Smoked a 91 mark3 bs

[MUD D. BOOTS]

First let me say I have a very basic knowledge of what goes on under the hood of a tube amp (enough not to get zapped )and I can solder. Recently my 1991 mark 3 blue stripe started acting up , noises , pops , and a fairly constant hum , suspecting a bad tube I tapped on the tubes with a chopstick , while tapping the el-34 farthest to the right I got a loud pop and one of the 6L6's flashed and smoke came out from the chassis . I immediately cut the power and opened her up to check the damage . the only obvious destruction I could find were what look like two burnt resistors . Again my knowledge is basic , am I correct in thinking that they are carbon comp resistors ? If so , what are their values , and could I replace them myself without risking life ,limb, or god forbid , AMP .. . I come seeking knowledge and guidance .

 

[FXR]

I don't recall ever being given the advice to rattle a power tube. But I know we do it to pre-amp tubes. But still, it's one of those things, yeah, hit something with a stick and you might find, pronounced create, a problem.

Anyway, assuming it was the OT that went short from the hit with the stick and that an OT can sound as you described when going south, perhaps replacing those resistors will be the fix with new OTs.

Be careful soldering. Use a good low wattage iron, use heat sinks and don't burn the traces on the PCB. Being able to solder a pot does not translate to de-soldering pcb components and replacing pcb components. Those are solder through mounted components, solder on both sides of the hole. It's quite a bit trickier then regular solder IMO.

You're gonna need the values of these resistors too. They look too burnt to read. I can't help there.

Trying to scare you? Yes, I am. Use caution. If in doubt, it's cheaper have the issue fixed then fix your mess ups and the issue if you burn the pcb.

 

[MUD D. BOOTS]

Thanks, sound advice ... regarding pcb board , will be taking it to a tech . Would still like to know the values of the resistors .

 

[>Photi G<]

Those look like the "Virtual CT Resistors" for the heater supply. It looks like the 39? (red/wht/org) one on the outside blew first, which was for the DC supply to V1 and the Rhythm 2 relay. This would have caused the loud hum you heard. Then, the 100? (brn/blk/brn) resistor on the inside blew, which was the AC supply to the rest of the tubes. This would have caused the 6L6 to flash, because the heater voltage exceeded 3.15VAC.

 

[MUD D. BOOTS]

Thanks , that analysis describes the chain of events very well ...are those "virtual ct resistors" something that a tech would normally have in stock . Do those resistors usually take other components out with them when they go or can they fail and have no other effect on the circuits components ...

 

[Steve@Russo]

screen resitors burnt out on a single I had, does boogie need to source better resistors?

 

[SonicProvocateur]

Doubtful, but though the value on the screen resistors are rated low from the factory and if they have been stressed from the blown tube you should have your tech check them; mine blew one tube and on the second it took out the resistor. There is an upgrade for that resistor, but its optional. You'll probably need to replace your fuse and carefully clean the PCB as it appears to be slightly scorched from the burnout.



edit: optional.

 

[>Photi G<]

Thanks , that analysis describes the chain of events very well ...are those "virtual ct resistors" something that a tech would normally have in stock . Do those resistors usually take other components out with them when they go or can they fail and have no other effect on the circuits components ...

Definitely, the round tan ones are regular Carbon Film resistors, and the brown cylindrical ones are Carbon Comp's. Basically, their job is to limit the voltage to the tube heaters. Now that they've blown, instead of supplying 3.15VAC/6.3VDC, they're supplying double that, which would damage the tube heaters.

screen resitors burnt out on a single I had, does boogie need to source better resistors?

Probably not. Resistors blow. Not uncommon for a 20+ year old amp like a Mark III. Mesa's parts are actually of a much better standard than most other companies, since they source USA made components. Other companies are focused on cheaper components that they can just shove in their auto-insert machines, and crank out more amps for less money. Granted, Mesa uses an auto-insert machine for uniform components such as resistors, but they don't use typical trash MIC components. All the bigger ones, such as filter caps, Orange Drops, sockets, pots, MOV's, etc. are installed by hand, so they can use anything there.

 

[JOEY B.]

The value of the resistor will be the same as the one on the far left of that group of (burned)four in the picture.

 

[Steve@Russo]

Mesa has sent a whole new board for that are, on the single recs they are on their own very small board so it looks like an easy swap