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Fender Dual Showman Diagnosis and Repair

Note: this web page is currently being created - as I tackle this project.

Fender Dual Showman silver face Model AB763 (1968)

This is a 'blog' of my efforts to repair and rejuvinate the guitar amp that I bought new in 1968 and has sat idle in my basement family room for 30 years.

Diagnosis and testing

  • Left channel works but volume is not good (maybe 1/3).
  • The right channel (with Vibrato) doesn't work at all.
  • All tubes light and there's no hum or noise.
  • All tubes test Okay and No Shorts. Tested with Seco Model 78 Tube Tester (see references)
  • All Caps are orriginal except for one electolytic (Cathode bias on pin8 12AX7 - vibrato).
  • No physical sign of any other work.

Some test voltages (yes I am very careful using current limiter and one hand in my pocket):

  • I noticed that the pilot light dims when switching off standby.
  • Transformer output is 337VAC (spec 343V) on each side of center tap. .
  • Output of Rectifier 476 VDC on standby and 341VDC switch on. (Spec 450VDC).