Mark III blue stripe 6ow - low output on clean channel

[kc_ksom]

Hi

Just wanted to see if there was an easy fix - I have had this amp fixed several times for other reasons to this problem - all though the last time that it was fixed I thought that he sorted it as it sounds very good but I didn't really turn up the volume past 2 but I now notice that the clean output is NOWHERE near where it was - I can turn the volume up completely and yea its loud but if I switched over the lead channel all hell would break loose it would be unbearably loud.

Is there something that I can look at without taking it back to the guy to fix as I am not going to pay for him to pull it apart again, this guy has had it 3 times now and all 3 times its come back with issues. I know that he is a very highly regarded tech here in Australia but he just hasn't fixed mine - so failing sending it back to the States which would be massive I thought there may be something I could look at or test and possibly find the fault.

I think it has something to do with this issue = this is a little hard to explain but if I am on the clean channel and I adjust the lead channel volume down below 2 the amp cuts out - if I turn up the lead channel volume up it actually effects the output of the clean channel very slightly kinda makes it a little more fuller sounding but still not where it should be, why would the lead channel volume have an effect on the clean channel ?? and would this possibly be the area to look at for fault finding..
Channel 2 is also low output (lower than channel 1) lead channel sounds as it should..

its had new tubes fitted etc

Casey

 

[Scott_hg]

Hey Casey,

I've been looking at the circuit for these amps and I can see where that fault could be occurring..

Just as a process of elimination does it do it when using the foot switch and the pull switch - or could the pull switch be faulty?

I don't mind telling you the circuit is way more complex than anything I have ever worked on before.

I love R2 on my amp, its my happy place.... hate to think you cant really use yours at the moment.

 

[kc_ksom]

Hi mate

I pretty sure your from AGGH - ha


Yea its frustrating but I don't have a foot switch to test - all channels sound great but just lower volume for clean and even lower for R2. All functions work and I believe that BJ Amps fixed the initial issue with the intermittent crackling, during palm muting etc - they found a solder blob intermittently shorting under a pre amp valve most likely from vibrations - when I seen the picture that they took I though hmmmm that looks like a fresh solder blob wonder if he actually done it the 1st time he looked at it ??
This new problem has come from the last fix I reckon as it never done it prior to any of the other repairs. I wished I had of fiddled with the knobs at volume before letting BJ amps know they done a great job fixing it, I only noticed the difference when I turned it up and was fiddling around with different tones, also because I know the lead channel volume had NO effect on the clean channel I didn't know there was a problem until one day I had the channel volume low but master high on the lead channel and I was noodling around I went to the clean channel and there was nothing working, I fiddled around switching channels and realised that the lead channel volume under 2 would completely shut the sound off on the clean channel and when it was turned up it had an overall effect on the sound of the clean channel almost like a full switch or something, it just widened the sound also gave a little more gain, I actually liked it but I don't get enough clean volume, so now if I go back there it will be like its played up again with a new fault and he will charge me.. Bit frustrating let me tell you - in saying all this - the actual amp sounds awesome still.
I have the receipt just not on me - they also replaced something on the lead channel it was some sort of LDR or relay or something like that - I cant remember what it was called but I think they blew it up along with the tubes during the testing of the initial fault, I was also charged for new set of power tubes I questioned it as they had already been replaced the time before that, they replaced the tube because it had completely blown the up, they gave me the old one and it was in fact my old tube, so I didn't question it anymore..

Casey

 

[Scott_hg]

Sounds like he may have wired the relay up incorrectly on your lead channel.

I am flying up to Brisbane for work on Wednesday, I can bring a footswitch with me so you can give it a go but I think that relay repair is the issue. Almost sounds like it is choking your clean channel off or dumping all of the guitar signal to ground? Seams kind of possible the way the gain stages are cascaded.

Pm me here or on AGGH if you wanna try the footswitch out, I have a few....

 

[jrb32]

I think it has something to do with this issue = this is a little hard to explain but if I am on the clean channel and I adjust the lead channel volume down below 2 the amp cuts out - if I turn up the lead channel volume up it actually effects the output of the clean channel very slightly kinda makes it a little more fuller sounding but still not where it should be, why would the lead channel volume have an effect on the clean channel ?? and would this possibly be the area to look at for fault finding..
Channel 2 is also low output (lower than channel 1) lead channel sounds as it should..

Sounds like they've fucked with it and don't really know what they're doing with these amps. Definitely don't go back unless they put it right for free. I'd also be questionable as to how competent they really are... leave a bad review etc if you like as that is NOT good service to charge you for stuff which they've broken themselves and 'fixed' an amp when it's not actually fixed. Anyway, the way the amp works is as you'll know is Vol, Treble, Bass, Mid (aka the Tone Stack) and Master affect every channel. This means R1, R2 (which is basically just R1 with a little bit more gain rather than a separate channel) and Lead all work off this initial Tone Stack.

