GRAPHIC EQUALIZER PROBLEMS
•Are rare indeed.
• Check switch-lug action on footswitch jack inside chassis, it should provide a ground
connection to the filter circuit with nothing plugged in.
• Check for 52 volt Bias across 50 mf. cap.
• Check transistors. Q4 is NPN, others are PNP. A blown transistor will upset bias voltage
to 6L6's as the equalizer circuit uses the same supply. Use min. 50V silicon replacements.
Transistor sockets make testing and repair simple.
• A low growling oscillation occurs if Q1 has too high gain, even though another of same
type make work OK.
• In case of real difficulty with the EQ board, contact factory for replacement. The board
can be hard to fix but is easy to replace. You may remove transistors and reset bias by
adding 5K to l0K resistance across bias filter cap. With EQ switch OFF, equalizer circuit is
completely bypassed.
ADDENDUM
A metallic sounding buzz in the tone of amps having Reverb may be traced to a parasitic
oscillation in the Reverb driver stage under certain settings. To cure, perform the following
modifications:
• Install minimum 220K resistance in series with the capacitor (.005) coupling signal to the
12AT7 grid (pin 7, or pins 2 & 7 if amp also has Graphic EQ.) There may be approximately
50 K in series already, but it might not provide enough isolation from the adjacent circuitry.
(If the buzz is greatly reduced but still persists, substitute 470K for 220K above, and
reduce .005 coupling cap to .001.)
• Replace the 470K (yellow-purple-yellow) grid leak resistor (also leading to pin 7 or 2&7)
with 100K.
• The cathode circuit of the 12AT7 (pins 8 or 3&8 if EQ) should have a 470 ohm to ground
with no bypass capacitor.
6
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