In replying, Brian Denley stated:

The Drake's a top gun HF receiver wit all the tools with all filters
included. The Ten-Tec is all DSP, also with a great set of DX tools and
about 34 filters included. The R75 is a good dx receiver but apparently
need several mods in order to work well and the other filters are optional.
The R8500 is an excellent wideband receiver but will not keep up with the
others on HF.


I would not disagree except to amplify his remarks on the R75's virtues (and
especially one big shortcoming.)

After more than a year's use I am beginning to become very annoyed with the
R75's AGC/Squelch "threshold" problem, which I haven't seen discussed anywhere
else.

Even with the AGC off, in envelope detection mode (or with the S-AM mode turned
on), one cannot really eliminate *another* gain control function: a sloping knee
in the transfer curve between "max RF gain" mode, and squelch mode. So, under
certain conditions of carrier level pulsing (due to propagation), very short,
sudden pulses of carrier level seem to exceed the attack time of the AGC circuit,
overshoot, and reach the sloping threshold of the squelching region, causing
very irritating jerks in volume level. Beyond that threshold, the quieting
squelch sets in and the audio level is first rapidly attenuated, and then shut
off altogether.

Now, I intuit that this is done to prevent severe overload of the IF system and
clipping. When the RF carrier level exceeds a pre-determined ceiling, the
squelch effect starts to activate. The automatic volume control circuit ahead
of it -- and indeed, any feedback AGC circuit in such a radio -- has both an
attack time, and a release time (very sophisticated circuit designs have complex
'simultaneous' multiple time constants, but the Icom doesn't seem to be one of
them; it has only two fixed release time constants.) In my Sony 7600 receiver,
for example, quick sharp positive carrier pulses during propagation
reinforcements cause very noticeable, irritating, sudden distortion bursts --
especially in SSB mode. Horrible! But on the Icom, instead of getting those
blasts of distortion, you get jerky, pulsing audio levels. To eliminate that,
you must turn the RF gain WAY down during some types of propagation disturbances.
The strange Icom design of the combined RF gain/Squelch control works this
way: RF gain increases rapidly up to about 10AM, and then flattens off to 12
o'clock; above that the squelch sets in. If the RF gain level is set from, say,
11 to 12, the jerky/slight squelch effect is prone to occur under some
fade/blast conditions. It does not matter much if the AGC time constant is set
to fast or normal, or even if the AGC is totally off; *or* if the S-AM (sync
mode) is on.

The solution is extremely careful fine-tuning of the RF gain control during some
broadcasts affected by such propagation-induced carrier level pulsing, or
turning the RF gain to 10 AM and losing much of the AGC effectiveness at evening
out differences of carrier when tuning the bands (and making very weak signals
too soft in volume.)

(Now: may we get a very critical, expert analysis of the Kiwa mods with respect
to this problem? I have not had them done to this unit yet.)

With steady internal carrier supplied by the receiver when in ECSS mode, the
problem goes away ENTIRELY. Then, under the worst fades/boosts of carrier, the
volume may indeed occasionally change but it does not severely jerk and nearly
squelch on the first few tens of milliseconds of a sudden carrier level peak.
So, I use ECSS mode most of the time, though this necessitates pretty critical
adjustment of the frequency offset to avoid pitch shift, and often a careful
setting of the IF passband tuning to maximize clarity by moving around the very
distinctive region of frequency/phase ringing at the sharp filter cutoff.

(I am an expert in designing feedback networks, integrators, and time constants
for AGC circuits used for electronic servo control systems, but since my R75 has
been in warranty until recently, and has delicate surface-mount components, I
haven't been willing to delve into modification experiments.)

FWIW, with extremely careful operation, the R75 is an extraordinary DX machine.
But obtaining its full potential of performance requires diligence.

Yours,
AUTEUR
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