Humming lead channel Mark III

[PhilBelanger]

I've had my Mark III for a while now and I have a humming lead channel (60hz).
The clean and R2 sounds fine, no hum at all.
I'm discounting the main filter as I figure the hum would be across all channels.

I'm not familiar enough with the layour to identify which filters caps are part of the lead channel, so I thought I'd try here.

Can somebody help me out?

 

[Boogiebabies]

That's a good question. Your lead channel is getting it's plate voltage from a different cap in the filter supply. The hum may be ripple current from a bad cap in the lead preamp supply. The MK III is a bit more modern than the older Marks and has the three blue 30uf/500V filter caps on the preamp board. The bad news is, I have no idea which one is for the lead supply, but you can buy all three for less than $ 21.00 or so. I would change them all for that price. Before I went hog wild I would call Mesa just to be sure it's not a leaking LDR or a common issue with your version. You never know. I always keep 4 30uf's, 4 220uf's and a few LDR's in stock for the unexpected.

 

[Shep]

the lead on mine III once the gain goes about 8 and 8 starts humming.. i am replacing the full filter cap set soon to see if that help's.. but i think it might be a model problem.. we will see.. i believe the lead supply cap is the one closes to the back of the amp out of the 3. but you should change them ALL!

 

[Restless Rocks]

Your MKIII lead circuit gets it's B+ plate supply from the "C" tap of the power supply filter stack. That would be the middle cap in the group of three 30uf blue caps. These guys are right on though you should just go on and replace all of them while you're at it including the 220uf and the bias caps as well. Sometimes however a different tube in the V3 spot can cure your LD channel hum. It's proximity to the power tranny's magnetic feild seems to be the culprit with certain tubes.

 

[daveg62]

What would be the next step after replacing the caps and swapping tubes? My Mark III blue stripe had a very bad hum on all channels, mostly on the lead. After changing caps and tubes it's gotten alot better, but still quite bad on the lead channel, slightly bad on r2 and barely noticeable on clean. What would be my next step?

 

[Restless Rocks]

If you are in an old building with no or poor grounding on the AC you can get hum. A good power conditioner with filtering sometimes helps. Also check the chassis ground connections ala the big braided wire soldered to the star washer by the power tranny. BB should be about ready to recommend his cleaning technique for this about now. Make sure it's not your guitar or cable as well and remove everything from the FX loop and eliminate all your effects and cables as possibilites. Does it hum with nothing plugged into it?

 

[PhilBelanger]

Thanks guys. I did replace all three caps and I messed around with tubes. I swapped v3 and the hum got a bit better.

I'll check the transformer ground connection (I didn't think of that )

The LDR? what does it stands for?

 

[Boogiebabies]

It could be dirty pots or sockets. You may have a bad resistor element in the lead master or dirt and grime. I've also had a few Volume 1's hum due to dirt and it was just increased in lead mode. My Volume 1 must have lived at 8 as it hummed there too.
I think you may have found your problem in either the lead drive or Volume 1 at 8.
It's time to blast them with Caig De-Oxit. Follow up with Caig Fader Lube on the pots. The star ground is right below the filter cap board and uses one of the tranny nuts. I just soak the begesus out of it as mainentance, but if the main ground was bad it would effect all three channels. The braided ground goes to the power board, the star and then to the preamp board where there's a trace that goes to the other side for the control pot grounding. For dirty sockets, I wiggle the preamp tubes with the amp on in clean and then lead mode. If it pops or give loud scratches and intermittent signal loss it's the culprit.

If you amp hums or sounds on with no guitar plugged in it's usually the input jacks shorting lug has been bent back and it does not short the grid to ground when unplugged. I just plug a cable in 1/2 way and bend the contact back with a mini screwdriver. I then release the plug to be sure it's making contact again with the tip lug of the jack.