YAESU * RX PROBLEM/TRACE/CURE
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Several Yaesu receivers have serious problems in that the design is not properly done. One of the most interesting - but rather difficult to solve - problems is carrier leakage into the receiver. It means that either the USB/LSB carrier oscillator signal can be measured at too high a level in some part of the receiver where it inhibits
normal receiver operation. The worst cases I have seen is in the FT-221 and FT-101/B. The problem exists also in the FT-200/250 (but in this transceiver I believe it spread out of the product detector, and thus it is easier to cure).
It is also a problem in the Drake R-4C, see Ham Radio nr 12/77, but even in the R-4C the AGC performs quite well.
In the FT-101 and FT-221 it seems impossible to improve the AGC.

I have traced the problem in the FT-221 as follows:

1) Radiation due to improper decoupling of circuits, biassing other stages with high RF voltages.
2) Improper PCB lay-out, wires above the PCB(PB1460) pick up RF fields.
3) RF radiation directly from carrier osc circuitry.

To measure the problem, you need a selective voltmeter for 10.7MHz or any other IF used. I used a selective voltmeter, Siemens Pegelmesser 3D344, 0.2-1.6MHz, with a down converter from 10.7MHz to 500 kHz. Sensitivity down to 1uV -120dB. 1.5k input. Converter is fairly simple with 3x BC547 and 11.2MHz xtal. Input impedance 1500 ohms, with series capacitor. To solve the problems as noted in para. 1 and 2, use an RFC in series, mounted on the PCB if possible, and sometimes also on the connector. The last problem is solved with better screening. In the FT-221 I measured with a 3x3cm square loop probe, a field strength of
-90dB at 1 cm above the components in carrier oscillator. On some PCB pins I measured -40...-50dB leakage. These pins were:

1) Keyline, 2) Audio wiring, 3) DC wire

Signals spreading out on open DC wiring is very bad.

Using the loop below the PCB sockets, I measured IF from the marker signal at -90dB at one point, this means that even the IF from the receiver spreads out through the wiring!

So it seems that rather a lot of work must be done to bring the Yaesu FT-221 or others up to an acceptable standard, such that AGC can later also be improved.

So far I have not found the above mentioned problems in the FT-7 and FT-902. In these two transceivers, both product detector and balanced modulator are mounted on the same PCB as the carrier oscillator. I believe that this is the only correct design.

73 Jan-martin LA8AK


PORTED AND REALIGNED FROM PC TO AMIGA FORMAT BY BOB G0LBQ - N6XDI.