Service manual

Page 19

(U20/U21 on Sheet 8 of the RXBPF board) is allocated as a bypass. This circuit represents the

DZKit exclusive Passive Signal Boost (PSB)

. By skipping the BPFs, any associated front-end

loss is eliminated at the expense of a potential increase in intermod, and an increase in the noise

floor, since more spectrum is allowed in. However, on a fading band, that extra 5dB of

“gain” (actually “lack of loss”) can spell the difference between hearing and not hearing a weak

signal. PSB is not intended to be used except under such conditions. We do not recommend that

you leave it enabled permanently even though signals will sound stronger.

Preamps, attenuator2

The selected bandpass filter’s output is point “B”. Referring to Sheet 9 on the RXBPF board, B

is routed directly to the first preamp (Q4 and associated circuitry) and to relay K4, where it can

be fed to the RF output without preamps. The first preamp’s output can be switched on by relay

K1 and appears at point R2, which feeds preamp2 (Q4 and associated parts) and relay K5. If K5

is disabled, so is K2, and thus Q4 has no power and provides only a weak load for preamp1. If

K2 and K5 are enabled, preamp1 is fed into preamp2 and then out to the RF output.

The preamps are low noise 2N5109 bipolar transistors set up for a power gain of about 12dB

and coupled via broadband toroid transformers T1 and T2.

Receiver first IF

The final RF output of the BPF board feeds into the Receiver board at J14 (Sheet 2 of Rx

Board). It is applied directly to a Minicircuits TUF-3 diode ring mixer (U6). Local oscillator

LO1 from the controller, which is the VFO, is set to the displayed receive frequency plus

70.455MHz (or 70.000MHz if the 20kHz roofing filter is used) and fed into the LO port. The

output, consisting of sum and difference frequencies and a number of other mixing products,

splits into two paths via a 50 ohm resistive pad, with one leg driving an Inrad 4KHz bandpass

filter at 70.455MHz and the other driving the FM receive and IF output circuitry.

The output of the bandpass filter is amplified by Q5, a dual-gate MOSFET with about 10dB of

power gain. The second gate of Q5 is derived from the AGC circuitry on Sheet 6, buffered and

inverted by U1 to provide a nominal 4V on gate 2, decreasing to 2V under full AGC action and

reducing the gain accordingly. The source is biased at 1.9V, so the 4V on gate 2 represents 2.1V

of gain enhancement. When the control voltage goes down to 2V in response to a strong signal

detected by the 3rd IF, the differential of 0.1V reduces the gain to its minimum level. Diodes

D11 and D12 provide temperature compensation while contributing to the source bias of 1.9V.

The output of Q5 feeds a Darlington transistor configuration, which provides high input imped-

ance and low output impedance, necessary to successfully drive the next RF mixer. Resistors

R101 and R126 set Q5’s load impedance to about 2K ohms to provide a moderately high im-

pedance load for the amplifier, whose nominal output impedance is about 200 ohms, while bias-

ing the Darlington stage at a reasonable level. This preserves the power gain while allowing the

stage to drive the low impedance (50 ohm) mixer load.