Boogie DC-3 headscratcher.

Wakculloch · 03-12-2008, 05:22 PM

I have a mesa boogie DC-3 that is laughing behind my back and giving me the finger.
It has a horrible low frequency farty distortion on both channels, eq in or out, through the line out and headphone out. Speaker and power amp have been ruled out. Valves have been swapped about. I keep thinking i've got my head around whats going on, have a cup of tea, and realize it cant be.
It looks like severe crossover distortion.
Does anyone know the common problems with these so I can feel i have a direction to head in.

cbarrow7625 · 03-12-2008, 07:43 PM

Got a schematic?

Enzo · 03-13-2008, 09:29 AM

I wouldn't assume it was a common problem. I'd just assume it was a problem and go from there.

Start with power supply. WHat are the B+ voltages? Are they clean? What voltages are on the power tubes? Are plate and screen up to solid B+ levels? Cathodes grounded right? Proper bias on grids? Clean bias, not rippled?

Go down the row of tubes. Look at voltage on the pins - B+ on 1 and 6, is it OK? Cathodes on pins 3,8 - all have some reasonable voltage? Look at the grid pins. ANy sporting DC voltages that don't belong? Unless they are a cathode follower stage or a split load stage, grids should be about zero DC, within a volt anyway.

Is there an efect loop? Is the signal clean at the send? Or farty? And a clean signal put into the return, is it OK? Or not?

APply a signal to the input, and scope stage by stage to see where the distortion occurs.

Wakculloch · 03-14-2008, 12:06 AM

Thanks for that Enzo. Pulled me out of "that is illogical captain" thinking, (and annoyance).
Am now feeling more like an incompetent doctor with a sick child who will soon die without diagnosis of an fatal but easily curable disease.
Metered all the anode, grid and cathode voltages; when i got to the phase splitter noticed a hiss from the speakers as i checked the anodes. Tried a guitar signal and all's well.
Assumed bad solder joints on the valve base and re-soldered.
Turned amp back on and problems back.
Checked PI voltages and as soon as the meter probe touches the plate connection it comes back to life; remove probe it fades.
Get narked, come back to it later, re-check voltage, apply probe hear it "cure itself", and now it won't go wrong, but I know all this is telling me something that i'm missing.
All theories received with thanks.

03-12-2008, 05:22 PM	#1
Wakculloch Senior Member Join Date: Jan 2008 Location: Galashiels, Scotland Posts: 148	Boogie DC-3 headscratcher. I have a mesa boogie DC-3 that is laughing behind my back and giving me the finger. It has a horrible low frequency farty distortion on both channels, eq in or out, through the line out and headphone out. Speaker and power amp have been ruled out. Valves have been swapped about. I keep thinking i've got my head around whats going on, have a cup of tea, and realize it cant be. It looks like severe crossover distortion. Does anyone know the common problems with these so I can feel i have a direction to head in.

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03-12-2008, 07:43 PM	#2
cbarrow7625 Senior Member Join Date: Mar 2007 Location: Philadelphia suburbs Posts: 366	Got a schematic?

03-13-2008, 09:29 AM	#3
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,258	I wouldn't assume it was a common problem. I'd just assume it was a problem and go from there. Start with power supply. WHat are the B+ voltages? Are they clean? What voltages are on the power tubes? Are plate and screen up to solid B+ levels? Cathodes grounded right? Proper bias on grids? Clean bias, not rippled? Go down the row of tubes. Look at voltage on the pins - B+ on 1 and 6, is it OK? Cathodes on pins 3,8 - all have some reasonable voltage? Look at the grid pins. ANy sporting DC voltages that don't belong? Unless they are a cathode follower stage or a split load stage, grids should be about zero DC, within a volt anyway. Is there an efect loop? Is the signal clean at the send? Or farty? And a clean signal put into the return, is it OK? Or not? APply a signal to the input, and scope stage by stage to see where the distortion occurs.

03-14-2008, 12:06 AM	#4
Wakculloch Senior Member Join Date: Jan 2008 Location: Galashiels, Scotland Posts: 148	Thanks for that Enzo. Pulled me out of "that is illogical captain" thinking, (and annoyance). Am now feeling more like an incompetent doctor with a sick child who will soon die without diagnosis of an fatal but easily curable disease. Metered all the anode, grid and cathode voltages; when i got to the phase splitter noticed a hiss from the speakers as i checked the anodes. Tried a guitar signal and all's well. Assumed bad solder joints on the valve base and re-soldered. Turned amp back on and problems back. Checked PI voltages and as soon as the meter probe touches the plate connection it comes back to life; remove probe it fades. Get narked, come back to it later, re-check voltage, apply probe hear it "cure itself", and now it won't go wrong, but I know all this is telling me something that i'm missing. All theories received with thanks.

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