Laney VH100R lost higs

[Emetal]

My Laney VH100R suddenly lost highs, lost sparkle. There is no loss of overall sound or volume, it simply became dull. I am talking about Distortion channel, the Clean channel I can not gauge with my ears because I almost never use it and I am not familiar with its proper sound. Anyway, the filter caps are new, installed about a year ago, the EL34 tubes are fairly new, nicely balanced and biased at 65%. 12ax7 are mixture but mostly newer ones.
So far I have:
- changed positions of 12ax7's and also tried brand new one on the first Dist. position
- checked all the voltage test points, everything is OK
- measured most of the resistors
Still did not find the problem.
- going right now to open and clean Treble and Presence pots just in case and measure that taper is fully functional
Do you guys have any other ideas, some electrolytic cap maybe?

[J M Fahey]

It is a VERY complex amplifier, PCBs are poorly identified (I cpild not find 9086-2 for example), ribbon cables criscross all over the place, a mess.

To boot itīs not "broken" , itīs a subtle "EQ change" and being a heavily equalized Guitar Amp, there is not a "flat signal" to be checked available anywhere.

In other amps first suggestion would be to send preamp outto another amp and feed a known good preamp signal into its power amp, at least to start splitting it into sections ... but here FX Send and Return are very far of the usual points and separated by a ton of stages ....

IF you wish, Iīd creat a splitting point by lifting the top lead of 1k R28 at the power amp input, feed known good signal (from a proper working head, any brand and model) there, which is the REAL Power Amp input and listen.
Does it sound good?
Is it muddy?, etc.

And viceversa feed the signal *reaching* the now empty pad into another amp.

But in principle, it will be a long tedious job because you do not have a before/after signal reference, nor an exact same model to substitute and compare both stage by stage to find the differences.

A mess.

I would much prefer a non working amp with smoking resistors, shorted caps or tubes, wild voltages, heavy hiss/hum/buzz or plain mute, which are at least *clear* problems.

But amps where Customer says "it *works* .... BUT ...." give me the shivers.

EDIT: you may build a Signal Tracer, which can be as simple as a shielded wire with crocodile clips plugged into a small practice amp , Iīll draw a suggested circuit below, inject some looping Guitar DI signal at input (you canīt play and test, to boot itīs DANGEROUS) and follow signal from input to output.
Warn you itīs a long tortuous path.
And PRAY there is some gross/obvious change somewhere.

[g1]

Also, you have to be sure it is the head and not the speaker cab. that is the problem. May not be likely, but needs to be ruled out anyway.

[Chuck H]

Not to mention that the guitar or anything between the guitar and amp could also be the issue. Does your guitar have a preamp? Have you tried it with another amp that you are familiar with to assure it's alright? etc...

I once had a customer call me, livid that the amp I built for him was failing and sounding wrong after only a couple of months. I had to make a trip to his place and see his whole signal chain and hear the problem to determine that it was a low battery in his tuner pedal that was the causing the problem. Happens all the time.

A low battery in a guitar preamp will cludge everything. A bad connection in the effects chain will too. Unless you're familiar with what your guitar sounds like run dry (no effects, just a cable) into the amp then it becomes harder to trouble shoot this sort of thing from a remote perspective.

[Emetal]

Not to mention that the guitar or anything between the guitar and amp could also be the issue. Does your guitar have a preamp? Have you tried it with another amp that you are familiar with to assure it's alright? etc...

I once had a customer call me, livid that the amp I built for him was failing and sounding wrong after only a couple of months. I had to make a trip to his place and see his whole signal chain and hear the problem to determine that it was a low battery in his tuner pedal that was the causing the problem. Happens all the time.

A low battery in a guitar preamp will cludge everything. A bad connection in the effects chain will too. Unless you're familiar with what your guitar sounds like run dry (no effects, just a cable) into the amp then it becomes harder to trouble shoot this sort of thing from a remote perspective.

Always been playing that amp straight without anything between guitar and amp, but cable. The only pedals (solo boost and delay) were in the Preamp OUT - Power amp IN loop. Easy to remove, has no influence on the problem. Guitars are perfect, I am guitar tech :-) Everything is ruled out but cab and the head, since the cab sits in the practice room I doubt that the speakers could change on their own. Sooo it must be the head...

[Emetal]

Also, you have to be sure it is the head and not the speaker cab. that is the problem. May not be likely, but needs to be ruled out anyway.

The cab just sits at the practice pad, I doubt the speakers could be OK at one rehearsal and then next rehearsal dull ?? I will of course check the head with the friends cab, but he has different speakers and impedance cab. Might be hard to gauge.

[Chuck H]

Guitars are perfect, I am guitar tech :-) Everything is ruled out but cab and the head, since the cab sits in the practice room I doubt that the speakers could change on their own. Sooo it must be the head...

Well, being as the head, the cabinet and the guitar are all "circuits" there's no more reason to suspect an instantaneous change in any one over the others. That isn't to say that the problem isn't the head. I'm just saying that the guitar, cabinet AND the head were all fine and now something is wrong. Nothing about the cab "sits in the practice room" rules it out as the head was, for the most part, just sitting too. Even if you transported it it wasn't operating. And I expect if you had dropped it or something you would have mentioned that.

Basis for comparison is the problem here for isolating the issue. If you have another guitar that you play with that amp and are very familiar with the tone then that should be done. If you have another amp that you play your regular guitar through and you are very familiar with the tone you could try that too. Either would rule out the guitar. Potentiometers can get hinky anytime. A lowered resistance on the guitar pot would squash the usual resonant peak in the signal from the instrument. And I've seen this happen.

None of this is to say the problem isn't likely the amp. Since it has the most complex circuit (and by implication the most things that can fail) it does seem most likely. But we should rule out everything we can before cracking it open.

[Helmholtz]

It's a rather uncommon amp problem and could have many reasons, like a bad connection somewhere or a small cap gone open.

Apart from the overall changed sound, do all controls do what they are supposed to, like treble pot increasing treble etc.?

[Emetal]

SOLVED !!!!!!!!
I cleaned the Presence, Treble, Middle and Bass pots (with medical 96% ethyl alcohol, didn't have anything else handy). Everything works fine now. Funny thing the pots were never making crackling noises or give any other bad sign, but somehow cleaning has solved the problem. It is an amp from '97 so I guess it is to be expected.
Thanks to everybody for the ideas!!!

[Chuck H]

Good deal

Basic trouble shooting technique could have resolved the issue a little faster. Helmholtz suggestion in post #8 would have likely revealed the treble or presence pot not behaving as expected. Checking to see if the controls are working right is pretty much step one in a situation like you were having.

[Emetal]

Good deal

Basic trouble shooting technique could have resolved the issue a little faster. Helmholtz suggestion in post #8 would have likely revealed the treble or presence pot not behaving as expected. Checking to see if the controls are working right is pretty much step one in a situation like you were having.

I know, but the weird thing is, before cleaning, all the pots did make changes to the sound. Actually the only pot that was funny acting was Bass pot, it had way too much deepness way to early. Kind of boomy sounding. Could not use it higher then 2. Now it has nice gradual change all the way throughout. And my highs are back.. so all good :-)