Yaesu FRG-7700, Survival Manual

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14
PAØPGA
I then build in the VFO a small circuit with a VHF varicap that was on hand (BB 209), and a small ceramic condenser
(12pf) over the existing variabele condenser. The tune voltage is HF decoupled by a 10k resistor and a 22n condenser. I
soldered everything spider-style to the tuning condenser. See picture.
The only critical part in this mod is the ceramic condenser, it must be a type with a neutral temperature coefficient (NP 0,
black dot). After testing, the VFO was retuned (with the trimmer only)! to 2500 khz with the variabele condenser fully
meshed. With the condenser fully open I had a frequency of around 3600Khz. The variabele tuning range changes
somewhat over the band: on the low side the frequency span of the fine tuning is around 12 khz, at the high side it is
around 3-4 khz, but because the fine tuning was meant for the hambands which are all on the low end, the variation is
more than enough for normal use. The stability of the VFO is just the same as before, and also the calibration of the
analog scale is not affected.
In normal use I have the fine tuning in the midposition, so I can tune either way of the station.
Should you need a smaller or greater variation, just change the ceramic condenser, I found that the variation at the low
side is around 1 khz/pf. The reason of the frequency span variation is that the new condenser at the low side has a
smaller effect, because it is in parallel with the whole main tuning condenser, and if the condenser is fully out, the added
cavaricap is doing the job.
A wire is soldered to the connection of the 10k resistor and the 22nf condenser and is brought out of the VFO, together
with the 3 existing wires. This wire has to be some 10 cm longer than the others. The variabele voltage from the fine tune
potmeter is on pin 13 of Plug 34, this plug is not connected if you don’t have a memory module, so we can solder a pin to
the end of the VFO wire, isolate everything with crimping tube, and plug this pin in position 13 of P34. For the pin I used
a old wirewrap connectorpin, it has the rigt size. The wire of pin 13 has a white/violet color. See pictures from this mod.
If you should obtain a memory module, the only thing you have to change, is pulling the wire off the plug, and connect
the plug to the memory module.
This modification is very elegant, because there is no drilling or extra knobs necessary, simply undone if neccessary, easy
to use and has no side effects in stability of the receiver.
I can recommend this mod and have now done it to all my FRG-7700’s, except one, who had the memory module
installed.
Another set of modifications are described on a very interesting Swiss internet site:
The adress is:
http://www.dr-boesch.ch/radio/yaesu-frg7700-mod1.htm
You find very much interesting stuff there about ham equipment and a lot of very good descriptions of ham equipment.
The pages are in German, so I will give a translation of the mods, for the benefit of all of you who are not fluent in that
language.
I did only the listed mods on the receiver board, there are some more on the powersupply, but i did not use them, so look
at the site eventually.
I used parts what I had on hand, special the minichokes, I listed the used ones.
After modification the receiver is to re-align according the service manual, for the best results.
The adjustments are very minor in a well-aligned set.
The original mods were described by Georg Lechner in magazine Weltweit Horen nr 11/1986
Description of the mod:
The FRG7700 is a half synthesiser, half conventional design, and is nearly identical to the Kenwood R1000. The circuit
design is ok, but can be improved. The set and memory module is very stable, but some changes improve the receiver
considerabely:
1) Put parallel to the SW socket a minichoke of 100-200 microhenry, and parallel with the BC socket a choke of 1
MilliHenry, to form a DC path to unload antenna charges.
2) Change R3 (47 ohm) near the antenna plug in 470 ohm, to make the attennuator more usable.
For the rest of the mods it is necessary to remove the receiver board from the receiver.
3) Remove R01 (4k7) and R03 (470 ohm) to lessen the coupling between the BC and SW inputs.
4) Change C07 (47nf) to 4n7, and C69 (10nf) to 2n2, for lower audio interference.
5) Change R46 (470 ohm) to 330 ohm, and put a 500microHenry - 1 MilliHenry minichoke in series, for a higher
collector current of Q1 to Q6.
6) Change R60, R61, R76 and R78 from 470 ohm to 330 ohm, for a better 1
nd
and 2
nd
mixer current.
7) Change R79 (100 ohm) in a 100-300microhenry minichoke for a better decoupling of the mixer.