[Premium-Rx] Improvements to the RA6790 / RA1792

[Michael O'Beirne]

Evening all,

Here is a further thought on improving these fine radios.  Everyone seems to have forgotten that there is small ceramic filter at 455kHz immediately after the MC1496 second mixer.  The manual says it has a b/w of about 35kHz.  I have no idea why it is so wide.  Sounds daft to me.  I think it was made by Brush Clevite.

G3YFK did a simple mod removing the filter and inserting a standard Toko filter of 6kHz.  The pin out is different so he made a small adaptor board.  I have one from him, but I have been too busy to install it.  I'll let you know how I get on.  It should help the skirt selectivity of the standard AM 6kHz filter to an extent.

A disadvantage of this mod is that it upsets the BITE programme and declares a fault, when clearly none truly exists.

I also mentioned a month or so back the very useful MA 1105B bargraph tuning accessory.  This hangs off the 455kHz IF output and displays the spectrum over a line of over 100 LEDs.  This gives accurate tuning at 10Hz intervals.  It is particularly handy for tuning in data.  The mark and space channels show up as a clearly defined cluster of bright lights.  For AM and CW, the carrier shows up too and it's so easy to centre the tuning without reference to the tuning scale on the receiver.  It really is a "must have" but they are now scarce in the UK.  I have the complete manual for this device.

It should work on any set with an IF output at 455kHz.  The original  MA1105 was designed for inputs at 1.4MHz or 100kHz (switchable internally).


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