Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Replacing Broken Potentiometer Shaft on the Vibro Champ Amp

You may recall from the first post about the Vibro Champ that two of the control pots have broken shafts.  They're CTS pots with plastic shafts.  One of them, unfortunately, is the intensity pot for the vibrato (tremolo).

Most Fender amps with tremolo used a 50K reverse audio pot.  On the Vibro Champ, however, a 25K pot is used - the circuit is entirely different than other Fenders.

Mouser does sell Alpha pots with that value, so I could put a new one in.  However, I have a small stash of old pots and I'm fairly confident I can just swap out the broken shaft for a good one from another pot.  Since the pot value and taper is determined by the carbon track in the pot, this should work.  That way I'll be keeping the pot mostly original.

I did a post a while back about opening up pots and 'modding' them to rotate faster to make them better suited to volume swell effects. 

This is the same process.  Here's my donor pot with a good shaft being taken apart.  First, bend back the tabs that hold the pot together.

Then disassemble the pot.  Note what goes where so you can put it back together!

You can see the grease on the bottom of the housing.  This gives the pot a nice feel with some resistance.  For the fast-spin mod, you'd want to take this out.  Since this is an amp pot, I'll leave the grease in there.

I then took the intensity pot apart.  Notice that CTS has a color code for the pot - the plastic disc and shaft are purple!

This is a good opportunity to clean the rotating contacts and the carbon track while the pot is in pieces.

Usually we spray contact cleaner into a pot through a hole to attempt clean it.  Once you've opened up a potentiometer like this, you realize that spraying into one with contact cleaner is a hit-or-miss affair.  It's a lot better to take it apart to clean it if you can.

I put the pot back together using the shaft from the donor pot, and the carbon track and housing from the original pot.

I tested the resistance and it's correct.

Here's the 'rebuilt' pot back in the amp.

The tremolo works!   And it looks original - if that's important to you.  (Oddly enough I believe I'm going to change the volume pot for an upcoming mod...go figure).

I did this for the Bass control as well.  I was just going to replace it with a new CTS 250K audio pot, but the schematic specifies a "J" (30% audio) taper.  So I just put a new shaft in that one also.  Interesting to note that the J taper control has a blue shaft and disc.


Post a Comment 0 comments:

Post a Comment