R5000.4

One other thing to note involves installing the optional filters.
First, I found that the AM filter which came with the radio introduced a
whistle into most AM signals.  I replaced it with the optional AM
filter, and it eliminated the whistle (and gave a "fuller" sound to the
AM).

Also, note that if you install other filters, you have to install them
"in order" -- in other words, the narrowest has to go in the N position,
the next narrowest in the M1 position.  Note that the M2 position is
already occupied (as shipped) by a high-quality SSB filter.  The reason
they have to be in order is that as you select narrower and narrower
filters, the wider filters remain in the circuit, so if you put a
narrower filter in M1 than you have in N, it will stay enabled even when
you select the wider filter, causing the wider filter to have no effect.

It's a little unfortunate that it works that way, since the M1 filter is
only enabled by the switch (not in AUTO mode), which would be ideal for
the very-narrow YK88CN filter.  I have identified a modification to make
M1 be selected only when the switch is in the M1 position; it involves
disconnecting 1/2 of a dual diode and soldering a diode between two
prodthe PK232.

--
Eric Roskos, IDA (rosko@CS.IDA.ORG or Roskos@DOCKMASTER.ARPA)