Service manual

Page 21
tweaked so that the loss through the IF Filter board is about the same regardless of the loss
characteristics of the various filters.
The crystal filters have various input/output impedances. The Inrad 2311 is about 400 ohms,
whereas the others are 200 ohms, so an additional matching transformer is required as well as a
different value pad.
The IF Filter board is designed to accommodate the filters (except the 2311) on Yaesu-
compatible plug-in boards. A cutout in the bottom chassis provides access to these plug-in
boards.
The Modewide control bit is set high in FM mode to de-select all filters and to turn on Q9
(Sheet 2) in order to provide a termination on the output side. Although the FM receiver follows
a separate RF path that does not involve any of the circuitry discussed so far, the IF amplifiers
do remain on, and this circuit helps keep them well-behaved during FM reception.
Receiver third IF
The filtered signal is brought back to the Receiver board via coaxial cable and immediately fed
into an NXP SA612 Gilbert Cell mixer. This active mixer has about 10-12dB of low-noise gain
and low input voltage requirements on the local oscillator (LO3). However, its input and output
impedance is 1800 ohms, so a step up transformer is required on the input and a step-down
transformer is required on the output. The third LO is set to 9.455MHz, so the primary mixing
products are at about 18.455MHz and 455KHz. The output is immediately fed into the IF Filter
board’s 455KHz crystal and/or Collins mechanical filters, which rejects the 18.455MHz sum
product and other secondary mixing products. An identical scheme is used for them as for the
previously discussed 9MHz filters so they will not be re-hashed here.
AGC
The 455KHz filtered signal is brought back to the Receiver board via coaxial cable, where the
50 ohm impedance is transformed by T1 (Receiver board, Sheet 6) to 1800 ohms to drive the
main IF amplifier (U9). This is the venerable MC1350, used in IF amplifiers in radio and TV
for decades. Although no longer made by Motorola, it is still available from NTE. This ampli-
fier has up to 50dB of power gain, and 60dB of AGC dynamic range. The AGC input on pin 5
provides dual slope AGC action that lends itself to direct observation on an S-meter, with 3.75-
4.75V representing linear steps from S0 to S9, and 4.5V-5.5V representing a steeper scale for
dB above S9.
The output of U9 is buffered by Q15 and fed to the final mixer. The emitter side of Q15 drives
the AGC circuitry. As discussed earlier, Q6 provides a variable gain amplifier that is used to set
the overall loop gain during receiver calibration. The output of Q6 drives a 10dB voltage gain
amplifier with a frequency response optimized for 455KHz operation, whose output is rectified
to produce a 0 to 76mV AGC control voltage (the cathode of D7). The output of D7 feeds a
very long time constant filter (R70, C37) of about 1 seconds. This is the “IF-derived attack” of
the AGC. Since this must drive an op-amp (U27 pin 3), the impedance must be low enough that