Mesa Mark IV lead gain pot noise

[punchy]

I just picked up a Mesa Mark IV combo and while going through testing, the lead gain pot shows an issue. Video linked below shows the issue, but at parts of the range (particularly in the lower range) the signal cuts in/out but not completely always as some signal gets through at times. The seller mentioned that he hasn't played the amp much, especially in the last few years, so I thought that maybe it's just a dirty pot. I pulled the chassis and sprayed it with Deoxit F5 but no difference in behaviour. I had a similar issue with a recently acquired Roland RE-201 and Deoxit fixed the issue, but before going any further I wanted to ask the knowledgeable folks here if they've experienced this in the past with Mesa amps or if they have some advice with troubleshooting the issue. I'm nowhere near an expert but I'm comfortable enough with electronics and getting inside amps to try to figure this out.

 

[stephen sawall]

I had a pot go bad om my IV about twenty years ago. It was difficult to get at but wasn't that difficult. Mesa sent me the pot and a few other small parts free.

The output knob now needs to be replaced. A few other older amps I have has the same issue. I have had a few pots wear out or go bad from not being used over the years.

 

[punchy]

I had a pot go bad om my IV about twenty years ago. It was difficult to get at but wasn't that difficult. Mesa sent me the pot and a few other small parts free.

The output knob now needs to be replaced. A few other older amps I have has the same issue. I have had a few pots wear out or go bad from not being used over the years.

In your opinion does this look like it could be anything other than just a bad pot? That's mainly what I want to make sure of since I'm not too familiar with Mesa amps. Getting to the pot isn't great but doable so if that's all it is then I'm not going to worry. The amp is stable, the only issue appears to be just when turning this one pot. Initially, the treble/mid/bass pots on the lead channel didn't seem to do anything when I first started testing the amp, but they all work well now so I'm thinking they were just dirty from the amp just sitting around.

 

[schnarf]

In your opinion does this look like it could be anything other than just a bad pot? That's mainly what I want to make sure of since I'm not too familiar with Mesa amps. Getting to the pot isn't great but doable so if that's all it is then I'm not going to worry. The amp is stable, the only issue appears to be just when turning this one pot. Initially, the treble/mid/bass pots on the lead channel didn't seem to do anything when I first started testing the amp, but they all work well now so I'm thinking they were just dirty from the amp just sitting around.

It's almost certainly a bad potentiometer. It's not uncommon. Given where it is in the circuit it's unlikely to be any other issue. If you email or call Mesa they'll send you the part.

On the other hand, if a GEQ slider was doing this, it could possibly be a bad tantalum capacitor, I've seen that. In that case, the capacitor was leaking DC which causes crackling when moved, and also was messing with the bias of the transistors.

 

[chumbucket]

Looking at it from the glass half-full side, it sounds pretty killer when it's working though. Sounds like a new pot and you'll be back in business.

 

[punchy]

Thank you all for the advice, really appreciate it. It looks like this particular pot isn't in production anymore and it doesn't seem like Mesa has any left, but I'll get in touch with them just in case. Either way, it's not a big deal and easy enough solutions exist.

And absolutely the amp sounds amazing so there's no doubt I'm keeping it. I'm not even in a rush to fix it since the pot works well at the gain setting I want (I'll fix as soon as I can though as I know the pot will just die one day).

 

[schnarf]

FWIW they almost certainly do have replacements for you. Their website isn't always up to date on stock, and if they don't have the exact original part they'll have some substitute that fits and has the right resistance and taper. Good luck and enjoy the amp!

 

[punchy]

FWIW they almost certainly do have replacements for you. Their website isn't always up to date on stock, and if they don't have the exact original part they'll have some substitute that fits and has the right resistance and taper. Good luck and enjoy the amp!

I have to give it to Mesa, their service department is great. Just called them now and they answered right away and was able to talk to someone that was very familiar with this issue. Unfortunately the pot controlling lead gain is not made anymore and the replacement they have won't physically fit. Their recommended procedure (and it's how the fix them) is to move one of the RHY1/2 gain pots to the lead position, then use the new replacement pot on the RHY1/2 position

 

[schnarf]

Ok wow, that's surprising! Good to know, thanks for the update.