Amp hum help

[glpg80]

for a second i thought it was ann landers speed, ann used to be able to blow through 3 pages in a single night, almost every night

 

[roadifier]

The caps came in today and I had them replaced. I had the filter caps replaced. That didn't help the amp at all. There is still the same amount of hum and buzz as there was before. I can take a picture of the inside of the amp if anyone wants it to see what is inside. I'm stuck at what could be causing this, I am probably going to take it to the tech tomorrow or saturday if I can't figure out whats wrong. but I got to play it at pretty high volume today and it sounded really good.

There is also another cap on the bottom of the circuit board the is vertical.

 

[steve_k]

The caps came in today and I had them replaced. I had the filter caps replaced. That didn't help the amp at all. There is still the same amount of hum and buzz as there was before. I can take a picture of the inside of the amp if anyone wants it to see what is inside. I'm stuck at what could be causing this, I am probably going to take it to the tech tomorrow or saturday if I can't figure out whats wrong. but I got to play it at pretty high volume today and it sounded really good.

There is also another cap on the bottom of the circuit board the is vertical.

I think you are digging too deep.

1) Does yours have a reverb circuit in it? If so, pull V4, turn the amp on and see the results.

2) Pull all the preamp tubes, except the input driver (V1) and turn the amp on and see if there is any change.

3) Play with the EQ with only the V1 tube in the preamp. Notice the hum changing?

4) Target the input ground, a bad pot or look for burnt resistors on the preamp board near the input.

5) Turn the volume up without an instrument plugged in. Still there?

6) Take your chop sticks and check around the solder connections on the preamp board and EQ pots.

7) If things are OK with only V1, add V2 tube and go from there.

Do you have a schematic? If not, go here: http://www.schematicx.com/schematic/mes ... schematic/


Steve

 

[glip22]

Chopsticks are my best friend. I would be looking at everything. Sounds like a bad ground but it could be allot of things. Also very carefully move your finger around inside the amp, not touching anything. Sometimes there can be an electromagnetic field in some areas. It's actually normal but can sometimes inject into the signal. I had one in an amp. When I put my finger in a certan area my body acted like a ground and the hum would all but go way. I found that their were two coupling caps picking this up. I bent the caps over away from the field and problem fixed. Look on your board for ceramic caps also. Check that large green cap to the right of the board. You can touch that one. Move your finger around near that one. They are more prone to picking this up. If nothing else works you need a tech for his O scope. It could be an open resistor and I have no clue how to isolate that without a scope.

 

[steve_k]

Chopsticks are my best friend.

Us and a gazillion Chinese and Japanese.

ROR (raff out roud)

 

[roadifier]

Steve I tried all the things you said to do, and nothing helped the hum and buzz.

Glip, I poked around in the amp and putting my finger near the top of the V3 socket near a few coupling caps made the buzz worse, but the hum was the same. Nothing happened when I touched the green cap on the right. I have an O scope, but I am going to take it into the tech later today, I'd rather just let them fix it and do it right and well. I will be without this amp for a few weeks

Thanks for your help guys.

 

[glip22]

Steve I tried all the things you said to do, and nothing helped the hum and buzz.

Glip, I poked around in the amp and putting my finger near the top of the V3 socket near a few coupling caps made the buzz worse, but the hum was the same. Nothing happened when I touched the green cap on the right. I have an O scope, but I am going to take it into the tech later today, I'd rather just let them fix it and do it right and well. I will be without this amp for a few weeks

Thanks for your help guys.

Good luck. You win some and you lose some. My last loss was an OT.

 

[roadifier]

I found out what was wrong with my amp, when I replaced the filter caps, I had to replace the bias caps, that should fix it

 

[glip22]

I found out what was wrong with my amp, when I replaced the filter caps, I had to replace the bias caps, that should fix it

Dude, They are more important than the other caps. I see the new F$t, and Sprague Atoms. But those orange ones are your original Sprague bias caps. No? That would definitely do it if one went bad,

 

[roadifier]

I found out what was wrong with my amp, when I replaced the filter caps, I had to replace the bias caps, that should fix it

Dude, They are more important than the other caps. I see the new F$t, and Sprague Atoms. But those orange ones are your original Sprague bias caps. No? That would definitely do it if one went bad,

Yeah they are original 20 years old

 

[glip22]

I found out what was wrong with my amp, when I replaced the filter caps, I had to replace the bias caps, that should fix it

Dude, They are more important than the other caps. I see the new F$t, and Sprague Atoms. But those orange ones are your original Sprague bias caps. No? That would definitely do it if one went bad,

Yeah they are original 20 years old

I recapped a MKIV from April 93. I replaced every cap I could.

 

[roadifier]

I was wrong...

I changed those 220 uF/63V caps, and that didn't help, the hum and buzz is still there.

One thing I noticed is when I turn up the gain control, either on the clean or lead channel, It hums and buzzes until the gain control goes past 7 or 8. When it is up all the way there is no hum or buzz whatsoever.

 

[glip22]

I was wrong...

I changed those 220 uF/63V caps, and that didn't help, the hum and buzz is still there.

One thing I noticed is when I turn up the gain control, either on the clean or lead channel, It hums and buzzes until the gain control goes past 7 or 8. When it is up all the way there is no hum or buzz whatsoever.

Weird. You said this happens with or without a cable plugged into the input.

 

[roadifier]

I was wrong...

I changed those 220 uF/63V caps, and that didn't help, the hum and buzz is still there.

One thing I noticed is when I turn up the gain control, either on the clean or lead channel, It hums and buzzes until the gain control goes past 7 or 8. When it is up all the way there is no hum or buzz whatsoever.

Weird. You said this happens with or without a cable plugged into the input.

Yes, I doesn't matter if the cable is plugged in or not.

 

[steve_k]

Gain pot?

 

[roadifier]

Gain pot?

I'm thinking that might be the culprit of the noise. I hope it is, it would be easy to replace.

 

[glpg80]

i think its a wiring issue at this point.

like i said in my very first post - troubleshooting an amplifier over a forum is like asking fhow to do triple bypass heart surgery texting on a cell phone.

 

[steve_k]

Gain pot?

I'm thinking that might be the culprit of the noise. I hope it is, it would be easy to replace.

Won't hurt to try, since you are becoming very good at the "chop stick" method of amp troubleshooting and haven't got bit yet

And, like Matt said, it's trial and error once you get it narrowed down especially from power to pre. With the cascaded gain on Mesa designs, it can be a little more trouble.

Keep us in the loop.


Steve

 

[roadifier]

My dad and I were working on the amp today, we couldn't find anything... I am just going to bring it into my tech on monday. Of course it will cost like $150, and find out it was some small easy thing to fix

 

[roadifier]

I replaced some coupling caps earlier and that didn't help, the only things I can think of are the power transformers and gain pot.

I did some more test, and I think the power transformer is causing the hum and the gain pot is causing the buzz. Anyone know of a way to test a power transformer to find out if it is bad?