Yaesu FRG-9600 VHF/UHF radio

~~~ DISCRIMINATOR MODIFICATION ~~~

By Donald Gray G3YPL/ Ex ZL1AZC

© Copyright Donald Gray 1998

Disclaimer: The following is given in good faith. I cannot be held responsible for any omissions or errors, or to any damage caused to any radio howsoever caused....All care but no responsibility!!!!

Tools required:

• cross head screwdriver
• 25 watt, fine tip soldering iron
• tweezers or needle tip pliers
• side cutters
• needle files
• sharp knife (to cut one pcb track)
• a small container to hold screws & washers.*

* The last item is very important. There are many screws to remove. It is all too easy to loose one. Make a habit of ALWAYS putting the removed screws & washers into a small container such as a 35mm film container, cup, coffee jar lid etc. NEVER ever leave them loose on the table or bench.

Components required:

• 1 x .01 capacitor
• 1 x small cable tie
• Approx 12 to 18 inches of smallish diameter Audio cable (coaxial screened essential)
• 1 cup of coffee

To gain access to the NFM discriminator on this radio is easy BUT only do it if you feel capable of doing one very fine solder connection....

Basically what you have to do is connect a short length of audio coax, via a small capacitor, from pin 14 of the mixer/nfm discriminator chip (MC3357P). to an unused socket on the back of the radio. There are several sockets on the back and I have used the "MPX" socket on my radio because I will never want to put a stereo multiplexer on it! The instructions below relate to the MPX socket. You can choose any of the sockets that you will not need but you must cut the tracks leading to that socket before adding soldering the coax thereto!

Step-by-step Instructions

1. Disconnect power and other cables at the back.
2. Remove top and bottom covers.
3. From the top, locate the N.F.M. board (4.25 inches [115mm] from the back panel)
4. Very carefully solder one leg of the 0.1µF capacitor to the solder side of the pcb where pin 14 pokes out. Be careful to correctly identify this pin AND be careful with the soldering, it is a fine bit of soldering and is easy to short out this to the adjacent pads. CHECK and double check that you have done a good "clean" solder job here -It's vital to be very careful - I cannot stress this enough! Leave thecapacitor hanging in the air for the moment...
5. Use a small needle file to drill a small hole in the top portion of the nfm pcb where there are no tracks. through this hole, thread a small cable tie and leave in situ for the time being. (Later, you will secure the audio coax here. See step 9 below.)
6. With a sharp knife, scrape a section of the solder resist from the top, right hand "Earth" track (as seen from the solder side of the pcb) and tin it with solder (To confirm that you have the correct track, it is the track also connected to the can of the xtal on that board!)
7. Prepare the coax by:- (a) Stripping back the outer insulation by about one inch. (25mm), twist the shielding together and tin it for about .25 inch [6mm] and then trim it down to this length. (b) Stripping back the inner by about 1/16 inch (2mm) and tin the inner conductor.
8. Solder the screen of the coax to the earth track so that it will present the inner conductor very close to or touching the free leg of the capacitor.
9. Now clamp this coax cable into position using the cable tie (as mentioned in 5 above) to secure it.
10. Solder the inner conductor to the free end of the capacitor
11. Orientate the radio so that the front panel is facing you and looking into the radio from the top, thread the free end of the coax to the bottom, by way of gap between the main pcb and the case at the back, right hand side (near the two white connector plugs) Tuck the coax under the big choke (the thing that looks like a transformer). BE CAREFUL not to put any strain on the coax. Leave a little slack ...
12. now turn the radio upside down and orientate it so that the dial is facing your right. The coax should be poking up at the left hand side nearest you! To use the "MPX" jack socket as the access point:-
13. Identify the 3 mounting pins of the MPX jack. They are 2 5/8 inch [65mm] from the edge facing you, of the main pcb . The pin on the left is the earth (ground) pin. The centre if the "live" pin and the right pin is not used electronically (only as a mechanical mounting point and is ignored in this mod)
14. Carefully cut the track leading to the centre "live" pin about half way along its length (This removes the unused "MPX" signal to the jack socket.
15. Prepare the audio coax appropriately and solder the screen to the earth pin and the inner core to the middle pin. Replace the top and bottom covers, making sure that there is no strain on the coax and that it is not being "pinched" by the covers.

IMPORTANT - Add a "MOD NOTE to the inside of the radio and also in the instruction book:-

• a small self adhesive label stuck to the metal screen on the underside of the pcb with words to the effect : "21 Jan 98 - track cut to remove mpx and coax added to give access to the NFM discriminator output"
• Similar label on the back denoting the MPX now to be "NFM Discriminator"
• If you have a circuit diagram, note the change also thereon!!!

The discriminator output is at a fixed level (about .7 volt) and totally isolated from the volume control - the setting of the volume control will not effect the discriminator level. The squelch control WILL cut in and out on the discriminator line in the same fashion as on the normal audio.

BTW: e-hum, why do you want a discriminator output on the radio anyway?:>)

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1. Open the lid on the metal box. Inside there's the circuit in the following figure.
   
   
2. Tune the FRG on 460 Mhz.
   On the PLL unit, below the IC MC 145158, you can see the pin marked TP02. With a digital tester measure the voltage on this TP02, it should be around 1,1-1,5 V. The PLL unit is, among the two high printed circuits, the one closer to our metal box, next to it there's written 'PLL UNIT'.
   
   
   
   
3. On the figure, next to the red arrow, there's a horizontal copper strip, with another vertical copper strip strip soldered at its end. These two strips make an angle of 90°.
   
   With a very fine tipped soldering iron melt the soldering which connects these two strips, and bend the vertical strip a bit towards the beginning of the horizontal strip, to shorten a bit this loop.
   
   A fraction of a millimeter (about 0,5 mm) should be enough. Measure again the voltage on TP02, receiver tuned on 460 Mhz. The voltage should be now around 0,6 V. If not, you can still make small adjustments to the copper link. Now, with the FRG tuned on 950 Mhz, you should have a voltage on TP02 of about 30,5 V.
   
   
4. Now, let's improve the reception sensibility in this high band.
   Connect the digital tester on pin nr. 12 of the IC MC3357, the FM discriminator.
   The voltage should vary from about 0,6 V with no signal received, to about 1,2 V with maximum signal, with the squelch unblocked.
   Tune the receiver to a frequency around 460 mhz, which has to be busy with some signal. (A repeater output would do fine)
   
   Next to the green arrow in above figure there are some pairs of copper strips, which make some resonating links. With a little NON INDUCTIVE screwdriver adjust these strips a little closer or farther from each other, until you read the maximum value on the tester, always whlie receiving the same signal.
   
   Repeat this step also on higher frequencies, the corrections on the copper links must be very fine.