SX-750 noisy fm stereo, mono clean?

Mcfly49

New Member
I picked up an sx-750 that has a scratchy hiss on fm stereo, but sounds great if I flip it to mono. Any troubleshooting recommendations? Thanks!
 
Clean all potentiometers and switches with DeOxit and Faderlube after, that is your first step.
 
FM STEREO is always going to be noisy compared to FM MONO, due to the MPX being on an AM carrier. DISTANCE from the tower will increase the noise, as will building obstructions, hills, and an antenna that is not oriented correctly, or is near electrical lines. FM is "LINE OF SIGHT", not over the Horizon like AM. So Range is a big factor.

These units DO NOT have a built in FM antenna. The antenna rod on the back is AM only. So you will need an antenna on it. A "T" Dipole from Radioshack is a decent starter and will work for most all but the furthest fringe transmissions and innercity transmissions with high amounts of Multi-path reception (google FM Multipath )

1.) Get it(antenna) up as high as possible.
2.) If a "T" dipole orient it so the "T" is perpendicular to the direction of the tower. If the tower is NORTH of you orient the "T" East/West.
3.) Look up the station (google the call letters) and look at the transmitting antenna coverage on the FTC site. If you are in the intermediate or fringe zones, you'll have an increased amount of noise in the Stereo position.

Cleaning all pots and switches is a good call, but probably won't help with distant reception.
 
Welcome Aboard!

Make sure the controls and switches are clean and that a good antenna is connected, and reserve the remote and random possibilities only if the problem still exists after that. A more remote possibility is that the separation adjustment has been tweaked just a bit too high, or the FM de-emphasis timing is too low.
 
Well, I mounted the antenna 2 foot higher and pulled it tighter and it seemed to help. I never realized that fm stereo could sound that horrible when mono sounded so good. I had just finished giving my new speakers a hard workout, then switched to fm and thought I blew my cd's. I would guess that a modern receiver would automatically switch to mono if stereo sounded that bad.

I did notice that the top 1/8 inch of a cap on the power board was visible. I assume that means it needs a recap of at least the power board? What all should be checked when an old receiver is put into service again? I think I read somewhere that the left and right pots for adjusting offset were reversed or something, so I really didn't want to monkey with it without asking here first.
Thanks for the help. I guess I have been browsing this forum for over a year now, and the only thing I have learned is that you all have all the answers and I am not as smart as I thought.
 
We don't all have all the answers, but working together we usually deduce the answers.

If you can see shrunken sleeves on some of the capacitors it's a sign that they have been pretty warm. The SX-750 is a nice unit, certainly worth the small effort and minimal cost in parts to refresh the power supply, and probably a few other items.

That power supply has a few oddities, and tips to improve them are discussed in many posts about the unit. One which comes to mind is that the voltage ratings of some of the caps are a little low for the application. There are also a few transistors which could be a little light-duty in some positions in the power supply. Search the forum archives and you should find lists of parts necessary, tips, and some detailed instructions.

If you don't already have a copy of the service manual for the unit, you can download it from the AK Database. Just to get you started, you can download a clean copy of the schematic here:

http://www.wwdsltd.com/files/SX-750SchematicWide.jpg
 
The forum is like a BORG collective, if you will. Like Watthour said no one of us has all the answers, but collectively we've got a pretty good batting average. Some are extremely knowledgeable, and some are up and coming. Some are extremely good generalists, and some specialize in specific units or Brands.
 
The forum is like a BORG collective, if you will. Like Watthour said no one of us has all the answers, but collectively we've got a pretty good batting average. Some are extremely knowledgeable, and some are up and coming. Some are extremely good generalists, and some specialize in specific units or Brands.

But the wonderful thing is...

All of us learn!
 
If you haven't done it yet. Open up the unit and with some can air blow out the fins of the tuner. Do not tighten any of the screws or use a spray cleaner on this.

An old set of rabbit ears work well for FM. Find one with the 300 ohm twin leads if you SX does not have the 75 ohm connector already.



Barney
 
Clean the tuning cap.
http://www.audiokarma.org/forums/showthread.php?t=144163
Your receiver may need an alignment. That would greatly improve reception. The adjustments and component values drift over time and eventually need to be re-adjusted. You need specialized equipment to do this so don't try to do it yourself. If you tell us your location we may be able to refer you to a qualified tech with rf experience.
 
Thanks for the support. I checked the dc offset last night and measured 61mv and 43mv. I know this is not adjustable on this unit, but I have not found any post about offset this high. I will try cleaning it up and reading up on this thing over the next week. I know I really shouldn't take on a project like this right now, but that has never stopped me before. If I do end up wanting to have someone else do the work I may buy a cleaner and more desirable unit to dump hundreds of dollars into. Thanks
 
I've always wondered if we replace R7/R8 with a 33KΩ and a 50K trimpot in series if DC offset could be adjusted safely and reliably. I haven't tried it, and haven't done the homework necessary to understand the full ramifications in the current mirror circuits, but on the surface it seems that it might work. Perhaps someday I'll give it a shot on one of my units to test the theory.
 
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