Musicman HD130 on the bench

[LooseChange]

I've got a Musicman HD130 on the bench.
This is the one with the Solid State phase inverter.
I get hum with good tubes in, powered on, in standby.
In fact I get hum when the power is off with tubes in.

This amp scares me a bit with it's plate voltage at 700v.

Any suggestions?
Schematic below.
Thanks!

[phsyconoodler]

Try tightening the power transformer bolts.If they come loose or the laminates are loose,it will hum.

[LooseChange]

Try tightening the power transformer bolts.If they come loose or the laminates are loose,it will hum.

Through the speaker??? When the power is off?
Will do.

It's a lot louder with the power on.
Thanks!

[Willabe]

In fact I get hum when the power is off with tubes in.

                     

Through the speaker??? When the power is off?

                                           



I'm gonna go take a walk, I think I need some freash air. Must be cabin fever.





                   Brad           


                                 


                       

[phsyconoodler]

The filter caps are holding a charge and I'm thinking it doesn't have a standby switch so it will make a noise until they are completely discharged.
  If tightening components doesn't do anything I would be investigating the power supply and filtering.

[DummyLoad]

The filter caps are holding a charge

that charge would be DC, no? how would that produce 60Hz hum? look for AC wires crossing speaker wires, etc...


unplug amp from wall outlet - does the hum go away?

[phsyconoodler]

Who said a 60hz hum? a buzz can be mistaken for hum too.

The filter caps holding a charge means there is something to produce a noise,not necessarily the CAUSE of the noise.A sprung input jack will sometimes make noises until the caps drain off.
 

[DummyLoad]

I get hum with good tubes in, powered on, in standby.

 

[Willabe]

I'm still out on my walk.       


           Brad         

[LooseChange]

Im not sure if you guys are just having fun or mocking me. 
I went back and listened again. I have better info for you...

I kid you not.. the amp buzzes when off.  But I think Phycho is right. Filter caps are bleeding.

I found if I wiggle any tube I can make the hum stop. And now I can SEE what is going on.. The heaters light up when the hum stops OR it hums when the heaters are not lit. Got to figure out why it's doing that!
The heaters are grounded on one side on every tube socket.

Anyone take a look at the bias circuit on this amp? The power tubes are cathode driven.

Willabe, come inside and play.

[phsyconoodler]

I don't see any standby switch in the schematic.A power transformer can 'vibrate' and it vibrates the speaker making it hum or buzz.

Big honking PT's like the musicman has can vibrate like a bastard.If all else fails hit the side of it with a ballpeen hammer. Not kidding.

[Willabe]

Willabe, come inside and play.

No, you guys are tryin to freak me out!                       (I'd put up the little guy hide'in behind the computer, but I think Doug cut the list of post-its?)


                  Brad        

[LooseChange]

There is a standby swith. Turns off the HV only.

[Willabe]

Big honking PT's like the musicman has can vibrate like a bastard.If all else fails hit the side of it with a ballpeen hammer. Not kidding.

                   Yep, you guys are tryin to make me loss my mind.         


                    Brad           
 

[phsyconoodler]

A transformer is made up of laminate layers stacked together and separated by a thin layer of an insulating material like shellac(I don't know what it is).If they get loose or excessively rusty,they can vibrate and create eddy currents that cause hums,buzzes and bad power.
  Hitting it with a ball peen hammer is not really a great idea but it has been known to stop the laminates from vibrating and making eddy currents simply by shifting the laminates.

  10 cents for the labour to hit it.100.00 to know where to hit and how hard.

[LooseChange]

Figured out the problem... All related to the heater supply. Cleaned and waxed and all is good.
While I was at it I lifted the Diodes from plate to ground (D7 and D8) and the 50pf caps from screen to ground. Any problem with leaving them out?

The next big problem was output... The 1458 was bad!

Let's talk about this cathode driven power tube stuff....

[km888]

Hi
I`m new to this forum thing, but I have a Musicman head with this problem - how did you get rid of the hum?
It seems like a really strange fault!
Thanks