Need Help-Hum from chassis when in standby

[MJ Slaughter]

My DC-10 has a hum that can be heard when in standby. Out of standby it is louder because it can be heard through the speakers with no cord plugged into the input jack. I've swapped out all tubes and it recently had a cap job. It appears that what ever is causing the hum in standby is just being amplified when out of standby. After 2 turning up the main level control doesn't make as much difference as the masters for each channel.

Any ideas what I can check?

 

[tetsubin]

My DC-5 has the same hum. I think it's due to a design fault (Sorry, Randall! Nevertheless the amp is fabulous!) : the power and output transformers are both aligned the same way and so they pick up the hum. Just take a look inside most other Boogies and you'll see the transformers mounted in 180° to each other.

PS.: To clarify what I mean. It's not the mechanical hum itself that I attribute to the non-rotated tansformers. The mechanical hum is normal for almost any tranny. But the hum in the speakers (even in standby) is what stems from that design fault. [BTW: I also had my caps checked, all are fine.]

 

[jonarobb]

My DC-10 has a hum that can be heard when in standby. Out of standby it is louder because it can be heard through the speakers with no cord plugged into the input jack. I've swapped out all tubes and it recently had a cap job. It appears that what ever is causing the hum in standby is just being amplified when out of standby. After 2 turning up the main level control doesn't make as much difference as the masters for each channel.

Any ideas what I can check?

Was it humming before the cap job?

The DC-10's are actually pretty quiet. Since there are so many flavors of hum I'll assume you checked to make sure the power transformer is securely bolted to the chassis.

If you have a noticable hum in standby that is coming out of the speaker(s) there are several culprits:

Both center taps off the power transformer need to be firmly grounded on the chassis. This would be the HT center tap and the heater filament center tap. If the filament winding doesn't have a center tap then you need to check the artificial center tap which would be the 2 100 Ohm resistors going to ground off of the filament strand.

All caps on the B+ rail need to be secure to ground, this includes the 220uf caps off the rectifier and the 30uf caps in the preamp.

 

[MJ Slaughter]

Sorry I wasn't clear with my initial post. The hum does not come through the speakers when in standby. I only hear hum coming off the chassis. When out of standby the same cycle hum is present through the speakers.

Hum was present before cap job. That's one reason I did it plus the amp is old and I just got it without knowing it's history. Transformers appear tight.

How do you check ground connections without resoldering everything? My experiance has been that just because it looks right doesn't mean that it is.

 

[jonarobb]

Sorry I wasn't clear with my initial post. The hum does not come through the speakers when in standby. I only hear hum coming off the chassis. When out of standby the same cycle hum is present through the speakers.

Hum was present before cap job. That's one reason I did it plus the amp is old and I just got it without knowing it's history. Transformers appear tight.

How do you check ground connections without resoldering everything? My experiance has been that just because it looks right doesn't mean that it is.

On a Boogie I always troubleshoot from the power section first



Can you confirm if the Bias Supply filter caps were changed as well?

Hum like you are describing, in Standby, but not audible through the OT, but only audibly from the tranny itself, is very easy to isolate. Something is pulling down one of the secondaries on your power transformer, and is causing a hum. By process of elimination you should be able to track it down.

My method for this type of hum, which I've verified to be coming from the power transformer, is to pull the power tubes, then I unsolder the secondary leads section by section. Bias supply lead first, turn amp back on, hum still present, turn amp off, resolder bias lead back on, move to the HT leads(pair of red ones), etc...

Once you found the offending secondary, then you troubleshoot that section alone. Hum ironically is the easiest thing to isolate. You just have to be methodical and patient.

Bias Supply
HT
Filaments
Relay Supply

Once you've found which area is causing the hum you move over to the passive components and start checking. Check grounds with a Multi Meter set to continuity. One tip to to the chassis, the other to the NEGATIVE side of the caps (cept' for the bias caps which have the positive going to ground). I'll warn you as I'm sure you know, some of those caps are charged with the full B+ so make sure to steer clear of the positive side of the caps. If you feel the least bit uncomfortable checking around the chassis, then take it to a tech.

After checking continuity then switch your multimeter over to resistance, lowest setting, and check the same negative ends of the caps to ground, as well as anything else that's going to ground. You should have almost zero ohms reading.

If I was a betting man I'd say the bias supply electrolytics caps weren't changed.

 

[MJ Slaughter]

I'm going to take my amp to the tech that did some work on it previously but talking with him he said one possibility is that the power transformer may be causing the trouble if all the caps and tubes are good. Depending on price, and if the transformer is the problem, I don't know if it would be worth it because though annoying at home it doesn't seem to effect me much at a gig.

Anyone have any experience with having to change out a power transformer before? What would I look for in a replacement as far as specs go?

 

[jonarobb]

I'm going to take my amp to the tech that did some work on it previously but talking with him he said one possibility is that the power transformer may be causing the trouble if all the caps and tubes are good. Depending on price, and if the transformer is the problem, I don't know if it would be worth it because though annoying at home it doesn't seem to effect me much at a gig.

Anyone have any experience with having to change out a power transformer before? What would I look for in a replacement as far as specs go?

Did he not hear the hum when he re-capped it? Highly unlikely it's a transformer but not unheard of. Same test will verify it. Lift all the secondaries, if the hum stops, then some other passive is either dragging it down current-wise, or the transformer itself is bad.