[drakelist] R-4C image problem

[Bob Henderson]

Dear reflectees   I am wrestling with a rather disconcerting problem on an R-4C I am refurbing and I'm wondering if anyone else might have seen this and can help me nail it.   I find I am receiving spurii of significant strength.  So far I have only witnessed this on 20/40m but I suspect the problem may affect other bands.   On 20m I hear multiple AM signals around the bottom of the band.  These peak by rotating the preselector a few degrees counter clockwise.  These signals are most troublesome as we approach dusk, which I attribute to the MUF falling to around 14 MHz.   I have a similar problem on 40m but the spurii can be peaked by rotating the preselector knob a few degrees clockwise.  Predictably these spurii are most troublesome after dark.   The slight rotation of preselector control suggests these spurii are from signals slightly LF of 20m and HF of 40m.  The fact that the antennas feeding the R-4C pass through a multiband Dunestar b/p filter, seems to further support this hypothesis, as the Dunestars would severely attenuate signals further away in frequency.   Today, I set up a TS570 running 5W at 14020 kHz into a dummy load on my workbench.  I attached a short pickup wire to the R-4C, peaked the preselector and found the signal on 14020 indicated S9+40 on the R-4C S-meter.  I reduced the frequency of the TS570 (which is transmit enabled 1.7-30 MHz) and when I got to 13970 I heard a carrier on the R-4C (still tuned to 14020) with an S-meter reading of S9+15 or 25db down.  When I reduced the TS570 frequency further to 13920 another carrier appeared on the R-4C, this time at S7 or about 50db down.   I am trying to figure out what is causing this but without much success so far.  One further thing I notice, which may well be related, is that if I tune in the calibrator signal to zero beat, I can hear another weaker carrier.  I have found I can change the pitch of this carrier by rotating tha passband tuning control.  At the 12 o'clock position the additional carrier disappears as it becomes zero beat with the calibrator.   The problem would appear to be caused by harmonics of the 50 kHz BFO getting into places they didn't ought to.   Does anyone have any ideas on what might be the cause of this problem?   Many thanks.   Bob, 5B4AGN