What can cause hum in DC-10?

[MJ Slaughter]

I've noticed a slight hum in my DC-10. Nothing too bad but it is there where I don't have any hum in my DC-5. It sounds like a "B" note and I only notice it when I'm not playing, channel 1 or 2. Does anyone know if being 100 watts will cause it to do this over a 50 watt? What might be some things to look into to correct this minor problem?

If I never had the DC-5 to compare against I wouldn't have even thought it might be a problem.

 

[cb101]

Are you running anything in the Loop? I've had some strange sounds with different effects and different mix settings through that parellel loop. Of course there's always the good ol' tubes.

 

[MJ Slaughter]

Thanks for the suggestions but I don't have anything in the loop at all and I've swapped out the tubes with no change.

 

[tetsubin]

Could be a capacitor.

 

[MJ Slaughter]

Could be a capacitor.

Would that be one of the big, blue I think, ones? Is there a way to narrow down which one or would you just replace them all?

 

[tetsubin]

Could be a capacitor.

Would that be one of the big, blue I think, ones? Is there a way to narrow down which one or would you just replace them all?

Yes, the big blue ones. I'm no tech, but as far as I know you can either see it if on is blown or you can, if you have the knowledge and tools, measure if it's in good condition. That should all be done by someone knowledgable because of the fatal voltages present in the capacitors.
Sometimes hum can be introduced by bad potentiometers, the gain kow for example. A good technician will know what to check.

 

[Monsta-Tone]

MJ, the amp is a little old, so it could be a capacitor. Mine is about 13 years old, so I will most likely change them next time I get in there for something.

I make it a point to change them all at a certain age, or if they start exhibiting any type of hum or loss of tone.


$100 now will save you $1,000 later.

I've seen some power tubes cause hum too, but it's rare.

Does the amp do it on Standby? If so, then this is your power transformer. You could try tightening the bolts down a bit.

 

[MJ Slaughter]

Hello Monsta-tone.

The hum is really not too bad unless it's turned up fairly loud and it is not present on standby. I did notice that with the volume turned up then placed into standby there is a little pop, almost like someone tapped a power tube with a pencil.

Though I haven't changed large capacitors before I am familiar with components and comfortable making simple repairs. What I don't know is a safe way to discharge the caps so I don't fry myself. Is just turning the amp off without turning it to standby first enough? This is an area I'm a little unsure about. Do you already know want caps I need to get so I don't have to open the head up? Sorry, I'm inherently lazy.

 

[stokes]

In the main or high voltage supply,that amp has 2x 120uf/300v in series and 3x 30uf/450v caps.You describe the hum as sounding like a B note.That could be 60 cycle or 120 cycle.The 60 cycle would be from your filament supply,if it is 120 it is from your power supply and the cure would be the filter caps.If the amp is old (more than ten years) it wouldnt hurt to change the filter caps,it will definately improve the amps response,and likely cure the hum.Just for the record,there really isnt a 100% reliable test for filter caps,I've seen caps "test" good and explode soon after,it is best to change them when they get old as a preventive maintenance thing.To discharge the caps you can wire a resistor between two alligator clips,connect one clip to the positive end of the cap the other to ground.The exact value isnt critical,1k to 10k will do.You can even go as low as 470ohm or so.I would recomend using F&T or Sprague,stay away from the asian manufacture caps.If you cant get an exact match for those 120uf's,you can use 220uf.In fact,the schem I have shows 120uf,but I'm not sure that isnt a typo,every Mesa amp I've seen has 220uf totem poled for that first node,I would suggest going with the 220's,a little extra filtering is better than too little.

 

[MrMarkIII]

Maybe it hums 'cause it forgot the words.

 

[Monsta-Tone]

They are 220 Micro Farad, and 30 Micro Farad. Email or PM me with an email and I will send you the correct drawings.

 

[stokes]

I figured as much.The schem I have is from a Mesa Tech Support Pack too.See a lot of mislabeled stuff in there.You'd think Mesa themselves would be a bit more dependable.

 

[Monsta-Tone]

I just looked really closely at my drawings. I had to blow it up to about 200% to see that it is a 220, but the 1st 2 doesn't look right for some reason. It looks like a 1 at lesser magnification.

Never noticed that before.

 

[MJ Slaughter]

Maybe it hums 'cause it forgot the words.

LOL, that made me laugh. I'm emailing you Monsta-tone. Thanks all.

 

[Monsta-Tone]

Done!

 

[MJ Slaughter]

It occured to me that the hard part of this proceedure isn't changing the components, it's getting to them! What is the best approach here. Pull all the pots, jacks and switches off the front panel and rock the PCB back and work on it there or unsolder all connections and pull the entire board out. I hope there is something really easy that I'm over looking.

It's like working on a car. Fairly easy to change out a water pump or altenator. It's another thing just getting to it.

 

[Monsta-Tone]

The solder pads should be on both sides of the board. I can easily change parts in mine without removing anything.

The only thing that you have to worry about is a steady hand. It is really easy to burn wires or other parts.

 

[stokes]

Theres no reason to remove any components but the caps themselves.Just be careful when heating the lead at the board itself,too hot or hold it there too long and you can burn a trace on the board.If you are new to the soldering thing,you can clip the lead on the old cap so you leave about 3/8" of lead on the board,then you can put a heat sink on the lead to the board and a sink on the new lead,just twist them togeether as neatly as you can and solder them up.Not the neatest look,but how often do you look inside the amp?

 

[stokes]

I just looked really closely at my drawings. I had to blow it up to about 200% to see that it is a 220, but the 1st 2 doesn't look right for some reason. It looks like a 1 at lesser magnification.

Never noticed that before.

I tried enlarging my copy and it still looked more like a 120,a little funky but not like a 2 at all.I would bet,though,that it is a pair of 220's.In fact if it were my amp and it had 120's I would go with the 220's anyway.Like I said before a little extra filtering at the first stage is a good thing.

 

[Monsta-Tone]

Yeah, they are definitely 220's. I've never even seen a Mesa that had 120's in it.


Another problem with the caps is that they have a silicone or similar substance that holds them in place. This must be broken so that the old part can be removed.

I usually cut the leads and then pry them up GENTLY with a screwdriver or dental pick.

I scrape it off with a razor blade and then re-apply a little automotive RTV silicone to hold the new parts in place. The clear stuff, not the red or blue.