74 Musicman 130 HD

[fdesalvo]

Long story short: received abused amp from customer. Spray painted black chassis. Two wood screws holding on one side of OT - customer said they are repacking NAILS that the PO used. Amp has massively buzzing PT, rattling chassis and blowing fuses. Replaced 2 diodes on rectifier board and all electrolytics. Resolved.


Next demon lies in these details I could use another pair of eyeballs on this!  Looks like only half the filament string is working at the tube sockets. Where am I going wrong?  Pardon font below. Pasted in from my notes app.


————//////


Uneven readings from heaters at the loaded tube sockets. Unsoldered filament leads:
Lead 1: 84vac to ground
Lead 2: 88vac to groundThere is continuity between both leads

V2-5: el341
1 .8mv
2  1.7 - 7mvac
3 480v
4 235.7v
5 -20.31v
6 -20.35v
7 4.41vac
8 288v

V1: PI (12ax7)
1 204v
2 10v
3 22.3v
4 31.1mvac
5 31.1mvac
6 167.2
7 0v
8 .8v
9 4.41vac

[shooter]

a schematic would be handy
Is one side grounded on your FIL, kinds indicated with mV on 4/5 n 4.4 on 9

Unsoldered filament leads:

84vac is not grounded in reality, I'd re-check that.   kinda indicates FIL and HT are intermingling, might be acceptable in the new world, but tube world still don't like it 

[fdesalvo]

Hey hey!  Long time, man. I took another set of readings.


V2-5: el34 hi
1 .4mvdc
2 13.7mvac
3 726vdc
4 365.5vdc
5 -40vdc
6 -40vdc
7 6.66vac
8 84.7mvdc

V1: PI (12ax7)
1 293vdc
2 20.6vdc
3 46.4vdc
4 39.4mvac
5 “
6 207.7vdc
7 .1mvdc
8 1.82vdc
9 6.72vac

I don’t understand what’s up with the heater circuit. The color codes on the schematic don’t exactly line up with what I’m seeing on this end. Can’t quite discern if there’s an CT on the heaters or not.  No sound whatsoever from output jack. No hum. No hiss. Nothin!

https://www.thetubestore.com/lib/thetubestore/schematics/MusicMan/Musicman-2475-130-2275-130-Schematic.pdf

[sluckey]

Nothing wrong with the heaters. Your voltage readings don't reveal any clues.

[fdesalvo]

Wondering why the majority of the voltage is on one side    if it’s not an issue then I can look elsewhere. Signal doesn’t seem to be making it through to the output. First amp I’ve worked on where measuring voltages at the sockets don’t produce an audible pop. Eerily silent even with volumes maxed.


Could the ot be suspect? I lifted the ct and checked the plate lead resistance already. Are there any other tests I could run to rule this out?


Ty

[fdesalvo]

Holy cow!  I just noticed something that’s been staring me in the face this whole time. Can’t wait to get the images up. I think this is it. 

[sluckey]

Wondering why the majority of the voltage is on one side    

Look at the schematic closely. It's very obvious.

[fdesalvo]

Will do. No more midnight sessions for me. Missed something so obvious. Owner used wood screws to mount the OT. Three of them narrowly missed hitting wires.  Two of them cut into the sides of the circuit board. One of them is touching the lower lug on the impedance selector.


https://i.imgur.com/8IVjynm.jpg


If it was a snake...hoping this explains the lack of signal at the output. Find out later. Update soon.

[fdesalvo]

The amp is back to acting like a guitar amp on the normal channel. Now I have to debug the effects channel.  Volume is almost nonexistent - about 1/10 of the normal channel.  I shorted the pot wiper lugs and there’s no difference.  It’s a dual ganged pot. Looks like it’s sending signal in to the reverb circuit right up front.  Oh, the odyssey this amp is becoming haha.

[Ritchie200]

Hey Stig!!!!!  How's it going?  Good to see you back!


  How's the car? 


Back to our regularly scheduled program....


I'm sure you did this but did you check tension and clean your tremolo/reverb footswitch jacks?


Don't be such a stranger!


Jim

[fdesalvo]

Haha you mean the minivan?  Oh yah it’s been a while!  Two kids don’t fit into an S2K or mustang lol.  I hope you’re doing well my man.  Things are getting crazy out there! 


I’ve isolated the problem down to the opamps in the second channel. Not op-ampin’! 

[Ritchie200]

Oh no!  What happened to the Honda?!?!?!


Jim

[fdesalvo]

It stayed out past midnight and turned into an odyssey. 

[Ritchie200]

Now you know, nothing good happens after midnight!  You stay out late and suddenly you have a family!!    From BF Goodrich track tires to soccer uniforms, club teams, and travel...  Haha!  welcome to life!  Your a good man!


Jim


PS, for your consideration - about $14k complete
https://www.dfkitcar.com/specs.php

[fdesalvo]

My dad was right lol! I do love these little guys, though! What an adventure. It’s like endless sleep deprivation and a disassociation with all space and time.