Quote:
Originally Posted by clifford wright
Good Day all!
I have recently got one of these sets and am rapidly upgrading the original
design.
So far it has a Kokusai Mech filter, manual and AGC Rf gain control on the
RF stage. It has also had improvements to its AGC system and I am about to
update the RF stage FET.
One problem has arisen and someone might be able to help me here.
The Kokusai filter is NOT actually on 455 kHz, it's actual centre frequency
is 454.2 kHz (measured against a calibrated frequency counter), Since the
self excited BFO is NOT drift free I want to replace it with USB/LSB Xtals.
Can you still get the old time FT241 types, or does anyone know a source of
odd frequencies in the 455/465 kHz IF range?
I calculate that the the old FT241 channels I would need would be 328 and
326. That would give me BFO frequencies of 455.55 (should be 455.7) and
452.7 kHz that fits in with the odd filter.
That way one of the main sources of drift should be almost gone.
I have found that the FRG-7 has its problems, but, as someone who has
designed and built many communication receivers over the years IMO it is
the basis for quite a useful general coverage receiver and a VERY useful
bit of test gear with noise bridges etc.
Any suggestions gratefully received.
73's Cliff Wright ZL1BDA ex G3NIA New Zealand.
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Gidday!!
Try messing with ceramic resonators instead of quartz.. These can be pulled very easily. My FRG7 has a few Cmos gates as the oscillator feeding the existing emitter follower into the product detector, works very well. It is not unusual for these older IF filters to have odd cFs. A few other mods help enormously with front end performance with these radios.... a more "normal" balanced mixer in the front end, and another at the second mixer. Doing this helps the cross mod, reduces noise quite dramatically. Also AGC the first IF amp, gain here is fixed, and much too high, also remove the source bypass cap. Increase the 2nd IF gain by removing the damping Rs across IF primaries, and short/remove the unbypassed emitter resistors. The transformers will now peak quite sharply, gain will increase. A voltage doubler AM detector too helps weaker signals in this mode. If running on mains, the regulator can be fiddled to supply a higher voltage to the signal circuits... helps dynamic range..... There are also mods to the Wadley loop to increase oscillator injection purity. This was implimented not very successfully in the FRG7000, the next and last model to use this system, but if you check out the 7000 schematic, you will see the differences. These sets perform bloody great!!