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Tom · August 23rd 06, 04:03 AM posted to rec.radio.shortwave

wrote:
First, there is no good substitute for having a professional hearing
test.
My speech pathologist friend agreed that out of the many that I found,
this one makes no claims and might spot "gross problems".
http://www.freehearingtest.com/test.shtml

Great collection of references, Terry. I have to endorse the above
statement from recent experience with my new FT-817ND. Right from the
outset, I detected a raspy distortion on CW and SSB, and later on
detected a more subtle level of it on AM. The FT-817 does not have AM
synch but you can use it very effectively in unsychronised ECSS mode by
tuning an AM station using LSB or USB - the carrier is well enough
filtered that even though the tuning steps are 10 Hz, the zero beat
error is not strong. However, I relaised that the raspy distortion was
being turned on and off or was being modulated by the zero beat error.

Went back to the dealer and found their demo unit has the same
distortion. Searched the Yahoo group and found sporadic references of
others detecting this distortion. Posted a message about it and a poll
and was surprised by the range of responses, especially after I have
formed the conclusion that it is a design fault. The vast majority of
respondents do not detect the distortion, even after I posted a sample
A-B comparison of FT-817 received audio with and without the raspiness.
By backing off on RF gain so that the signal strength falls below the
AGC threshold, the distortion rapidly diminishes; it appears to be at
its worst just at and a little above AGC threshold and eases a little
with stronger signals. Even QRN can trigger the AGC and as a result the
noise is distorted and worse than it would be through a linear system.

This distortion looks to be mainly harmonic and at its worst is about
1%THD - doesn't look like much but it sounds worse! Perhaps because the
harmonics are all of similar amplitude. When simulating a distorted CW
signal using a multitone generator, the addition of the fifth harmonic
had a pronounced effect.

If I'm right that it is a design fault, then it would appear that many
of the respondents who said that their FT-817 does not exhibit the
problem must have faulty hearing or a faulty monitoring system.
Certainly, passing a CW (or for that matter, any) signal through a low
pass filter will suppress harmonics of a high enough fundamental
frequency and have a mitigating effect. This would be most pronounced
on CW because harmonics of bass speech spectrum and of QRN will still
fall within the passband.

I very much want to fix the problem but continued warranty coverage
and tiny SMD's are rather large barriers. And so far, Yaesu has not
allowed anything more than that this may be "more or less nominal for
this radio"!

73, Tom