pioneer SX680 tuner questions

[thetube0a3]

have an SX680 which works pretty decent except for the multiplex section in the tuner. The best way to describe the problem is excessive stereo. I've heard the same effect on 1960s fisher tuners when the dimension knob is turned too far.

The stereo on the 680 is exaggerated and hissy but I do not see any adjustments? Perhaps caps or resistors have drifted? Mono on the tuner sounds near perfect. Tuner is relatively sensitive so the problem is directly related to the stereo decoder.


Any help would be appreciated. At one point i will probably replace most of the electrolytics throughout the receiver. But i'd like to get this problem solved first.

[thetube0a3]

No responses yet? Surely someone out there has experience with these tuners. It appears to be the same basic tuner that was used in the 780 880 and 980 as well.

I am not sure if this is due to a weak frontend or not yet although the tuner does seem relatively sensitive.

It does seem if I tune in a really strong station the noise drops substantially. But on 90% of the strong NYC (30 miles from me LOS) stations here its pretty noisy.

Using a typical folded dipole indoor antenna.

[sakis]

number one is the replacement of electrolytics in the tuner area and secondary power supply

by tomorow i will have a schematic and tell you a bit more

regards sakis

[thetube0a3]

thank you much Sakis. I appreciate it.

[sakis]

the first thing you do is not to touch any of the trimers insie if you done that allready the tuner can be only brought in original state by only precise instruments ....and procedure

replae all electrolytics from inside .... as said it might be wise to change amplifier , preamplifier , auxiliary psu and so on

if problem is not solved with that there is one cap to play with and one coil that effects the mpx operation ...

sent me your e mail for schematic
kind regards sakis

[thetube0a3]

Sakis : have already properly aligned the front end of the tuner. I am having trouble getting my hands on a MPX generator so that has not been done yet. Will be doing all of the caps.

You mentioned there is a cap and a coil to *play with*. not sure what you mean?

[sakis]

do you have or need a schematic ????

[thetube0a3]

Sakis : Just got the schematic online thanks.

[unclejed613]

exaggerated stereo can result from one of the channels getting inverted, which may be from a bad electrolytic causing a preamp stage to misbehave or causing the MPX chip to misbehave. it could also be (IIRC those were some of the earliest PLL stereo decoders) that you have 19khz noise leaking back through into the incoming stereo signal, which could be causing problems with the stereo decoding.