• Check diodes. A blown rectifier diode will cause low voltage at the STANDBY switch
(should be 450-465). Test each diode in circuit (power off) with an ohm meter. Resistance
reading in one direction should be much greater than reading with meter leads reversed.
Use heat sinks whenever soldering is done to diodes. You may remove the 3 Phillips
screws holding down power supply board to gain better access.
• Output transformer. If all voltages are correct and good clean drive appears on the 6L6
grids, (pins 5) but not on plates (pins 3) or on plates but not at the speaker output, then the
output transformer is suspect. To verify, use areplacement, available from us.
NOISE PROBLEMS: Buzzing or loud hum with no signal plugged in
• Unplug instrument and see if buzz goes away.
• Try GRND switch, center position is off, left or right switches cap in between chassis
ground and A.C. supply.
• Make sure reverb cables are securely plugged in.
• Make sure tube shields are in place, particularly over the metal Fetron and that its case
is grounded by stretching out the spring inside the shield.
•Check pre-amp tubes, loud hum most often caused by a shorted pre-amp tube.
• Check the 100 ohm 12 watt resistors (2) located on the output circuit board nearly
beneath the speaker jacks. These resistors connect either side of the 6.3 volt A.C. filament
line to ground and can be blown out by a defective 6L6.
• Visually follow the filament string beginning with the twisted green leads at the power
transformer and look for shorts or bridges to any other parts.
• With the MASTER up and its input lead removed from the right end lug, you can quickly
ascertain board where buzz is originating. You have removed the pre-amp signal so if
buzz continues, it's on output board.
SPORADIC CRACKLING OR RUSHING SOUND
This is nearly always a tube noise (often the 12AT7) and will be the worst while warming
up. Less frequently it is a resistor in the pre-amp: the higher the resistance the more likely
it is to be noisy. Check the 100K plate resistors first by simple replacements. If noisy only
when TREBLE is up, replace 250 pf disc cap leading to TREBLE control.
REVERB PROBLEMS (See addendum for modifications in Reverb circuit)
• Make sure springs are unlocked.
• Check tubes and cables. output,
• Tap on the spring unit, hollow echo sound indicates spring output cable and reverb -
return circuitry are functioning.
• Remove spring unit and check for a lead broken off one of the jacks inside.
• A bad reverb driver transformer can be disconnected or replaced by small 4 Ohm output
transformer with single ended primary for temporary service.
• Arcing at the 12AT7 (reverb driver) socket may be remedied by squirting control cleaner
on the socket.
• Excess reverb feedback may be reduced by loosening (or sometimes tightening) the 4
screws which mount the reverb pan to the back panel.
• Certain speakers cause premature feedback by interacting magnetically with the reverb.
• Reduced MASTER setting results in less feedback with more reverb.
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