Mesa DC3

[Valvehead]

Ive got 2 burnt resistors in a DC-3 and my schematic doesnt show any of the power amp section (of course) ....... anyone familiar with these? Or have a complete scem ? Hard to tell where they are connected to without getting the main board out .

Pcb is not labeled (of course its not ) , so i took a pic . These 2 are connected to a group of 6 resistors , 0.68 ohms it looks like .


EDIT--- i found the scem. I dont see any of the network of 6 resistors,,,,except maybe in the power supply ?? Ill try to attach the scem, but it shows a bunch of 470 ohm in series....???? maybe " A" from the OT ?

[The Dude]

Does this help?

mesa-boogie-dual-caliber-dc-3-schematic.pdf

[Valvehead]

scem....

[The Dude]

Are they maybe the 220K balancing resistors parallel with the filter caps? Check continuity to the filter caps to see if they are connected as the schematic shows. It might save you from removing the board.

[Valvehead]

Are they maybe the 220K balancing resistors parallel with the filter caps?

thats got to be them... but why do they use 6 , 1/2 w series resistors ? heat dissipation ? and what would melt those 220k ? bad filter cap?

[The Dude]

" ......why do they use 6 , 1/2 w series resistors ?......"

Because it's a Mesa.

"......what would melt those 220k ?......"

Bad caps, shorted rectifier diodes, Resistor wattage under spec'd, etc.

What are the symptoms of the amp? What do the resistors measure? If they test near proper value, they might simply be not large enough.

[Valvehead]

" ......why do they use 6 , 1/2 w series resistors ?"......

Because it's a Mesa.

"......what would melt those 220k ?......"

Bad caps, shorted rectifier diodes, Resistor wattage under spec'd, etc.

What are the symptoms of the amp? What do the resistors measure? If they test near proper value, they might simply be not large enough.

the owner said it "smelled hot" .....

and sounded "off" .

im not going to listen to it in this state...they measure around 630k each ...

ill see if he wants to spring for a new filter cap job ..and ill check the diodes. The amp did have a low freq hum to it before . I re-tubed it a while back but he didnt want to spend any $ on it

can i substitute a couple few larger wattage resistors to replace those 6 in series? Or should i do it the Mesa (proper? ) way ?

[The Dude]

Are those series 470's bad, too?

[Valvehead]

Are those series 470's bad, too?

you can see them starting in the bottom left in the pic...they all measure 00.7 ohms in circuit ...thats as far as i got tonight

blue / silver / silver looks to be the color code ?

[The Dude]

Resistors don't short, so there's a discrepancy somewhere. Not likely bad resistors.

Edit: The color code doesn't look to be 470 ohm (looks maybe .68), so maybe there was a version change, if I'm looking at the right resistors.

[alexradium]

that string of resistors to drop 100V is designed to fail,,if you want to keep the same effect (sag) i would substitute them with maybe 3x10k 5watts in parallel or a 25watt armored one.

[Pedro Vecino]

I think it's possible that it's a DC-3B.

[Enzo]

100v dropped across the 8 resistors. They each dissipate a little over a quarter watt each.

[Valvehead]

can someone tell me what i need here--ive got a part thats definately cracked....if you look at the scem its right next to the pilot light. Labeled on the scem = MOV 130 vac.

Thermister ?

the actual lettering on the part is Z 131 , 2 OUL , and measures 3.2 ohms ....looks like a yellow quarter .

[alexradium]

Varistor,it limits the incoming overvoltage.