Crate B80XL - Hums - Need Tips or Schematic

[xtrmdrm]

This is my first post on this forum so please be gentle 

I was given a Crate B80XL bass amp (80W/1-15 combo).  It works quite well but it hums whether my passive Peavey Grind NTB-5 is plugged in or not.  The guy that gave it to me said it 'always' hummed.

I haven't gone beyond preliminary troubleshooting but so far (I think) I have determined the hum is coming from the power amp section as the hum doesn't get louder or softer or change character when I move any one of the controls.  I hope I am correct in my presumption(s) so far.  Does anyone here know where I can get a schematic for this amp?  Or does anybody have any experience with this amp (or its relatives) in curing the hum problem?

Otherwise, when I finally get to open it up (not soon - my work area is being painted), I will:

1) Clean every control, switch, jack, etc. I can access.
2) Check all of the grounds to make sure there are no cold solder joints
3) Test all of the power supply caps, diodes, etc....

Any other thoughts would be helpful!

AAC   

 

[Jack1962]

A lttle more info is needed bro, is this a solid state amp or tube , does it hum with the cord not pluged in , is it a loud hum or does the hum change pitch, is your wall outlet grounded? some amps have a slight hum to them this is called the amps breath.

                                           Rock On

[Enzo]

It is your basic Crate solid state Bass combo.

Isolate the problem - preamp or power amp.  Plug something into the Line In jack.  You have a pair of jacks Line In and Line Out, your effects loop basically.  They are next to the headphones jack.  The Line In is the power amp input.  Plug the guitar into it and turn it to zero volume. Still hum?  Or did the hum diminish or leave?

Likewise send the signal from the Line Out jack to the input of another amp for a listen.  Is there the hum on that signal?

The power amp is all transistor, no op amps, so the whole thing runs on the +/-40v rails alone.  There is also a +/-15v set of rails that serve only the preamp.

If the hum is in the power amp, then check the two 40v rails.  Are they both up to about the same voltage - like within a volt of each other?  And is the ripple level about the same in each?  Best is to scope them for ripple, but lacking a scope, set your meter to AC volts and measure the 40VDC rails that way - the meter will ignore the DC and just measure ripple.  Well unless it is a $5 meter.

The low voltage rails for the preamp - the +/-15VDC - derive from the 40v rails through dropping resistors and zeners.  They are actually 16v zeners, but check right on one of the op amps in the preamp.  Are both 15v rails up to the same voltage and free of ripple?  If one is several volts off, that is a problem.

Does the heatsink get pretty warm just idling?  If the amp is baised wrong - on the too hot side - it can hum.

Scope the output for RF oscillation.  That will usually make it run hot, but it will also add hum, and usually distortion as well.

If turning the controls on the amp all to zero doesn't kill the hum, then I'd agree it is in the power amp.  Check the solder on the main filter caps - reflow the solder anyway - and make sure the screws are tight.  Note the mounting screws for the circuit board.  At least the one in the rear corner behind the input jacks has a ring terminal for the screw to go through.  And I think maybe also the one behind the power switch.  These ground the circuit to the chassis.  A loose screw there could hum you.

A thought: C29 is immediately behind the ribbon cable to the graphic controls.  Check for rippple on it - there should be none at all there.  To the left or it are a resistor and then a diode.  The end of the resistor towards the front is connected directly to the + on That cap, a handy place for a reading.  For that matter, right behind the High Boost button is one bare wire jumper aiming front/rear.  That is the ground for that cap.  It would be a better spot for the meter ground for this than the chassis.

And are the output transistors Moto or Texas Inst?  It matters to the bias.

[xtrmdrm]

Solid state.  It hums whether my bass plugged in or not.  The hum doesn't get louder or softer or change character when I move any one of the controls.  Outlets are grounded.  This is more than a slight hum.  My Crate BX-15 has a slight breath, the B80XL has severe respiratory illness...    

A lttle more info is needed bro, is this a solid state amp or tube , does it hum with the cord not pluged in , is it a loud hum or does the hum change pitch, is your wall outlet grounded? some amps have a slight hum to them this is called the amps breath.

                                           Rock On

[xtrmdrm]

Enzo,

Thanks for the 411.  I don't have a scope but I have a fairly good VOM and DVM to start checking voltages.  I won't get to it for a few days but I will post what I find....

AAC