IMPROVING THE IC202 RECEIVER

IMPROVING THE IC202S 144MHz RECEIVER
Dave Robinson, WW2R, G4FRE

After discussions at Microwave Update 1994 about I.F.s for the microwave bands, It became obvious that the well circulated (in Europe) IC202 RX improvements were unknown in the USA. Two lots of receiver modifications have been published, the first by YU1PKW in DUBUS the second by G4DHF in Ham Radio Today, May 88. The following modifications are mainly from the YU1PKW, the G4DHF mods, which are much more extensive, including IF modifications involve major surgery. The "as sold" noise figure of the IC202 was between 4 and 7.5dB. There are 3 main sources of this higher than necessary noise figure:-

1. The antenna filter (L31, L32 and C148-C152) This is the LPF for the transmitter but is also in line on receive, without much benefit. Around 3dB loss has been measured in the filter. To use the filter on RX only, cut the anode lead of D26 from the junction of C148 & R101 and connect it to the junction of C151 & C152, using a short piece of wire.

2. The ferrite core of L1 is lossy at 144MHz (0.5 to 1dB) Remove the core and re-resonate L1 with a miniature 10pF trimmer connected from the hot end of C2 to the metal can of L1.

3. Whilst Q1 was state of the art at the time for a 2m receiver there have been better devices since. I replaced my Q2 (3SK40) with a BF981 dual gate FET which fits in its place and is easy to rebias (just short out R6). Dalbani (1-800-325-2264) are one stockist I have found of the BF981 in the USA for those who claim the device is an unobtainable European device.

The above component designations refer to the IC202S and are different for the IC202 and IC202E. With the above modifications my 1978 IC202 has a noise figure of 2.0dB and my 1985 IC202S is 2.3dB. If you are worried about the noise figure contribution of the pin diodes try using a small relay instead of the pin diodes. I used a 2P2T TO5 can relay, which fits easily near the output connector, using one pole to switch the RF and the other pole to provide +12V on rx though a 4k7 resistor. Alternatively the spare pole could be used to provide a more usual earth on transmit for an amplifier.

Whilst on the subject of IC202, a warning. All the IC202 family produce +9V DC on the antenna centre pin on RECEIVE, a byproduct of the pin diode biasing scheme. The FT290 produces +9V on TRANSMIT, a deliberately designed interface voltage for accessories. Therefore a transverter designed to use this voltage from an IC202 for switching IS NOT COMPATIBLE with an FT290 and vice versa. Some recent transverter articles I have seen give the impression the two are interchangeable, THEY ARE NOT. For this reason when I use the IC202 with a transverter, the switching circuit is so arranged so that it's IF c/o relay is in the TX state in all conditions other than an IC202 in the RX state being connected. A suitable circuit was published by me at Microwave Update 1992 which ensures RX mixer FET longevity.

An annoyance on the transmit side of all IC202 is that with the switch in the cw position a key has to be plugged in to generate a carrier. This is easily corrected. Remove the cover on the component side. There are two 3.5mm sockets, one for the headphone, one for the key. The key socket has one metal tab which is unconnected. Solder a wire from this pin to ground. Putting the 202 into cw position generates a carrier, plugging a key in operates the transmitter as normal.

Having improved the Rx sensitivity of the IC202, the only remaining area of improvement is in it's frequency readout/resetting accuracy. Currently G3WDG is working on bringing a design based on some ideas of G4DHF to realization which allows a digital 1kHz resolution readout to be fitted to the front panel of the radio (or in a small box on top of the radio for the squeamish)!