Fender Super Champ-XD User manual

Fender Super Champ XD >> DSP (Digital) Board Bypass Instructions
PLEASE READ EVERYTHING ENTIRELY BEFORE YOU START!
This instruction is designed to modify a Fender Super Champ XD amplifier such that the DSP
board will be eliminated from the circuit. Typically, it would apply in those cases where trouble
with the digital portion of the amp is experienced yet the remainder (analog/tube portion) is
working. But, before blaming the digital board, it is good practice to verify that the tubes are good
or there is not some other problem such as a disconnected speaker wire or blown fuse.
Although I modified a Super Champ XD, other amps (such as the Vibro Champ XD) may be good
candidates for this modification as well.
Understand that what is being accomplished here is to (1) bypass the DSP (digital) board and (2)
create a tone control (unfortunately, the factory tone adjustment was handled in the DSP board).
To accomplish this, R84 (“GAIN”) and R85 (“VOLUME 1”) are re-purposed.
See Appendix 1 - Schematic.
When completed, the procedure described below will allow you to adjust VOLUME and
BRIGHTNESS (TONE) –nothing else! It assumes that you want to retain the “tube amp” sound
and that any added effects will be inserted between the instrument output and the amp input.
See Appendix 2 –Frequency Response Data to see the effect of the Brightness Control.
I accept no responsibility in the outcome of your project (since you are the one doing it), but I am
confident that if you follow these directions studiously, you will end up with a reliable great Fender
tube amp!
If you should have questions, comments, or you have adapted this procedure to another amp,
please email me at: [e-mail]
This procedure expects the technician to have good workmanship/soldering skills and the
necessary tools to work on printed-circuit assemblies and leaded parts, including
low/medium power soldering iron and heat gun. Additionally, a Dremel type cutting tool or
X-Acto type knife is required. An ohm-meter/multimeter is desirable.
Please be safe and wear protective eyewear!
PARTS LIST:
1 ea. 10,000 ohm, ⅛, ¼, or ½-watt resistor
1 ea. .03 micro-farad (µF) capacitor, 100 VDC or more –can be fabricated and material is not
important. Below are examples of two combinations that may be used:
A) Use a .01 µF and a .02 µF (100V or more each), see Figure 10a
B) Use three .01 µF (100V or more each), see Figure 10b
C) Any other combination that is equivalent to .03 µF total at 100V or more
1 ea. 4.7 or 5.0 µF capacitor, 100 VDC or more
1 ft. 24 or 26 gauge hookup wire (color not important; I used blue)
1 ft. Heat shrink tubing (HST), size 3/32” or 1/8”
3 tie wraps or some lacing twine
Label maker (if you want to re-label front-panel controls)
STEP-BY-STEP INSTRUCTIONS:
NOTE –where the cutting of PCB traces is indicated, use a Dremel type tool or an appropriate X-
Acto type knife and blade.

Unplug AC Power and Speaker plugs from chassis.
Remove the four screws securing chassis to cabinet and remove chassis. Do not remove any
printed circuit boards from the chassis! Be careful of the tubes –you may wish to remove them.
Orient bottom of chassis as shown in Figure 1a.
Unplug the ribbon cable from P5 and the DSP board and set aside (it is no longer needed).
Figure 1a –Orientation of Chassis
The “Breakaway” board with all controls is hidden by chassis lip above but shown in here in
Figure 1b.
Figure 1b –Orientation of Breakaway Board
On the Breakaway board, there are 3 thru-hole points (A, B, C, in photo below) of contact for R84
(GAIN) and R85 (VOLUME 1). The blue wires and jumpers come later so ignore them for now.
After making the following cuts in the traces, the trace should be checked with an ohm-meter to
verify it is indeed “open”; if the meter indicates zero ohms, the trace hasn’t been completely cut.
Figure 2 –R85
FOR R85 ONLY (the left-most control next to
INPUT jack when oriented as in Figure 1): cut
PCB trace where circled, between R85
terminal A and the via (thru-hole in PCB) as
shown in Figure 2.

Figure 3 –R84
Figure 4
On ribbon cable PW3, the red-striped side is conductor #1 for our purposes.
Figure 5
A B C
PW3
1
8
FOR R84 ONLY: 1
A. Cut PCB trace where circled, between R84 terminal A and
the via (thru-hole in PCB) as shown in Figure 3.
B. Cut PCB traces (2 places –where circled)
above and next to terminal C, thus isolating
this terminal.
Separate the cut conductors from the rest of the ribbon and
install heat-shrink-tubing (HST) on each lead as shown in
Figure 5.
Near the middle of the ribbon cable (see Figure 5), cut conductors
#1, #2, #3, and #8 (#8 being the farthest from the red striped side).

