Realistic STA-860 Tuned and Stereo LEDs Not Illuminating

[Brivan]

I sure hope someone is looking for a sleuthing project. I've spent countless hours / days on this, but have just ended up scratching my head. I have a Realistic STA-860 that won't illuminate the "tuned" or "stereo" LEDs. From other threads I've seen here on AK, I'm not the only one to have experienced this problem. I also have a "sister" receiver - the STA-790 with the exact same problem. In fact, the STA-790 was the first receiver to exhibit these issues, but, due to no available schematics, I had to rely on the STA-860 schematic, which made things a bit murky. I recently acquired an STA-860 that won't illuminate the "tuned" or "stereo" LEDs, so I now have an accurate schematic to reference.

I've tested the LEDs, and they will illuminate if voltage is provided to them, so it's not a wiring issue. As you can see from the attached spreadsheet, the problem is somewhere in the tuner, although I can't pinpoint exactly what.

I've replaced the LA1231N FM IF and UPC1161C FM MPX ICs several times with new replacements, with no difference in the results. I've checked TR202, 203, 204 & 205, which have tested good. I gotta think the problem is somewhere further upstream.

I've attached the STA-860 schematic, which I've highlighted for my use - with green showing the "tuned" LED circuit path, red showing the "stereo" LED circuit patch and blue showing possible common circuit paths for both. I've also attached my spreadsheet showing voltages between my "bad" STA-860 and a good STA-860 in both tuned (to a strong local FM stereo station) and de-tuned. Both ICs mentioned above, as well as all the transistors in the tuner circuit are shown, with discrepancies highlighted in light red.

A lot of work has been put into this and I aim to find this problem, but I could use a little help understanding the circuitry involved - especially in the TR202, 203, 205 and 205 area. If someone would like to help solve a mystery, I'd appreciate it. I'll do whatever voltage checks/component swaps you think are necessary.

 

[Tom B]

How does the signal strength look??
If the signal from the FM front end is too weak, you won't get stereo and may not illuminate the "Tuned LED"

 

[Brivan]

How does the signal strength look??
If the signal from the FM front end is too weak, you won't get stereo and may not illuminate the "Tuned LED"

I had that thought and just went through the front end transistors to see if there was any difference between being tuned into a strong station and none at all, but all the voltages stayed the same. This was done on the "good" receiver. The only thing I have to go on is the signal strength meter and how much the tuning dial can be moved around while still receiving a station. Audio-wise, the signal strength "sounds" the same from a weaker station I use to adjust the trimmer caps with during a regular tune-up. I know neither one of these is a concrete indicator, but it's about all I have for now. I'll do some probing of the FM ICs with the scope and see if I can see a visual difference in signal strength. Tuners are obviously not my forte!

 

[Brivan]

Comparing the signals on the LA1231N FM IF IC between my good and bad receivers, everything looks about the same, with exception of pin 6 - there is a huge difference in audio signals there between the good and bad receiver.

 

[Ylli]

Noting the voltage on pin 7 of the LA1231, I would say the quadrature coil is out of adjustment. Alternately, there may be some issue with the varicap diode D103. I don't find a manual online, do you have the full manual? What does it say about tuning the quad coil/detector?

 

[Brivan]

Noting the voltage on pin 7 of the LA1231, I would say the quadrature coil is out of adjustment. Alternately, there may be some issue with the varicap diode D103. I don't find a manual online, do you have the full manual? What does it say about tuning the quad coil/detector?

I tested D103 and it checked good. The adjustment for T201/T202 is attached. It does require use of an FM signal generator, which I'm afraid I don't have. Previously, I noted positions of both T201 and 202 and slowly went through the length of both adjustments without any difference in my tuning or stereo LED illumination.

 

[Brivan]

This is what I'm seeing on pin 7 of the LA1231 in my good receiver.



Here's the best I could obtain from my bad receiver, adjusting T201 for the cleanest signal.


Voltages on pins 6 & 7 of the LA1231 of the bad receiver measure the same as before. I feel I have T201 adjusted correctly, but adjusting T202 still makes no change - either with my LED illumination problem or the signal on the scope. I realize I would be able to set T202 properly using a signal generator.

 

[Ylli]

Step 2, 'centering' can be done without specialized equipment.

Put your voltmeter across TP6 & TP7. Disconnect the antenna and tune the receiver to an unoccupied frequency near the middle of the dial. Adjust T201 (primary) for zero volts on the voltmeter. [comment: I have always done this with a strong signal being received. Many manufacturers seem to ask to have this adjustment made with no signal - they must be centering the tuning on noise. Try it per the manufacturer, but if that doesn't seem to work, then try it with a strong received signal.]

Then redo the chart you did showing LA1231 pin voltages tuned/untuned on the bad receiver. Need to figure out why the LA1231 pin 12 is not dropping to near zero.

 

[Brivan]

Step 2, 'centering' can be done without specialized equipment.

