22
If it is still not working, unplug the power cord from the mains, then remove the fuses from the fuse holders on
the rear panel of DX-3SP and check them with an ohmmeter. If a fuse is blown, replace it with a fuse of the
same type and try again. If the fuse blows again, or if the fuses were not blown in the first place, contact your
supplier.
12.2
Fan not working
As soon as you switch the power on, you should notice two things about your DX-3SP: the display should
indicate about 3000V of plate voltage
and you should be able to hear the fan running. The amplifier is not
supposed to be powered without the fan, even in idling mode. If the fan does not operate when you power up, or
if it stops while using the amplifier, switch the mains power off immediately and contact your supplier.
12.3 If the FAULT light turns on
The FAULT (red) light turns on in two situations:
Over-current in the plate circuit
This could happen due to a flash-over in the tube, or as a result of a momentary high plate current during tuning.
This completely disables the amplifier for about 2 minutes. The same timer is activated as when the amplifier is
switched on, causing the READY light to turn off. If the OPR/STBY switch on the front panel is put on STBY,
the FAULT will turn off, but the READY will be also off. When the READY comes on again, the FAULT will
disappear (if the amplifier was left in OPR), and the DX-3SP is ready to operate again.
Over-temperature cut-off.
See below.
12.4
Over-temperature cut-off
The operation of the amplifier is disabled, but the timer is not activated. The READY light turns off, and the
FAULT light turns on. When the tube cools down, the FAULT turns off and the operation can resume. This type
of fault should never happen. DX-3SP has a very efficient air cooling system. When the ventilation is not
restricted, in normal ambient conditions, when properly tuned, a DX-3SP can operate at full power for extended
periods of time, with the fan not even going to high speed. The temperature cut-off happens when the tube
reaches 150º Celsius, with ample safety margin to the specified limit of 200º for the FU-728F tubes.
12.5
Over-current cut-off
If the plate current exceeds even for a brief time a pre-set safe limit, an over-current detector will activate the
power up timer, disabling the amplifier for about 2 minutes. During this time the READY light is off, and the
FAULT light is on, if the amplifier is left in OPR mode, as explained under 12.3, "If the FAULT light turns on".
12.6
Ig2 blinks fast - No plate voltage
If the high voltage is lost due to a fault in the amplifier, the screen current tends to reach very high values. This
is an unlikely situation, but if did happen, this would normally guarantee a tube failure in unprotected amplifiers
that do not have a current limiting circuit. The screen current limiting in DX-3SP is so efficient, that the tube is
fully protected even in such a situation and test bench DX-3SP's have been extensively run with no plate voltage
and maximum screen voltage, with no effect on the tube. If this loss of high voltage should happen to your
DX-3SP, the Ig2 display will have the red LEDs flashing, due to the screen current exceeding the pre-set limit,
with no input power applied to the amplifier, while switched to OPR.
