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Wanting to make a modification to a Realistic STA2600

B

Brian Stone

New Member
I have a very nice STA-2600 that I use daily out in my shop, but I am displeased with one detail about it. We live out in the country, and even with my roof mounted antenna on a rotor, only a few stations will come in at 5 bars on the meter. Unless it had 5-bars, it will not tune in stereo and turn on the stereo indicator. I attached the schematic and wanted to see if anyone had an idea if maybe one of the resistors in that circuit could be changed to a different value (or a pot) to make it tune in stereo on weak signals. If too much static on weak stations in stereo after modification I can always manually press mono button. IMG_0670.png
 
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IC301 is the stereo decoder chip. I would suggest that you first try adjusting VR303 to the center of the range of positions for which a strong stereo signal does turn on the stereo indicator. Mark the existing position first so you can go back to that if necessary.
 
Turning on the FM mute completely mutes any signal less than 4 bars, so I leave that off!
audiotemp said:
IC301 is the stereo decoder chip. I would suggest that you first try adjusting VR303 to the center of the range of positions for which a strong stereo signal does turn on the stereo indicator. Mark the existing position first so you can go back to that if necessary.
Click to expand...
Thanks for the info! I will give that a try this weekend.
 
Hi new member Brian,


Try this, lift one leg on D303(DS4148)?
Do not play with vr304, 19kHz vco free run adjustment.
Not very many tuners use the NEC chip set but they are high quality parts.
Your tuner has a multiplex filter "FM MPX" switch that is a stereo blend feature to minimize stereo noise with lower antenna levels but at the expense of reduced separation.
Good luck
 
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VR201 (by IC201) adjusts the mute level. VR202 adjusts the sensitivity of the signal level meter (bars). Again, before you mess with either make sure you mark or record their original positions. You might want to look at STA-2380's SM. That tuner is very similar (not identical).
 
Adjusting the vr202 ( signal level) is not going to change the operation of the stereo/mono switching threshold.
Now adjusting vr201 (muting level) will affect both the stereo/mono switching threshold and the search/scan functionality. Some tuners have the muting level pot mounted to the front or back panels to make it user adjustable.
By lifting D303, you are disabling the muting threshold from affecting the stereo switching threshold. You could have a switch that enables/disabled D303 function that uses the muting threshold as the stereo switching threshold, so that you have either auto stereo/mono switching threshold or none and rely on the FM mono switch ( which kills the vco) or the internal 19kHz pilot detection level internal to the upc1161 stereo decoder chip.
Kinda hard reading the upc1161 spec as it's mostly in Japanese.
 

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@rcs16
I think your proposed mods are well beyond what OP has in mind. I think he just wants the auto-stereo switching to work right.
 
Actually not too much of an effort to move the mute pot and put a 50k pot on the back panel, adjust away as you see fit. Perfect mod imo.
Either that or play with the mute pot as it sits on the board and hopefully it solves the stereo switching threshold issue as was requested to fix the issue.
 
The fact that you state that you only get a few FM stations might suggest that you are in a bit of a broadcast dead zone. With that in mind it might be worth installing a powered RF booster amplifier between the arial and the receiver to try and give it a stronger signal to lock on to.
 
rcs16 said:
Hi new member Brian,


Try this, lift one leg on D303(DS4148)?
Do not play with vr304, 19kHz vco free run adjustment.
Not very many tuners use the NEC chip set but they are high quality parts.
Your tuner has a multiplex filter "FM MPX" switch that is a stereo blend feature to minimize stereo noise with lower antenna levels but at the expense of reduced separation.
Good luck
Click to expand...
All tuners should have a high blend switch. Blending those higher frequencies can get rid of a lot of the hash. I live in a bowl. High levees all around the town I live in. Even with my outdoor antenna reception is limited to just a couple strong stations. 8 years ago I moved from the Northern Sierra mountains and reception was even worse. I had a couple Marantz tuners with a high blend switch and always listened with the high blend on. I would have to look at the schematic to be sure, but I belive the Dynaco FM3 tube tuner I have is built with a high blend circuit that can't be switched off.
 
Not all tuners have a high blend switch but probably should, it a simple cap across the left and right channels. The pioneer sx-1250 should have had one, my sx-950 has one. Do not know what pioneer was thinking.

Is the OP poor reception specific to the st-2600? Has he tried any other tuner, having the same results?
Did I scare him away :-)
 
Lavane said:
All tuners should have a high blend switch. Blending those higher frequencies can get rid of a lot of the hash. I live in a bowl. High levees all around the town I live in. Even with my outdoor antenna reception is limited to just a couple strong stations. 8 years ago I moved from the Northern Sierra mountains and reception was even worse. I had a couple Marantz tuners with a high blend switch and always listened with the high blend on. I would have to look at the schematic to be sure, but I belive the Dynaco FM3 tube tuner I have is built with a high blend circuit that can't be switched off.
Click to expand...
My company used to add variable blend to higher end tuners. That way YOU could decide how much blend was enough.
 
Ed240 said:
My company used to add variable blend to higher end tuners. That way YOU could decide how much blend was enough.
Click to expand...
Nice idea. That would be a great feature. Thinking back, I should have kept the Marantz model 120 scoped tuner that I gave my son. I should have given him the model 112 instead. The 120 has a high blend switch.
The 112 is a nice tuner though.
 
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