ac.4. Receive Improvements
The receiver’s strong signal-handling capability and audio quality can be vastly
improved by the following changes:
During alignment check to see if, while adjusting T-102 (see Figure 14. IF Circuit Board
Modifications on page 84), there are two points where the transformer can be peaked --
one spot for transmit and a slightly different setting for receive. If so, peak the
transformer for maximum transmit drive instead of maximum receive gain. The receiver
has an abundance of gain, however the transmitter could use a little extra. This will get
rid of some of the receiver gain but WILL NOT affect receiver sensitivity.
Also change the value of the screen dropping resistor R113 in the second I.F. amplifier
V4 from 1KΩ to 10KΩ (see Figure 14. IF Circuit Board Modifications on page 84). This
will reduce the gain of this stage a bit and improve gain distribution.
ac.5. BFO Level to Product Detector
The next step in receive improvements is to increase the BFO drive to the product
detector. The BFO (carrier oscillator) injection to the product detector should be about
five times the level of the I.F. signal for low distortion. On these rigs, the I.F. signal can
equal the BFO injection level under strong signal conditions resulting in a very raspy,
distorted audio quality. I have even noted at times some pulling of the BFO oscillator
frequency. The major problem is that during receive, the BFO signal is coupled to the
product detector cathode through C17, a 12pf silver mica capacitor which is connected
to a piece of coax running to the product detector cathode. The capacitance of the coax
cable exceeds the value of coupling capacitor C17, and the combination of the two acts
as a voltage divider, greatly reducing BFO drive level to the product detector V13C.
Correcting this involves increasing BFO drive and decreasing the I.F. drive to V13C.
To increase BFO drive, simply replace coupling capacitor C17 on the modulator board
with a 100pf silver mica, and replace the 33KΩ resistors R6 and R7 on the modulator
board with 27KΩ resistors (see Figure 15. Modulator Circuit Board on page 86). To
reduce the I.F. drive, change resistor R123 on the I.F. board from 470Ω to 75Ω (see
Figure 14. IF Circuit Board Modifications on page 84).
Note: Rather than replacing C17, it may be simpler to add a 47pf silver mica cap in
parallel with C17. The added capacitor can be soldered on the foil side of the Modulator
board.
Modifications Page 90
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