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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
GU78B tube.
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 12.4, "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-3 is so efficient, that the tube is
fully protected even in such a situation and test bench DX-3'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-
3, 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.
In such a case, it is possible that the high voltage is still shown on the meter, but it does not reach the tube. In
any case, this is a major fault. Put the amplifier on STBY and the flashing will stop. Then switch the power off
and contact your supplier.
12.7
Sparks / Discharges in RF area
Very high RF voltages are present in the amplifier during the operation at high power. Incursion of dust or even
suicidal insects can cause such temporary problems.
12.8
SWR cut-off
The factory adjustment for SWR protection is about 2.6: 1. This adjustment can be changed, but the best is to
operate with a properly matched antenna. Should your DX-3 'see' bad SWR, it will shut down for 3 seconds (in
bypass mode).
12.9
Frequent over-drive protection cut-off
As previously described in the tuning and operation sections, there is a protection system acting on DX-3 input
overdrive. When the drive reaches the limit where signal distortion begins to occur, the overdrive indicator on
the front panel will warn you to reduce the drive. If you increase the drive even further, a cut-off protection is
activated.
After about 2 seconds of amplifier bypass, the operation is resumed, but if you have not reduced the drive, the
protection will be activated again. To stop it, you must reduce the input power drive. This protection is not
adjustable - you must not exceed the limit. If this happens too often, you obviously do not have a fine control
over the transmit level of your transceiver. In this case, try using the ALC, See section 10.2.
12.10
Tube replacement
It is very unlikely that you will ever need to replace the GU78B (4CX3000A). If there is a need for that, the tube
is easy to replace. However, contact your supplier, because a bias adjustment is required for the new tube.
13
WARRANTY / SERVICE
The full FOUR YEARS warranty for DX-3 amplifiers covers all parts and labour, except for the tube. The tube
is under a limited warranty, also for FOUR YEARS.
