terms of isolating the problem, how would you go about that? I don't seem to be able to induce the noise to be worse or to stop/start by mechanically tapping components, and I don't have the equipment to probe for noise on the board (eg an oscilloscope or similar). What would be a good way to find the responsible component?

[Enzo]

Oh good, you beat me to it. OK, V2 kills it but V1 doesn't, so V2 circuits are suspect. V2 is two stages here, so short across R12, ie ground grid of V2a. I don't care if that increases hum, but does it kill the noise?

We can't ground the grid of V2b, but we can add a cap to our ground clip, and then use it as an AC ground. Using a cap, does grounding the grid of V2b kill the noise? We are determining which section of V2 holds the problem. You can also use our cap clip to ground the cathode of V2b, what the heck.

I happen to have a ton of 0.047uf 630v caps, so I use them for this, but the value is not important. A cap of sufficient voltage rating and reasonable large will do.

[Enzo]

Don't tell me you are a doctor, I just had a colonoscopy yesterday, and am not so positive about your profession today.

Thanks for the link, it was not your fault the file won't open here. For now, your partial is working for me.

The reason I don't focus on "the part" is because it can just as easily be a connection, and in that case we could possibly replace every part and still not cure it.

Look up signal tracer. A signal tracer is essentially an amplifier with a probe for an input. We add a cap to block DC,..And we then probe a circuit and we can listen to the signal path. A scope for your ears. You can build or buy a dedicated tracer, or just make up some probe, and plug it into some other amp you have. SOme guys make one from a computer speaker.

Was my previous post about technique clear for you?

[markyb]

Thanks Enzo, this is great stuff and completely clear. I probably won't have time to attack this today but I now have some things I can try. I'll report back with more info once I do that. I'll look up how to make a signal tracer - I have some spare bits of solid state small amps knocking about the place. Thanks again for the super quick and helpful response.

[Enzo]

If you google signal tracer you will find a million of them. SOme include a diode, and that has the purpose of making RF signals audible. You only need a cap.

[markyb]

Well. Today I got around to taking a look at this problem. I stripped the amp down to get at R12, however after removing and replacing the tubes (to get the PCB out of the case) the amp stopped making the noise. I've tried all sorts to recreate the noise, including tapping on components with an insulative probe, and I left the amp running for several hours to warm up thoroughly. Fiddled with all the controls aplenty. No cracking/sputtering at all. It's a lovely little amp and a great warm clean sound for jazz, with tons of headroom and just enough compression. All for £70 spares & repairs!

So I am back to thinking it may have just been poor contact on a tube pin. Hopefully that'll be the last I hear that noise, but thanks Enzo for the advice, and of course those concepts of shorting out resistors, grounding through a large cap, and signal tracers will be valuable generally. Even more so if the noise comes back one day!

Thanks

[Chuck H]

Well. Today I got around to taking a look at this problem. I stripped the amp down to get at R12, however after removing and replacing the tubes (to get the PCB out of the case) the amp stopped making the noise. I've tried all sorts to recreate the noise, including tapping on components with an insulative probe, and I left the amp running for several hours to warm up thoroughly. Fiddled with all the controls aplenty. No cracking/sputtering at all. It's a lovely little amp and a great warm clean sound for jazz, with tons of headroom and just enough compression. All for £70 spares & repairs!

So I am back to thinking it may have just been poor contact on a tube pin. Hopefully that'll be the last I hear that noise, but thanks Enzo for the advice, and of course those concepts of shorting out resistors, grounding through a large cap, and signal tracers will be valuable generally. Even more so if the noise comes back one day!

Thanks

Not uncommon to have oxide or some other crud cause poor tube pin contact. Enzo called it out in post #6 when he said "The reason I don't focus on "the part" is because it can just as easily be a connection,..". This can actually be a dangerous situation for the amp because it can affect tube bias levels causing overdissipation. So this is not exclusively a signal problem, but can also be a operating condition problem. You should clean your sockets and tube pins with contact cleaner. The odds that only one contact in your amp was problematic are remote and the problem could occur again in a more dangerous way. Cleaning the mechanical circuit contacts is standard maintenance and cheap insurance.

[markyb]

Hanks Chuck H good advice definitely. In this case, I already did a thorough clean of every tube socket pin, with contact cleaner, and abrading the inner surface slightly to ensure the oxide had been removed. The problem went from permanent to intermittent at that stage, but was still there. I still don't know for sure what made it go away in the end, but at the point it went all I had done was take out the tubes to remove the PCB from the chassis, and put them back in. It's possible the problem might rear its head again, but so far so good, and I've given it a long soak test and played it a fair bit now. Thanks both for the very helpful and practical advice.

[g1]

Should the problem recur, if the tube sockets are directly mounted to the circuit board they could probably use resoldering.

[Perkinsman]

Your amps sound/symptoms are exactly the same as an amp I had in recently. Did you measure the 2 plate resistors on V2? I’ll bet one of them is way off value.

[HaroldBrooks]

Hi all, it's been 14 years since my last post here, which I can't quite believe. But hey.

I have a little 1x12 Laney LC15 which I bought spares/repairs on eBay a few months back - it is working but has a loud intermittent crackling/hissing noise. Sometimes it won't make any noise at all, and functions perfectly. It's a great little amp, 3 x 12AX7 and 2 x EL84. I have the schematic [ATTACH]56552[/ATTACH]

I'm fairly happy with doing my own mods and tube amp maintenance, I've been maintaining my Carvin Nomad and Bel-Air amps for 20 years.
I'm located in Yorkshire, UK (the sticky suggests to put your location), (240VAC 50Hz)

Following troubleshooting guides in Tom Mitchell's excellent book, I reckon I've traced the issue to something around V2, when I pull V2 the noise stops, and pulling V1 makes no difference to the noise. I thought initially it was bad contacts on the tube sockets but I've tightened these all up and cleaned thoroughly and although initially I though this had worked, after a bit of playing the noise came back. I wonder if it's a bad cap or resistor around V2.

I've attached an audio file of the kind of noise, because I think someone will recognise the sound and will be able to have a good guess what is causing it.
[ATTACH]56551[/ATTACH]

So my question is: does anyone have any good advice about which components to start replacing first? Or any other thoughts about cause.

If the problem re-occurs soon, I would check the value of the V2 and other further down stream plate resistors. I've had to replace many of those on my vintage amps, and they create snap crackle and pops, along with hissing noises. But all the other stuff like cleaning sockets, pots, and chopsticking solder joints would come first, as it's quick and easy and standard service on old amps, IMHO.