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![]() 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 -- Ce message a ete poste via la plateforme Web club-Internet.fr This message has been posted by the Web platform club-Internet.fr http://forums.club-internet.fr/ |