IC-746 FAQ
The RF driver transistors may have failed.
The driver stage is common for 2 meters and HF. The two driver transistors (2sc1972) are
right under the top cover. You can easily measure the voltage on the basis, or feel the
temperature of the 100-ohm basis resistors.
2SC1971 transistors are much cheaper than 2SC1972 transistors (and are used in the
common pre-driver), but can only provide half the power (50 watts). If your rig is used then
it’s possible, though not likely, that a previous owner did a repair using the wrong part.
Replacing these parts are not for the faint of heart. Have the service manual on hand and be
prepared to readjust/realign everything afterwards. Don’t try to replace the transistors
without a safety resistor in the +13 volts; otherwise they will be gone before you notice.
After you properly adjust of the bias current, you can remove the safety resistor.
The RF power amplifier transistors may have failed.
In the IC-746, there are separate PA's for HF/6m and 2m. Each PA uses a matched pair of
2SC2694's. If you’ve lost power from one but not the other, then this is a strong possibility.
As with the driver transistors, have the service manual on hand and be prepared to
readjust/realign everything afterwards.
Blown Output Detector Diodes.
Blown diodes in the SWR detector can fool the power-fold-back circuitry into cutting back
power unnecessarily. The blown diodes will be shorted. This can easily happen by playing
around with an external antenna tuner in conjunction with the internal ATU.
Your ALC circuit may have failed.
A failure within the ALC circuitry can cause a drop off in power.
SWR Foldback
The IC-746 is very sensitive to any sort of SWR mismatch. Even a mild mismatch of 1.3:1
can have a significant impact. In fact, a few hams have even encountered this when feeding
amplifiers. Turning on the internal ATU is a simple fix.
Confused Meter Readings
When checking RF power output, you should always use CW. It is normal for the output
meter to read considerably lower during voice transmissions than during CW transmissions.
The average male voice has a peak to average power ratio of about 14 dB, which means that
an indicated average output of 10-20 watts is about right for 100 watts PEP.
Processed speech will have a high average value and the meter will read higher, of course. A
loud tone or whistle will read higher yet. A lot depends upon the characteristics of the meter.
Some meters will follow voice peaks better than others.
Page 68 of 113
2003-02-08
Содержание IC-746
Страница 36: ...IC 746 FAQ Page 36 of 113 2003 02 08 ...