Mesa .50 Caliber + Fried resistors

[01redcobra]

First question to all is, What is the easiest but least invasive way to pull the board from this amp. I understand that it's a nightmare.
Next would be, can anyone identify the value on the scorched resistors in these pics?

[mozwell]

one of these may help you

[01redcobra]

for some reason, these PDF's are coming up as blank pages?

[Enzo]

I can't open them to anything but blank either.

In many cases if I have to get to the other side of a board like that, I pull the nuts off the controls and whatever else is holding that side, and flip the board over with the socket wiring as a "hinge." Will that work here?

[The Dude]

Is it really necessary to pull the board? I would think you could replace the resistors from the top side.

[Enzo]

You can in many cases, but looking at photo 2 bottom center, that burnt up resistor looks to me to have destroyed its pads, leaving little to solder to, and I'd probably want to lay the leads of the replacement down along the traces to improve its dissipation.

[The Dude]

That's entirely possible Enzo (something the OP will have to assess). It'll be easier to tell once the original component is removed and everything's cleaned up. Things are often not as bad as they look once you "remove and clean". Either way, I would stand the replacement resistors away from the PC board a bit when you replace to keep the heat away.

[01redcobra]

Enzo, In the time I had, I ended up doing exactly what you suggested. I had to de-solder the leads to the power switches and the EQ board as well as the nuts on the controls and input jacks. The one pad for the big resistor may be partially usable, but the other is wasted. It appears, from what I can tell, that that resistor is feeding the LDR's for the gain and Eq switching.

[Jazz P Bass]

Mesa Boogie 50 Caliber+ Schematic

Schematic.
Note: I rewrote the file using BullZip.

[01redcobra]

Thanks Jazz! That's a big help.

[01redcobra]

So the 1500 5W resistor got taken out too. I assume the carbon resistor that burned up was a 680 2W from the schematic. Gonna assume that the 2 diodes right there have been compromised as well. Are these amps noted for this. I haven't had much experience with LDR equipped amps.

[slawkowski]

AFAIK the fix has been issued for studio .22+
caliber and studio share the same switch supply scheme
last page on pdf http://www.captainbob.de/SCHEMATICS/...io.22_Plus.pdf

[01redcobra]

Thanks for that update. I am curious to know what causes the problem in the first place. I take it that this is common among these amps.

[01redcobra]

I need to ask what would cause these resistors to burn like they did? Is it a problem with the vactrol LDR's? I saw in another post that someone was replacing the LDR's in the amp they worked on. I can't even find a number for the vactrols that I could use to order new ones. Any input here?

[Bluesandgrits]

What value is the resistor labeled R512 on this board