Put your voltmeter across TP6 & TP7. Disconnect the antenna and tune the receiver to an unoccupied frequency near the middle of the dial. Adjust T201 (primary) for zero volts on the voltmeter. [comment: I have always done this with a strong signal being received. Many manufacturers seem to ask to have this adjustment made with no signal - they must be centering the tuning on noise. Try it per the manufacturer, but if that doesn't seem to work, then try it with a strong received signal.]

Then redo the chart you did showing LA1231 pin voltages tuned/untuned on the bad receiver. Need to figure out why the LA1231 pin 12 is not dropping to near zero.

The best I could do with the antenna disconnected and de-tuning to a "quiet" area on the dial was 0.1v between pin 6 & 7 adjusting T201. Either side of that gave me a higher voltage reading. New voltages on LA1231:
Pin 1: 3.0
Pin 2: 3.0
Pin 3: 3.0
Pin 4: 0
Pin 5: 0
Pin 6: 8.8
Pin 7: 8.8
Pin 8: 5.6
Pin 9: 5.6
Pin 10: 5.6
Pin 11: 12.5
Pin 12: 5.0
Pin 13: 3.7
Pin 14: 0
Pin 15: 4.9
Pin 16: 0.9

I'll update my spreadsheet shortly.

 

[Ylli]

TP6 is the same as LA1231 pin 10
TP7 is the same as LA1231 pin 7

You show pin 10 as 5.6 volts and pin 7 as 8.8 volts. That's a 3.2 volt difference. Something is inconsistent.
Are the values in the last post with a signal being received or no signal tuned?

 

[Ylli]

The best I could do with the antenna disconnected and de-tuning to a "quiet" area on the dial was 0.1v between pin 6 & 7 adjusting T201. Either side of that gave me a higher voltage reading.

Did the voltage pass through zero, e.g.l did it go from, say +3 volts to -3 volts as you adjusted it, or did it go from +3 to near zero then back positive again?

 

[Brivan]

Okay, my silly mistake. I was testing pins 6 & 7 on the IC, not TP 6 & 7. The lowest voltage I can obtain on TP 6 & 7 adjusting T201 is 3 volts. Also, my IC voltage readings were with the receiver tuned in to a strong stereo station. Here's the IC voltage readings with the receiver tuned to a "open" space on the dial:
Pin 1: 3.0
Pin 2: 3.0
Pin 3: 3.0
Pin 4: 0
Pin 5: 0
Pin 6: 8.5
Pin 7: 8.6
Pin 8: 5.6
Pin 9: 5.6
Pin 10: 5.6
Pin 11: 12.5
Pin 12: 5.1
Pin 13: 1.6
Pin 14: 0
Pin 15: 5.1
Pin 16: 0.8

 

[Brivan]

Did the voltage pass through zero, e.g.l did it go from, say +3 volts to -3 volts as you adjusted it, or did it go from +3 to near zero then back positive again?

The voltage at TP 6 & 7 always stays positive (depending on test lead orientation). With the slug pretty much completely out, it dips to about 2.5v, then increases to a little over 3 volts while I turn the slug back in, then decreases to about 3 volts with the top of the slug level with the can, then increases again until the slug hits bottom, where it sits at about 2.5 volts, again.

 

[Ylli]

Either T201 is not tuning though 10.7 MHz, or the AFC is for some reason driving the signal to the edge of the bandpass.
I'm not sure how the tuning of T202 affects the tuning of T201, but changing the setting of T202 might be worth a try. Note exactly where T202 is currently tuned and note the direction and how far you turn it, so that in the end you can return it to it's original position.

You've got that nice scope, do you by any chance have a signal source that can output at 10.7 MHz? I'll assume not.

You will be measuring the voltage between TP6 an TP7, receiver tuned to a quiet spot with the antenna disconnected so the detector is seeing only passband noise. Attempt to tune T201 with the goal of seeing it go from a positive voltage, through zero, to a negative voltage. Change the tuning of T202 by 1/2 turn (note the amount and direction). Now try adjusting T201 again. Better, or worse? If better, try another 1/2 turn in the same direction, if worse, then back off that 1/2 turn and go 1/2 turn in the other direction. If this is having no effect, then return T202 to it's original position.

 

[Brivan]

I'm afraid I'm fresh out of 10.7Mhz sources, but I wouldn't mind getting my hands on an FM stereo signal generator. I tell ya, I'm at the point with this problem that I'm willing to throw more money at these two receivers than I could get out of them - I need to find out why this happens to these receivers with no known cure. Anyway, T202 will give me a fine-tune adjustment that will vary the voltage one way or another by a few hundredths of a volt. T201 seems to vary by a few tenths of a volt. Both behave about the same, with a low voltage reading occurring when the slug is about level with the top of the can. Unfortunately, I'm still at 3 volts and not even close to going past zero to the negative side.

I used to have a good STA-790 that I was using to compare voltages between it and the bad STA-790 that I still have. I'm pretty sure I socketed the two FM ICs in both receivers then tried both ICs from the good receiver in the bad receiver with no change. I haven't tried pulling my two known good FM ICs from my working STA-860, but I'm willing to do that, just to eliminate the possibility that the new replacement ICs in the bad receiver aren't working. I've had it happen on more than one occasion where a brand new tuner IC didn't work, but brand new one from a different production run worked fine - it's a weird thing.

