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).
References:
- Cap Set From Tubes & More (Sprague Atoms) This supplier is far cheaper than Vintage Fender Repair.
- Schematics for Model AB763 (PDF)
- Layout for Model AB763 (PDF)
- Seco Model 78 Tube Tester Manual (Cape Old Radio Web site)
- Tube Tester Instruction book (PDF)
- How Tube Amplifiers Work (an article on PremierGuartar.com ) (PDF)
- YouTube video repair Fender deluxe (fixing repair)(link)
- Great video on how to make a transformer (youtube link)
- Vintage Fender Amp Repair [web site] Offer advice along with parts (re-cap kits, etc)