The RA1792
Page 5
The 2nd Mixer

There is more to this little board than initially meets the eye. Q1 and Q2 form a two-stage tuned amplifier at 40.455MHz, the gain of which is AGC controlled by U1, U2 and PIN Diode CR1. The output is then fed through a band-pass filter at 40.455MHz on the input to U3 (MC1496P), which is configured as a double-balanced mixer. The resulting 455KHz output is selected by the tuned transformer T2 and ceramic band-pass filter FL1 before final amplification by U4. This signal is then fed to the main IF/Filter board.  


If there is one thing to really love about the RA1792 it has to be the Power Supply. Nothing complex here. Just a simple linear PSU using TO-3-style regulators. Not a switcher in sight … unlike the PSU in the RA1795 which is pure evil!

Actually, in hind sight, there is one awkward feature of this PSU and that is the inclusion of two wire links which act as fuses. These are on the outputs of what is termed the +5V (unreg) and +15V (Audio) … both of which come from the input of the respective regulators … i.e. the unregulated inputs. Given the 1792’s appetite for ‘blue tants’, at least one of these wire fuses is guaranteed to blow during the life of an un-refurbished receiver. Replacing the wire fuse is physically simple but who do you know carries exactly the correct gauge of wire for the job? All I do is take a length of 36 strand wire, remove one strand and rewire the fuse with that.

The photograph on the right is the PSU in my own RA1792. Note the two resistors mounted on insulated stand-offs on the outside of the box. These are in series with the AC feed to the back-lights. My 1792 was originally a non-back-lit model. However when I got it, both the displays were unreadable and when I obtained a replacement front panel assembly it turned out to be back-lit. Modifying the PSU is very simple. Refer to the schematic in the service manual for actual details.