At any rate, I'm about ready to head home (my shop is at my workplace) for the weekend, so I won't have access to the receivers. I thank you very kindly for your help, so far, and would appreciate it if you wouldn't mind picking this up again next week - at your convenience. If you think of anything over the weekend, I'd be more than happy to give it a try on Monday. Thanks!

 

[Ylli]

To sum up TR202, 203, 204, & 205......

TR202 & TR203 form a 'Schmitt Trigger' circuit. It is driven by the 'Mute' output of IC201, squaring it up and introducing a bit of hysteresis. When IC201 determined you are receiving a good signal, pin 12 is driven low, and drives the base of TR202 low. That low will propagate though to a low on TR203 collector. A low on the collector of TR203 turns on TR204 and the high on TR204's collector passes through TR205 to light up the tuning LED. TR205 is simply a switch that shuts off if the receiver is not in a radio mode.

If the signal is too low, the collector of TR203 will be high. That high is coupled through D205 to pin 16 of IC202, disabling the FM Demux.


You mentioned that you already replace IC201 a couple of times. Unless they came from a very unreliable source, I doubt that is the problem. If you are comfortable with your soldering/desoldering skills, and things are accessible, you could try swapping T201/T202 from the working to the not working unit. DO NOT CHANGE THE TUNING OF T201/T202 ON THE WORKING UNIT. Also, swap L206.

 

[Brivan]

To sum up TR202, 203, 204, & 205......

TR202 & TR203 form a 'Schmitt Trigger' circuit. It is driven by the 'Mute' output of IC201, squaring it up and introducing a bit of hysteresis. When IC201 determined you are receiving a good signal, pin 12 is driven low, and drives the base of TR202 low. That low will propagate though to a low on TR203 collector. A low on the collector of TR203 turns on TR204 and the high on TR204's collector passes through TR205 to light up the tuning LED. TR205 is simply a switch that shuts off if the receiver is not in a radio mode.

If the signal is too low, the collector of TR203 will be high. That high is coupled through D205 to pin 16 of IC202, disabling the FM Demux.


You mentioned that you already replace IC201 a couple of times. Unless they came from a very unreliable source, I doubt that is the problem. If you are comfortable with your soldering/desoldering skills, and things are accessible, you could try swapping T201/T202 from the working to the not working unit. DO NOT CHANGE THE TUNING OF T201/T202 ON THE WORKING UNIT. Also, swap L206.

Okay, I'll get that done (hopefully) Monday morning and will report back. It's nice to be working with someone knowledgeable on these receivers - I posted on AK about my issues with the STA-790 awhile back and got nothing but crickets. The 860 is one of my favorite receivers, but I've found a few with weak sound in either the left or right channel (or both) and finally got it narrowed down to a coupling capacitor or two in the preamp. In fact, the good STA-860 I have is exhibiting signs of that very problem, with weak output from the left channel - that's going to be coming up on my fix-it list, too. Thanks for the awesome explanation of TR202...TR205. Much appreciated!

 

[Ylli]

Just so you know where I'm coming from, and so you don't think I am some kind of audio guru or whatever. I retired 6 years ago from a medical imaging company. I was an engineering tech doing analog and RF design/tech work on MRI systems. An MRI system contains what amounts to a transmitter and a receiver. So my experience is actually in industrial electronics, and I don't have a lot of hands-on experience with consumer electronics. But a receiver is a receiver, and I do pride myself in being pretty good at troubleshooting, but I am certainly not infallible. I took up receiver 'refreshing' and repair as a hobby about a year ago.

 

[audio3000]

Sounds like Ylli is very knowledgeable about this receiver to guide you thru this.

Your problem is also very interesting to me to. I just throw in my 2 cents here.

How about isolating whether the problem is before or in or after the LA1231? Since you have a scope, a meter and a good receiver. How about trying the following:

1. Scope pin 1 of LA1231 on both good and bad receiver to see if you are seeing same level of signal on both on a strong local FM station. If the signal is much stronger on the good unit than the bad unit. The problem may be before the IC. I have seen similar problem on my NAD 7140 due to a damaged 3SK MOSFET which is not a uncommon problem. Also similar symptom on pioneer SX-1010. But this time was bad filter caps producing huge ripples on the tuner VCC . Go figure...
2. You can also compare pin 13 voltage which shows the level of signal the IC receives. The lower the stronger according to the spec..



Other checks suggested:
1. Does pin 16 change when you scan thru stations? If pin 13 changes but not pin 16, you may have a bad VR201 or C215 (depends on the voltage)..

Have a good weekend to you all...

 

[Ylli]

1. Scope pin 1 of LA1231 on both good and bad receiver to see if you are seeing same level of signal on both on a strong local FM station.

Good idea. The signal level at that point should be in the tens of mV with a strong signal. The data sheet for the LA1231N refers to signal levels of 70 dBuV (= 3 mV) as a weak signal and 100 dBuV (= 100 mV) as a strong signal. The scope he has should be up to the job.