If Lead Drive or Lead Master has ANY affect when in R1 mode then you definitely have a problem. There are 3 LDRs which specifically turn Lead ON/OFF isolating it from R1. The 3 LDRs switch together all at once and control:

Lead Input ON/OFF
Lead Output ON/OFF
Lead Master ON/OFF

There is also an additional LDR which makes sure Treble Shift only occurs in Lead Mode.

My gut instinct is that if the amp wasn't doing this before, and it is now, and you said an LDR was replaced (why?) then the guy has probably put the LDR he replaced on backwards... in fairness they're backwards in the official Mesa schematic but you would have thought you just match up how the original looks or how the others look but hey... If this is the case and you can verify it then they should be able to fix this for free easily without too much hassle, or you could yourself if you're decent with an iron.

If you can take the chassis out and take some pics of the board PM me and I'll send you my email address so I can have a look at the pics to see if I can see anything obvious.

Thanks,

Jon

 

[jrb32]

If you also let me know the original fault it was in for repair for and the history of the repairs on the amp that'd be great.

 

[kc_ksom]

Hey fellas

Thanks for the replys and the help - I haven't pulled it apart just yet but I will do that in the next week or so - I fired it up yesterday as its been sitting for a long time as I mostly play the Mark V 25 now, but gezz this amp sounds good, I have always loved the sound this amp has, its very John Mayer at low gain - I should note to you that this amp had been modified by the previous owner, it has EL34 power tubes & I think it has Bias screws or something inside. The initial or the first time it was looked at was because it had been sitting for a long time in the old owners house near the beach, when I turned it on it did make some noises but it was crackling and popping as it would heat up, little flashes in the power tubes etc etc the pots were crackling when you moved them & the input socket was rusted but I still brought it as I didn't pay much for it.
The initial service was replace tubes, sockets service - clean pots - input socket and a good clean.
Once I had it home everything worked as it should, it sounds good but it would still crackle & pop on start up and at volume - specially palm mutes etc, I replaced the C90 speaker with a UK V30 - but it still crackled.
Then out of the blue while playing it popped & crackled then there was a flash, it smoked up for a sec and I seen a bit of fire but it stopped just as quick, I pulled the chassis and took some pics, I wrote a thread here on the boogie board, I sent it in for repairs - the screen grid resistors blew - the board was scorched but he got it working again - he replaced the power tubes and a pre amp tube, a couple of resistors etc and I took it home.
It still had issues - so sent it in again and he found that there was a blob of solder that was intermittently shorting - I also started a new thread on this forum with that issue, but anyway I thought it was all sorted but I soon realised that this new problem was apparent - the guy said that in the process of testing the lead circuit blew up and he to replace a tube and a little black switching thing but I cant remember what he called it.

Anyway this has been over a long period of time, as I said I don't play the mark III much anymore - Scott what part of Brisbane are you working in mate

Casey

 

[Scott_hg]

Casey,

Ill be in the city, staying in the mall and working at West End.

I will be there Wednesday and Thursday night, may have to do a work dinner one night there but I'll pack the pedal and you can post it back when you are done..

Scott

 

[jrb32]

it has EL34 power tubes & I think it has Bias screws or something inside

Fine but you need to rebias the amp EVERY time you replace the power tubes. Might be better to go back to 6L6s and have the bias fixed to save on more amp tech trips to do this but it's up to you.

the guy said that in the process of testing the lead circuit blew up and he to replace a tube and a little black switching thing but I cant remember what he called it.

Cool I'm almost certain he's probably put the new LDR on the wrong way round or got the pins backwards (the little black switching thing, well technically called an optocoupler) or there's a blown diode in the LDR circuit. Easy things for a tech to check though and the guy should 100% fix it for free if this is the case. Easy thing to visually check if you could send some board pics.

The LDRs act like a relay switch but with less noise. So one of them is probably stuck in the ON position when it's meant to be OFF.

Using a footswitch shouldn't make any difference.

Thanks,

Jon

 

[jrb32]

Hey just wondering if you had any luck with this?

Cheers,

Jon

 

[kc_ksom]

Hi Jon


I still have not pulled the chassis out to check or take a pic but if you send me your email I will flick you a picture as soon as I take one - I don't have an online picture storage account anymore for forum pics so it might be easier just to do via email etc

cheers mate


Casey

 

[McBarry]

Hey Casey,
just back on after ages away..

Totes agree with Jon - LDR , esp if one was replaced..
A pic will help tell if wrongly orientated.
I'd also wonder if he pulled the preamp board to do the LDR. Not strictly necessary, but for 100% guarantee the underside has no anomalies it is to only way to visualise the area(s).

I'd really recommend John at Pro Audio. As prev discussed, it'll cost, BUT it's as close to returning to Mesa as you'll get. Period.
Failing that, I'd recommend Ivan Richards Audio. Central Coast.

Keen on some closeup pics of the LDR's..remember 2 subtly-different ones used also..

Cheers, Dave