MkIII Ch Switching Issue/Question - Can't switch RHY/LEAD

[nor]

ISSUE: Won't switch from pull-knob, mesa footswitch, or rockman octapus. Normally works with all of these.

Q: Would a broken LEAD DRIVE pot cause the footswitch jack to fail as well?

TIA

 

[MrMarkIII]

Check V3. That's the Lead Channel 12AX7.

 

[nor]

Fresh bottles.

Pulling the knob disables the footswitch, right? Think I need to source a fresh pot.

 

[MrMarkIII]

The Lead Drive pull knob should override the foot switch.

With the foot switch plugged in, pulling the Lead Drive knob should turn the foot switch light OFF and engage the Lead channel.

Someone correct me if I'm wrong, but I believe the pot does not do the actual switching, but an LDR which performs this function. These do go bad occasionally.

 

[McBarry]

There's a few LDR's involved in the lead channel switching..
Not hard to check if it's the actual switch behind the pot.. otherwise it's time to get it on a bench and start chasing..

Plenty possible causes, and the LDR's can be a pain to access without pulling the board out...

Just changed a MkIII treble shift LDR 2 days ago in a long head- major PITA, but a fair alternative to pulling the whole board out..

Good luck, though it sounds like it needs some "exploratory surgery..."

 

[nor]

Ya, taking it in to the shop this week. It's due for a refurb anyhoo.

 

[McBarry]

Hey Nor,
what did it end up being - may be of help to future readers too..
Dave

 

[nor]

Just talked to the tech. A resistor decided to become an Assistor, lol. My understanding is this component is supposed to convert 30v to 3v-ish. Nice little char mark where it popped. Lowell at Hunt Amps is 90% sure this is the cause of the switching issue (and a couple others). Sounds brutal, but is an easy fix. He's gonna recap and clean.

Should be around $250 out the door. Then my Mark will be master of the universe again!
MMMMUUUUHAHAHAHAHAHAHAHAcoughHAHAHAHAHA!
:twisted:

 

[nor]

Zener diode failed, and smoked the resistor. All better now. Fresh caps, fresh bottles (kt77s on the outside).....

I'm going to have to learn how to set this up, all over again.
8)

Props to Lowell at Hunt Amps for the great work and speedy turn!

 

[nor]

OK, how did I come to the decision to stop using this amp?! My Stiletto sounds like a toy compared to my old Mark. I'm even getting great tones at bedroom levels (.0025 vol, lol). The Kt77s really took the edge off the high-end, and the rest of the cocktail from Doug's sound great! The recap did wonders for the noise levels, even my gained-up axes sound pretty **** quiet.
The real test will be band rehearsal, and our next gig in June. This amp never really played nice with others. The other guitarists would always ***** that I was burying them with level, when it was the tonal clarity and note definition that was making their fanboys sad. We will see how it plays with the Roadster on the other side of the stage.
:lol:

Lowell measured the 'actual' output in SimulClass mode - 98w! :shock: The original manual specs 75/15.

:idea: TECH TIP: Lowell noticed that I didn't have the reverb tank plugged in. He suggested either leaving it plugged in or pull v4 - not a great idea to run that part of the circuit without a load of some type, apparently.

 

[McBarry]

Ahh, the old Zener-turns-to-***** trick.. hehe...
Good it's back to former and future glory..
A quick Q - U mentioned bedroom levels...
With a gtr straight in, on clean channel, say input 5, treb 5,mid 5,bass 5, at what m/vol does it get "painful" loud from say 5 feet-ish ?
Would m/vol be around 2-3ish, or around 6ish ?
Reason I ask is I've a MkIII that seems gutless, and the only thing I can find is the m/vol pot may be incorrect - log rather than reverse log..

Cheers,Dave

 

[nor]

Anything past Master-3 in an enclosed space is crazy-stupid-wtf-lol loud.

 

[McBarry]

Cheers, thx for that, DB