70s Fender Vibro Champ ... Funky Treble Knob

[Farringdon]

Hi, my Fender Silverface Vibrochamp's treble knob is only sounding normal between 0-2 and 9-10 .    Anything in between causes a huge dip in volume.  Here's a video of it

https://www.youtube.com/embed/prxPISjIOxM

Any idea how to fix it? 

Thanks in advance!

 

[TheJames]

The treble pot on a Fender tone stack is the "output" to the next stage of the amplifier.  My guess is that the pot is worn out and needs to be replaced.

Schematic

I don't know what type of shaft your treble pot has, but I would guess it's a solid-shaft 1/4" type.  It needs to be a 250k audio taper.  Remove the treble knob and see the shaft type then get a suitable replacement.

Pots here

The Alpha pot should be fine.  I typically use CTS pots in amps myself.  I've never used the PEC pots, but they're supposed to be great.  You might consider getting pots for the other controls as well to have on hand.

Volume 1M Audio taper
Bass 250k Audio taper
Treble 250k Audio taper
Speed 3M Reverse Audio
Intensity 25k Reverse Audio

 

[Farringdon]

Thanks for your advice.

This is a 1975 Vibro Champ, I assume this doesn't make a difference to the pots needed.  Will look into getting a new CTS pot as replacement.

 

[CJ]

make sure it is the pot before you replace it,  a silver face vibro can fetch $1000 bucks nowadays so you want to keep it's original form, which means don't change the pot unless you have to,

if you have already drilled holes in the chassis, cut out the baffle board for a bigger speaker and changed the grill cloth, then don't worry about it,

to see if it is the pot, i would try removing the wiper lead, connect an ohm meter to the ground leg and center terminal, turn the knob and write down the readings 1/10th  turn at a time, (use the numbers on the knob)
report back immediately.

 

[Farringdon]

I think I know where the wiper lead is and I have an ohm meter, but can you please let me know  where the ground leg and center terminal are?

Many thanks!

 

[CJ]

the wiper lead and the center lead are the same thing, me bad on using two terms for the same thing,

ground will be the lead on the left as you are looking at the pot from the back (opposite the knob side) assuming that the terminals are facing upwards,

 

[Farringdon]

Okay, did what you suggeted and here are the readings from turning the treble knob (starting with the treble knob on 1, ending with 10):

Multimeter: 2000 ohm

1: 016
2: 135
2.5:1147
2.7: 1730
2.7-10: no reading

When played with a guitar, the treble knob works from 1 to 2.5 and then it works again around 10.

 

[CJ]

you need to go up a range on the ohm meter after you reach 1730 ohms,

anything over 2000 will not register,

 

[Farringdon]

Here's a new reading:

Multimeter: 2000 ohm

1: 016
2: 135
2.5:1147
2.7: 1730
2.9: 12.2 200k
2.9-10 no reading on any multimeter ohm setting

 

[CJ]

not sure what this means>  2.9: 12.2 200k
you mean 2.9  200K?

ok at this point you probably have one  more opportunity to save the pot replacement,

get some contact cleaner or if you do not feel like spending 10 bucks, grab some iso alcohol and ppour a bunch inside the pot and spin it a few times to see if you can get it to work again.

 

[Farringdon]

Sorry, here's a clearer way of listing the readings:

Multimeter: 2000 ohm

1: 016
2: 135
2.5:1147
2.7: 1730

Multimeter: 200k ohm

2.9: 12.2
2.9-10 no reading on any multimeter ohm setting

I've sprayed the pot and given it a spin but unfortunately it still doesn't work.  Guess I need to buy a new pot....  I'm in the UK, they're a little more expesive here. 

Just to be sure, is this the right pot?

http://www.ebay.co.uk/itm/Genuine-CTS-250k-Solid-Shaft-Pot-Potentiometer-Audio-Log-/281444542057

Also saw this:

http://www.ebay.co.uk/itm/Fender-250k-CTS-Pure-Vintage-Log-Pot-Choose-Split-Shaft-Or-Solid-Shaft-/281474256929

Thanks again for your help.

 

[TheJames]

I found a few shops dealing with guitar-centric parts in the UK, but didn't seem to find exactly what I wanted.

If I had to guess, you have 24mm, 1/4" shaft, 3/8" bushing, 250k Audio taper pot for the treble pot.  You can determine shaft type by removing the knob from the pot.  Spline shaft can be used in place of solid shaft provided the replacement pot is not a "split" spline shaft.

I would think that either of the pots you listed will work.  Their might be a "best" pot for vintage nuts, but I would just find something that physically matches, and meets the electrical requirements.

 

[Whoops]

I would think that either of the pots you listed will work.  Their might be a "best" pot for vintage nuts, but I would just find something that physically matches, and meets the electrical requirements.

Buy a new CTS pot you're done, can buy them here:

http://www.banzaimusic.com/CTS/