Laney LC15R problem

Chalky

My Laney LC15R has the following symptoms:
1 Harsh grainy nasty distortion even when set to very low gain
2 Gain level and master volume work as expected except nasty distortion present at all volumes, but overall output of the amp is much quieter than normal
3 Nasty distortion generates very obvious 'out of tune' overtones on most individual notes
4 Nasty distortion on both hi and lo inputs.
5 With no lead and gain and vol on max there is a bit of noise but nothing more than you'd expect on a cheap valve amp.

I have another faultless LC15 (not the R) so diagnosis carried out so far includes comparison between both amps having swapped guitars, cables, speakers, and all valves. Also compared all voltages - problem amp is running approx 10% higher voltages at all points measured so far.

Tried measuring voltages on coupling caps to see if any leaky DC getting through but nothing detected on output side of the caps. Electrolytics have the usual silicon goo on them but otherwise no visible problem. No obvious scorching or deterioration on any components.

Just about to bridge out the LC15R reverb and FX loop PCB to exclude that from equation but wonder if any amp gurus had any advice?

ICBM

Compare the volume of both amps when plugging the guitar into the FX return.

If the bad one is much quieter and/or still sounds bad, the problem is in the power stage. If they're the same it's in the preamp.

Chalky

Many thanks - will try tomorrow!

Chalky

@ICBM - Thanks. Test results are than the LC15R FX Send into the return loop of a third amp works fine. Now browsing the schematic after that point...

Forgot to say that the contra of third amp send into the FX Return of the LC15R still had nasty distortion.

Ah the schematic puts me into solid state territory so any further differential diagnosis you can advise will be much appreciated.

ICBM

If the preamp output into another amp sounded OK it's unlikely to be the solid-state part of the circuit, since that is driven by the other half of the same IC that does the FX return (U1A and . It *is* possible that just one half has died though.

Ater that it's just the power amp, and if you've tried changing both the phase inverter and the power valves, it begins to sound like what I first suspected from the fault description.

Blown output transformer.

Chalky

Loved the dramatic pause! So I could swap in the OT from the working LC15 to check. Or just get another? Where is the best place to get one?

ICBM

The very first thing to try is to run it on only one power valve, one at a time. (This is safe with this amp, it's fixed bias.)

While it's not conclusive that it's the transformer, you should get one of several results:

Pulling either valve makes the output even quieter and even more oddly distorted.

Pulling one valve makes no difference, pulling the other makes a big difference.

Pulling one valve makes no difference, pulling the other produces silence.

Chalky

Ok thanks, will test and respond. Thought there would be many suppliers but only somewhere called livinginthepast came up when googling.

ICBM

You could try Laney, although I think you may have to go via a dealer these days, they changed their (previously helpful) policy. They have the amps manufactured in China now, so don't be surprised if you have to wait a while if they don't keep UK parts stock.

Chalky

Pulling either valve makes no discernable difference - volume and tone are the same. That wasn't one of your expected outcomes from what I can see...

ICBM

Just a more extreme version of the first one…

Blown output transformer, almost certainly.

Chalky

Ok, thanks. So if you were me would you go to the trouble of taking the OT out of the good LC15 and putting into the bad one?

ICBM

Chalky said:

Ok, thanks. So if you were me would you go to the trouble of taking the OT out of the good LC15 and putting into the bad one?

Yes, if its not too much hassle - before going to the trouble of ordering a new one.

It might be possible to hook it up by jumpering across between the two chassis without physically removing it, if you're lucky.

Chalky

@ICBM - Many thanks. OT finally arrived today so I fitted it and all is fine. Reverb not working but FX loop is. Will investigate the reverb later. Many thanks for your kind help.

ICBM

No problem! It is a bit of a process of elimination - I'm always hesitant to jump in and buy a new OT immediately until it's fairly conclusive .

The reverb is most likely the tank if it's the older spring version.

Chalky

Yep its the spring version. Will investigate thanks.

Chalky

@ICBM - can I try another tank on there without damage? I have another short tank in a Trace Elliott.

ICBM

Yes, should be fine. Both amps will have solid-state reverbs so the drive and return impedances should be OK.

Chalky

Thanks again @ICBM - the Trace Elliott tank worked fine. So its the Laney tank. I get output from it if I tap the strings and obviouslt the Trace Elliott tank has proven everything works up to the tank input. So I should be able to see the broken connection but can't. Intellectual interest piqued!

ICBM

Chalky said:

Thanks again @ICBM - the Trace Elliott tank worked fine. So its the Laney tank. I get output from it if I tap the strings and obviouslt the Trace Elliott tank has proven everything works up to the tank input. So I should be able to see the broken connection but can't. Intellectual interest piqued!

Does it have push-connectors on the transducers? I'm guessing it does.

If so, if you pull those off you will see two pins. The wire from the coil goes over the edge of the bobbin and is wrapped around the pin and soldered to it. The weight of the push-connector is enough to vibrate the pins and break the very fine wire at that point. It's a design fault with newer Accutronics reverb tanks. It's almost always the output end that breaks though - very unusual for it to be the input end.

Sometimes if you're very careful, you can scrape the insulation off the top of the coil wire, solder a whisker of fine wire to it, and reconnect that to the pin. I've tried this a few times with a success rate of under half, even being very careful - it's just too fine and fiddly and there's a great risk of snapping the wire closer to the coil, in which case you're stuffed. Usually it's easier not to bother and just change the tank.

If you do replace it, buy a TAD or MOD tank, not an Accutronics or Belton - those don't have the same design fault. (Nor did Accutronics up til about 20 years ago, or Belton before they bought Accutronics…)

Chalky

Thanks @ICBM - might have a go at repair if I have time later in the week. Just going lateral here - is it possible to take off 9 or 12v off the loop/reverb pcb or off the PT heater feed to power a reverb pedal that I would then attach inside the cab on the side? Would use the fx send/return and remove the reverb tank cables completely of course. Just thinking...