Fender Vibro-King-SM Service Manual

VIBRO-KING

CUSTOM SHOP SERIES

SERVICE MANUAL

Fender Musical Instruments Corp.

7975 North Hayden Road Scottsdale, AZ 85258

VIBRO-KING

(This is the model name for warranty claims)

SERVICE MANUAL

JUNE 1994 REV A

TYPE CSR 4

IMPORTANT NOTICE:

The information contained herein is CONFIDENTIAL and PROPRIETARY to Fender Musical Instruments Corp. It is
disclosed solely for use by qualified technicians for purposes of equipment maintenance and service. It is not to be
disclosed to others without the expressed permission of Fender Musical Instruments Co. All specifications subject to
change without notice.

For warranty repair service, only Fender specified part numbers are to be used. It is recommended they also be used
for post-warranty maintenance and repair.

Parts marked with an asterisk (
and SAFETY requirements. DO NOT USE A SUBSTITUTE!

A coded naming convention is used in the description of certain parts. The codes and what they mean are as follows:

CAPACITOR CODES HARDWARE CODES

CAP AE = Aluminum Electrolytic BLX = Black Oxide
CAP CA = Ceramic Axial CR = Chrome Plated
CAP CD = Ceramic Disk HWH = Hex Washer Head
CAP MPF = Metalized Polyester Film M = Machine Screw
CAP MY = Mylar NI = Nickel Plated
CAP PFF = Polyester Film/Foil OHP = Oval Head Phillips

RESISTOR CODES

RES CC = Carbon Comp SMA = Sheet Metal "A" Point
RES CF = Carbon Film SMB = Sheet Metal "B" Point
RES FP = Flame Proof SS = Stainless Steel
RES MF = Metal Film TF = Thread Forming
RES WW = Wire Wound ZI = Zinc Plated

*) indicate the required use of that specific part. This is necessary for RELIABILITY

PB = Particle Board
PHP = Pan Head Phillips
PHPS = Pan Head Phillips Sems

VIBRO-KING

THEORY OF OPERATION

J1 and J2 are summed by R11 and R12. These 10K Ω resistors are a lower than usual value for a tube
amplifier. This value was chosen for two reasons. 10K Ω is smaller than 33K Ω or 68K, Ω therefore the
amplifier will produce less noise at high volume settings. Secondly, when linking two or more amplifiers a
common trick is to plug the guitar into Input 1 and use Input 2 as the output to the next amplifier. With 68K
Ω resistors, 1/10

After the input resistors, the signal is split and fed to V1A and V2A. V1A serves as a preamp for the
Reverb Drive, or Dwell control. The Dwell control feeds V3, a small power tube which drives the Reverb.

V2A is a Cathode-Follower circuit which creates a low impedance, isolated (from the input) signal to drive
the Mix control (R2).

V2B amplifies the Reverb return (sense) signal and sends it to the Tone control (R3). High frequencies are
sent (shunted) to ground via C17. Varying R3 will move the wiper closer to or further from C17, thus
making the Reverb brighter (Clockwise) or duller (Counterclockwise). The Reverb signal then sums with
the Dry signal at the Mix control (R2).

After the Mix control, the signal travels to the Send jack (J3) of the Effects Loop. The signal is then
Normaled to the Return jack (J4) and drives V4A. This configuration allows the Reverb functions of the
Vibro King to be sent out to other amplifiers (via the Send jack). It also enables a player to insert a delay
unit between the reverb and the amplifier for a Rock-A-Billy type effect. A Volume pedal can also be
inserted to control the Guitar and the Reverb.

th

of the signal is lost with each link.

The V4A circuit contains the Fat switch (S1). The Fat switch connects C4 to ground to provide a 3dB
increase in gain, mostly in the low and mid frequencies. R17 is there to prevent “popping” of the Fat switch
when actuated. NOTE: the Fat switch may pop the first time it is switched. This is due to the warm­up/stabilization cycle of the tube.

The Volume control R4A is linked to R4B. R4B reduces the gain of the Reverb at high volume levels. This
allows adjustment of the Volume without readjusting the Reverb Mix control.

From the Volume control (R4A), the signal is fed to V4B, the Tone control driver section. The signal leaves
the tone controls through the Treble control (R5), and is fed to the Vibrato and Phase Inverter circuits.

V6B is a low frequency oscillator. The Speed control (R8) varies the oscillation frequency. V6A is the Neon
Lamp driver. The lamp flashes faster or slower based on the position of the Speed control (R8). The
brightness of the neon bulb is sensed by two LDRs (Light Dependent Resistors) in parallel. The resistors
connect to the Intensity control (R9) which varies the amount of attenuation. Because the Vibrato circuit
connects to the amplifier directly after the Treble control, the high frequencies will attenuate before the
bass frequencies. This simulates a rotating speaker effect.

(TROUBLESHOOTING TIP: If the Vibrato does not function, first check the footswitch jack. The switching
part of the jack serves to short one side of R57 to ground. If R57 is not grounded, the Vibrato will not
operate.

If the Vibrato exhibits a clicking sound, the problem is caused by the position of the neon bulb in relation to
the LDR’s. Adjust the bulb by gently pulling on the leads. Test and repeat as necessary.)