The filter board uses diode switching to prevent long signal lines. The diodes on the in/outs of each filter are either forward or reversed biased via a DC voltage to activate, the half rail voltage at the cathodes reverse bias the unselected filter switching it hard off.. The DC voltages are switched via the mode switch section S3a, this section was intended to switch in the pretty useless automatic? noise limiter. Mine is construcetd on a small PCB hard wired into the holes of the removed original filter.
The LED lock light glows red, green when locked. This simple arrangement uses just 3 components. When unlocked, the current from R132 in the FRG7 flows through the RED LED to ground through the 100 ohm resistor.. This raises the voltage of the green LED cathode, switching it off. When locked, the current passes from the 10 volt rail, looses 3.9 volts through the zener diode, through the green LED and to ground through the common 100 ohm resistor, When transitioning, the small window of overlap will give an orange indication on the dual LED.
The BFO, as already explained is vital when using a narrow filter on the FRG7. This design uses a ceramic resonator taken from a cheap transistor AM radio. These are cheap and nasty components, you may need to try several to get the frequency of oscillation correct. S3c on the mode switch is modified to forward bias the diode in USB mode, this switches in the 470pF capacitor to lower the frequency. Again, diode switching is used to prevent signal radiation through long wires. The BFO output is fed into the existin emitter follower of the FRG7 after the removal of the original BFO components. Mine was constructed on a small PCB and mounted directly in place of the original.
The Synchronous detector too is very simple, and although using an"obsolete" IC, the NE567 or equivalent, these are still easily available on Aliexpress.
This amazing chip has all the functions required to really improve AM reception. The existing switch position on S3b? that selects the normal diode? AM detector is rerouted to the 567 output. The 567 455kC input is connected to the C428 on the FRG7 IF pcb.? Adjustment is critical the 2k2 potentiometer must be set to 455kC, the closer the better to ensure good weak signal locking.
Mine was also constructed on a small PCB and placed above the existing IF to keep the wires very short.
Finally, to aid in quieter reception, remove emitter bypass capacitors C419 and C423, This mod will reduce the excessive IF gain and improve the AGC response. Remove R404, this will narrow the first IF response. The Smeter will likely need a small adjustment to get back its former signal level.
And very finally, on the RF signal pcb, remove R109 from gate 2 of Q104 and return the gate2 to R133 using the same resistor, this will apply AGC control to the first IF. The second mixer is pretty nasty, this will help with overload anjd cross modulation.
These mods are quite simple, but any mistakes here will damage your radio....... take care, work neatly and methodically.
Apologies for the crappy image.... I will do better if noone can figure it out.?
MikeM
