MY DC-5 IS HUMMING LIKE CRAZY ... CHGD CAPS ... NEED HELP

[STR440]

My DC-5 has a really bad hum. the power (wattage) seems ok. Still powerful. But regardless it has a really loud hum. Here's what I've checked/done so far ...

Changed every electrolytic cap with new original mesa caps. Just for maintenance and hoping it was a filter cap issue

Changed power and pre-amp tubes.

Checked all voltages , Plate and screen V's seem ok.
Not sure how to check the Power transformer. Each pin on the transformer gives me zero voltage with one side to ground. Also the 220ohm 5W resistor (which i cant seem to trace on the schem) is getting super hot (maybe thats normal)

Here is a pic of the amp http://i64.photobucket.com/albums/h171/mypicspics/DC-5.jpg

B+ voltage = 440V (OK) ; Checked V1a to V6b voltages ....

On V4a I get 222v. that's high. According to the schem it should be 188v. Not sure if this is my culprit.

Just wondering if you guys can help lead me in the right direction. thanks

Here is a link to youtube that shows my amp's noises ...

http://www.youtube.com/watch?v=cgewTdGr1m8

I am just starting to learn about amps and electronics so any help would be extremely appreciated

 

[STR440]

Do I have a rare problem? Or is it that I've tried everything there is to try? Please at least give me some feedback even if it is to say "Hey that's a tough one, I'm not sure" or "This is the wrong forum guy"

Thanks

 

[zeeman]

My DC-5 has a really bad hum.

How did the hum start? What I mean is...

1) Did it get worse over time or did it just start one day out of the blue?
2) Is there anything that you can think of that might have caused this? For example.... Did someone spill something on it? Was it dropped? Did someone try to modify it and the modification went horribly wrong? etc.
3) Did you buy it used and it was that way when you bought it?

I am just starting to learn about amps and electronics so any help would be extremely appreciated

Maybe its time to take it to a qualified tech.

zeeman

 

[Blueracer]

As zeeman posted some of the history of when and under what circumstances the hum started would be helpful.

On the surface it sounds like a grounding problem of some sort. Assuming you properly replaced the filter caps and tubes these may for the time being be ignored. From the youtube link it sounds like the gain control (the one next to the input jack) decreases but does not eliminate the hum while the master pretty well kills it. This is kind of important. If the gain control next to the input jack actually kills the hum then you can isolate the problem to the first stage associated with V1. To check for this turn that gain control down ALL THE WAY (shouting for effect). If it doesn't kill the hum but decreases it then it is a more generalized problem.

One thing to check and is often overlooked is to make sure the center tap of the filament transformer is grounded. Check the cable from the input jack to circuit board. Is the shield connected.

Here is a link to a troubleshooting site that may be helpful.

http://www.geofex.com/ampdbug/ampdebug.htm

Try some of these things and I will think a bit more about your problem. Post back your results.

 

[JOEY B.]

I noticed that it hums without anything plugged into the input jack. This is a longshot, but the input jack is supposed to ground itself whenever the guitar cable is unplugged. If the amp has sat with a cable plugged into it for a long time, it might not be grounding properly when the cable is removed. Try plugging in a guitar, turn down the guitar volume to "0", remove any effects from the loop, then turn off all lights and ceiling fans that are tied to the outlet that the amp is plugged into. If you have tried all this, then it might be time for a professional tech.

 

[Blueracer]

I noticed that it hums without anything plugged into the input jack. This is a longshot, but the input jack is supposed to ground itself whenever the guitar cable is unplugged. If the amp has sat with a cable plugged into it for a long time, it might not be grounding properly when the cable is removed. Try plugging in a guitar, turn down the guitar volume to "0", remove any effects from the loop, then turn off all lights and ceiling fans that are tied to the outlet that the amp is plugged into. If you have tried all this, then it might be time for a professional tech.

This is a possibility. Turning the gain all the way down would help isolate it to this this. Plugging a guitar in should make this effect go away if the grounding without a cord in the jack were the problem.

 

[JOEY B.]

This is a possibility. Turning the gain all the way down would help isolate it to this this. Plugging a guitar in should make this effect go away if the grounding without a cord in the jack were the problem.

As long as the guitar volume is ALL the way down, and any other electrical sources are eliminated. A fellow board member had issues with dimmer switches in other rooms of his house (downstairs). It can get crazy sometimes :shock: .

 

[lesterpaul]

yes, and I LOVE it when the lights are OUT!

 

[STR440]

Hey guys. thx for the responses. It's late while reading this so I will re-read the posts again tomorrow and try all the suggestions.

What I did try is to see what actually completely kills the hum-the gain or the master volume- as per Blueracer's suggestion.

Ok with nothing plugged in and everything on zero. No hum or hiss
Ouput Volume control turned up and Mas and Gain on Zero: very slight hum(maybe thats normal)
Output Turned up Gain turned up to 10 Master on zero: again very slight hum
*Output up, Gain to zero & master turned up : thats when i get a nasty hum

**But when I turn up the master and gain controls together the amp hisses really really bad

And Joey B I thought we're all super techs here

 

[JOEY B.]

And Joey B I thought we're all super techs here

I'm no amp tech, for sure. I have had to learn these things the hard way. Plenty of "WTF ?????" moments during the years :lol: :lol: :lol: .