Channel 2 DEAD With Wrong Pot Value

Hey all,

Here I am posting my stupid mistake of swapping the BASS pot on CH2 with a 100k pot thinking that the original was a 250k based off a schematic I found on here. The original was really a 1Meg pot. Now I get a literal whisper-level volume out of the 2nd channel. Channel 1 and 3 are fine.

Would anyone have a good schematic of the Mark V 90W?
Would anyone know what I could've fried by changing the value of the pot to a 100k?
My guess is I burnt out a transistor. The transistor closest to the bass pot, though not directly connected to it is reading approx 90ohms across legs 1 and 3. Odd, but then again, what do I know at this point?

I appreciate any help directly related to troubleshooting which component I may have damaged.

Note that I've got a handful of switching transistors, J175's. Going to try to get a night's rest before testing again.

Thanks all,

Jason

The schematic you have is probably the only one that was released. It says the bass pots for all of the channels are 250K audio taper, which I guess is incorrect.

The manual also has the Mesa part numbers of the pots at the end, and it gives all three as 598794. This is Pot 598794 - 450/S 250K 5% Short Shaft

So, no idea what is going on.

tbonesullivan said:

The schematic you have is probably the only one that was released. It says the bass pots for all of the channels are 250K audio taper, which I guess is incorrect.

The manual also has the Mesa part numbers of the pots at the end, and it gives all three as 598794. This is Pot 598794 - 450/S 250K 5% Short Shaft

So, no idea what is going on.

Click to expand...

My mistake. You're right. I mistyped the part number and it returned the 1meg pot of the same number on the Mesa site. There are different pots used, *all 250k, however the differences are the tolerances (5%/10%) and not the resistance. In this case it's a 250k log with a 5% tolerance.

Which further confuses me as to why the channel failed from swapping it out with the other pot value (100k).

Trouble shooting further tonight:

• Tapping on the preamp tube sockets I'm hearing a full thump.
• The FX loop delivers full signal to the power amp.
• Gain on CH2 has no affect. When full off, CH2 passes sound, but low volume
• None of the EQ affects the sound.
• Presence affects the sound when something is plugged into the FX loop.
• None of the mode switching, EQ, or power toggles has an affect on the sound (other than power causing it to get a little louder)
• Having tapped on most of the components, not including resistors and small caps has no affect to the signal, like a cold solder joint or stuck relay would.

Thoughts?

One of terminals of the treble pot were touching the gain pot! Sighhhhhhh! That is a sigh of relief!

batflash said:

One of terminals of the treble pot were touching the gain pot! Sighhhhhhh! That is a sigh of relief!

Click to expand...

Did you get this resolved? Not much room between the pots and leads. If a wire broke off or is just hanging in there but not fully connected can set you up for some grief in finding the issue. I assume you had to move the pots in the way to replace the one you took out.

bandit2013 said:

Did you get this resolved? Not much room between the pots and leads. If a wire broke off or is just hanging in there but not fully connected can set you up for some grief in finding the issue. I assume you had to move the pots in the way to replace the one you took out.

Click to expand...

Yeahhhh, it was just one of the legs of the treble pot touching the gain pot. Realized with when I was tugging on one of the wires connected to the legs. Absolutely relieved.

On another note, I swapped out the bass pot for a 100k on CH2 with interesting results. Essentially making the bass knob not a touchy, easier to dial in, and tightening up the low end a little. I like it.

Thanks for following up.