Laney VC15, treble control knob has decided to become an inverse volume control?

[jm2]

I have a Laney VC15, which I like. I do not gig with it, and it sees only bedroom levels. It worked without issue for almost two years. Not long ago, it suddenly developed a strange action. The treble control no longer controls the treble. If the treble knob is set to zero, the amp functions, but with virtually no high end. As the treble is advanced, the volume dies. At treble=10, there is no sound. The quality of sound at zero treble setting does not seem out of the ordinary, just no treble.

I am obviously going to take it to a shop, but I thought I would first ask if anyone has any thoughts on what might have gone wrong. It seems like such a peculiar malfunction. Any thoughts would be appreciated.

[Enzo]

Maybe you broke C14 off the board or cracked its solder?

[jm2]

I found a fuzzy vc15 schematic online, and if I am reading it correctly, C14 is a 470pf capacitor. That would be nice as it doesn't sound like a complicated fix. Although the amp doesn't even get moved around much, so I don't think it would have failed from anything I did, but maybe it failed on its own. Thanks for the reply.

[jmaf]

Yeah C14 is where you should focus. If it's leaking DC it might be sending high voltage to the treble pot so beware.

[Mick Bailey]

Check the solder joints on the treble pot. A bad connection here (or a faulty pot) will give the same issues as a defective C14. Is the pot loose at all?

[jm2]

The pot feels secure. Based on the replies, I might root around with a meter to some small degree.

[52 Bill]

The pot feels secure. Based on the replies, I might root around with a meter to some small degree.

You sound like you are worried about getting in too deep, which is a good thing. There can be dangerous things inside an amp. As long as you follow a few basic rules, you should be fine.

Make sure to unplug the amp from the wall and allow the amps filter caps a few minutes to discharge. The schematic shows no standby switch or bleeder resistors on the main filter cap, so give it a little time discharge. You may actually want to read the dc voltage on the power supply before you start to do any probing.

To get to the circuits, you will need to remove that entire chassis from the cabinet. I suggest that you take a few photos of the assembly before you start to remove panels and plugs and wires. That way you will have something to reference when re-assembling the amp.

Work slowly and carefully and you should not have any problems. If you need or want specific instructions on how to measure things or what to look for, please feel free to ask for help.

Good luck.

[jm2]

That's great. Thanks for all the considerate replies. I am very mechanically inclined, and I can do some very simple electronics, but it's not remotely my strong suit, and I know where my limits are. Having said that, I have had the chassis out on this amp before, and even biased it myself, but did so with an AMP-HEAD bias tester-MPD, which takes the pin locating out of the equation by using sockets. I'll proceed on the simple diagnosis in the next evening or two and will ask again if something particular jumps out at me.

[jm2]

I just finished as much probing as I could manage. My digital meter has relatively stubby probes, and the components were tricky to access. I assume most bona fide amp people have a set of long narrow leads for tight places. In any event, the only thing I could confidently locate was the C14 capacitor. It's a 471 or rather a 470pf cap, and that's what the schematic specifies. It measured 820pf. I had to bend the cap to get access to it.

In any event, I put the amp back together, and lo, problem solved. The treble is back and rolling off fine. My best guess is a dry solder joint at C14, which I disturbed by bending the cap. I suppose I should open it back up and touch it with an iron, but I am compelled to enjoy it for a short time lest I screw something up. I am still a bit puzzled how the cap could be that far out of spec. unless my meter is faulty. Mind you, it's fairly new, and I verified its function and apparent accuracy on some .015mf guitar tone caps. Could checking the cap in circuit throw the reading? In fact, I just realized I forgot to pan the treble and checked the cap at just one setting, whatever it was.

[The Dude]

You won't get an accurate measurement of a capacitor's capacitance in circuit. You need to remove it to test. If the amp works, probably not necessary, but I would resolder it.

[Enzo]

In any case, your problem would be because it was open or disconnected, not that it was a wrong value.

[jm2]

I played it for a short time, let it cool and re-soldered it. So far so good. I'm glad it was a simple problem.

[jm2]

I really like this amp. The only thing that would make it better is a standby switch.

[jm2]

I had more strangeness take place. I touched up the solder connections on the C14 cap shortly after I had bent it, and the amp worked fine for about four hours of playing. Then, right while I was playing, it failed. I could actually hear the treble roll off, and it took about 3-4 seconds in all. After that, it was back to the way it was, with the treble acting as an inverse volume.

I touched the connection with the iron again, but it did nothing this time. Now I am wondering if the capacitor itself is at fault, having some kind of internal flaw that I temporarily fixed by bending it. Do capacitors just fail like this? And does it not seem strange that I would hear the cap fail gradually over 3-4 seconds?