Figure 6a Figure 6b
.
Figure 7 Figure 8
Connect a bare wire from P5 pin
11 (L-out-/R-out) to P5 pin 5
(AGND) as shown in Figure 6a.
Trim one end of the 4.7 (or 5) µF cap to ½-inch. Tin P5 pin 13 and the trimmed lead
on the cap. Slide a ¼” piece of HST over the lead. Solder the lead to P5 pin 13. Slide
the HST down over the P5 pin and shrink it there. Finished step is shown in Figure 6b.
Cut the free end of the cap and one end of the 10K resistor to ¼”. Tin each of
these leads, lay them against each other lengthways and solder them together.
Slide a 5/8” piece of HST over the resistor and the connection all the way to the
cap body and shrink it there. See Figure 7 circled area for the finished step.
Slide a ¼-inch piece of HST over the bare wire from the 10K resistor.
Solder the wire to P5, Pin 1, as shown in Figure 8, keeping the HST clear of
the heated area of the wire until cool. Then, slide the HST over the P5 pin
and shrink it there. The finished step is shown in Figure 8.

WIRING TO R85
Figure 9
CREATE A .03 µF CAPACITOR (two different combinations shown)
A) Using a .01 µF and .02 µF to create the .03uF needed, solder the caps in parallel like this:
Figure 10a –Parallel .01 µF and .02 µF Capacitors
B) Using three .01 µF caps to create the .03uF needed, twist the leads together like this and
solder them:
Figure 10b –Parallel .01 µF Capacitors
AB C
R85
1. Cut a wire about 4” long. Strip 1/16” from each end. Connect one
end to terminal C of R85 and the other end to the left (lower) end of
R52.
2. Cut a wire to about 4”. Strip
approximately 1/8” from one end of
the wire and solder it to the right
(upper) end of R63.
3. Strip approximately 1/4” from the
other end of the wire & bend it at
about 90º and tin it. Now solder it to
terminals A and B of R85 as shown.

WIRING TO R84
Figure11
Figure 12
A B C
A
B
C
1. Cut a wire about 4” long. Strip about 1/8” from one end and ¼” from
the other. Bend the ¼” stripped portion to 90º in relation to the insulated
part. Tin both ends and the three terminals of R84.
4a. Slide piece of HST over the entirety
of one lead of the .03uF capacitor pair
created in a previous step to cover all but
about 1/6th inch at the end.
R84
R84
2. Solder the 1/8” stripped end to the
bare wire between the 10K resistor
and P5, pin 1, close to the resistor as
shown in Figure 11 (refer to Figure 7
also).
3. Solder the ¼” tinned end
to terminals A and B of R84
as shown.
4b. Solder this end to terminal C of R84.

Figure 13
Figure 14 (a and b)
Assemble the chassis into the cabinet in reverse order of disassembly. When re-assembling
chassis into cabinet, make sure all components and wires are below the chassis edges so they
aren’t able to contact the cabinet. Remember, the cabinet is lined with foil in the chassis area!
Here is how I re-labeled
the two affected controls
on the front panel:
5. Trim the other end of the capacitor pair
to reach to the midway point on the bare
jumper between J5 pin 5, and J5 pin 11.
Slide piece of HST over this lead to cover
all but 1/16th inch at the end. Form a
hook in the end of this lead and clamp it
around the jumper and solder. See
circled area in Figure 13. Shrink all HST
in place. NOTE: If your .03 µF cap
doesn’t reach, you will need to add
enough wire so it may be connected as
shown.
To stabilize the flying components and wires, tie them off with
lacing twine or cable ties at the points shown in Figure 14.

Appendix 1 –Schematic showing changes
● RED INDICATES CHANGED OR NEW CIRCUITRY
● “X” INDICATES CUT TRACE OR WIRE

FREQUENCY RESPONSE DATA
(Fender Super Champ XD with DSP bypass mod, measured at speaker terminals)
FLAT
(Dial=1)
BRIGHT
(Dial=10)
dBm (8Ω)
dBm (8Ω)
FREQUENCY
3.98
3.06
50
7.50
5.92
70
10.00
7.79
100
9.78
7.79
500
9.89
9.89
750
10.00
10.00
1000
10.42
13.52
2000
10.00
14.86
3000
9.08
16.55
4000
8.59
17.51
5000
8.06
18.38
6000
7.50
18.38
7000
6.26
18.38
8000
5.19
17.96
9000
4.40
17.51
10000
Appendix 2 –Frequency Response Data
0.00
5.00
10.00
15.00
20.00
50 70
100 500 750
1000 2000 3000 4000 5000 6000 7000 8000 9000
10000
BRIGHT
FLAT
▲
dBm
Frequency (Hz)
3 dB area
With FLAT/BRIGHT dial
at "1", the output at the
speaker terminals varies
less than 3dB from 70
Hz to 7kHz.