"Noisy-Hissy" FM Stereo reception on SX-626

[timinky]

Did a bit of research on this "issue" but thought I'd solicit a few thoughts here.
FM stereo position yields noisy/hissy signal reception. Signal meter shows full tilt signal strength
of most stations. I switch to the Mono listening mode noise/hiss completely gone. Switch back
to stereo mode noise/hiss returns. The selector knob on 626 allows me to choose Stereo-Auto or
Mono. I also notice when I have function selector knob in Stereo-Auto setting, and elect to choose
Mono on the independent Stereo/Mono push button on 626 that too will eliminate hissy FM reception.
Just trying to determine if unit needs servicing. FYI- all other output of sources (CD, turntable,) clean as a whistle in stereo listening mode.

Thoughts appreciated.

 

[Joe Roberts]

Quite possibly "self noise" from HD channels superimposed on the signal you want to hear. Most vintage tuners have this problem but not all. Depends on the filtering in the circuit.

Sadly, if this is the issue, there is no simple solution except the mono switch.

 

[audiojones]

Sounds like ordinary FM Stereo MPX behavior to me. I can never get totally silent FM stereo reception even with a rotating high gain chimney antenna and a signal amplifier unless the station is very close by, not even on my Sansui TU-717 or Pioneer TX-9500II tuners. That's just the nature of FM stereo.

 

[larryderouin]

It's the AM carrier that the Multiplex signals sit in and possibly the filters that are supposed to suppress the AM carrier once in the MPX section. Usually there is a slight hiss in FM Stereo, but it shouldn't be excessive. More often than not the MPX circuit need alignment along with the IF section.

 

[markthefixer]

Since the meter indication can be borked with, the swing of the meter is no indication.
LOOK at the tuner board, and tell us if there is just one integrated circuit on it, or if there are several.
That model year(s) have several incarnations - and in one the stereo decoder is done almost completely discretely, and the other is highly integrated.
The discrete one can be adjusted, the IC one can't.

FM signal to noise ratio is specified with a particular amount of FM signal in at the antenna. Stereo magnifies this effect.

 

[rcs16]

This is why some receiver/tuners, usually higher end, have a FM blend function, which shorts out the L/R at higher audio frequencies where the st noise is more apparent. The best solution is a variable blend control and changing the freq point where the blend takes effect. The blend function does reduce the L/R separation but it is one or the other, st noise or separation.
Sansui used such a ckt in their higher end rx/tuners. It op as a LDR across the L/R, in which the LDR control was driven off the IF signal level.

I always wondered why Pioneer did not have the same blend function on the SX-1050,1080,1250,1280,1980 like they did on the 950. It would have only cost them a switch and a R,C, really near sighted on their part.

As far as IBOC(HD) noise, there is a ckt that was posted on fmtuner forum for a IBOC filter, it is simple notch filter and a opamp buffer that goes between the detector and the st decoder.