Kenwood Receiver Does Not Lock onto Stereo - diyAudio
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Old 11th August 2013, 04:09 PM   #1
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Default Kenwood Receiver Does Not Lock onto Stereo

The stereo beacon does not light up and it only tunes in mono. I've attached the schematic. I pulled the bulb and it looks ok. Strangely, if I dial off a strong channel, the bulb flickers and I read a bit of millivolts on the meter, but it goes to 0 mv when I'm on a channel. It is almost like the tuner likes static. I measured 8V at test point 15. I do not have the SM, but I found someone claiming the bulb is 7V/50ma and that 8V/50ma would work.

PS: I can't get the schematic uploaded, it exceeds the limit, hifiengine has it.

Last edited by DreadPirate; 11th August 2013 at 04:27 PM.
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Old 11th August 2013, 04:23 PM   #2
Mooly is offline Mooly  United Kingdom
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If there is a PLL adjustment in the stereo decoder (a preset pot), mark its position and try giving it a spin end to end, then slowly turn it around the original setting and see if its OK. Common issue on countless FM decoders of all makes and models.
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Old 11th August 2013, 09:12 PM   #3
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That schematic just won't scan well at the upload limit. I'll check that pot out, measure it before changing its value. The tuner probably needs an alignment, but its probably not worth the cost on this particular unit.
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Old 11th August 2013, 09:19 PM   #4
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Which might be the pot, VRG1 or VRG2? I'm guessing VRG1...
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File Type: png kenwood fm.png (343.7 KB, 133 views)

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Old 11th August 2013, 09:29 PM   #5
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Quote:
Originally Posted by DreadPirate View Post
Which might be the pot, VRG1 or VRG2? I'm guessing VRG1...
Hi,

VRg2 looks like the one you need. ICg2 is the stereo decoder chip.

VRg1 sets the input level to the decoder from the FM ratio detector (demodulator).
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Last edited by currentflow; 11th August 2013 at 09:32 PM.
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Old 11th August 2013, 10:03 PM   #6
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Do I have to work blindly on that pot or can I monitor something else to set it (for example, shoot for 2.1v on pin 12 or 13- 19Khz Loop Filters- when I have got a strong signal on a channel? or monitor output on pin 10- the 19Khz output?). I'm getting good results with the tuner in mono, it is just stereo that is not coming in. The chip, by the way, is 76115N.

On a separate subject, will part number 76115 sub in for the N version? ICG2 is SN76115N. The non N version crosses to NTE801, is this a generic chip or is an exact N version required? It appears that it pin out the same.

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Old 11th August 2013, 10:12 PM   #7
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Originally Posted by DreadPirate View Post
Do I have to work blindly on that pot or can I monitor something else to set it (for example, shoot for 2.1v on pin 12 or 13- 19Khz Loop Filters- when I have got a strong signal on a channel? or monitor output on pin 10- the 19Khz output?). I'm getting good results with the tuner in mono, it is just stereo that is not coming in. The chip, by the way, is 76115N.

On a separate subject, will part number 76115 sub in for the N version? It crosses to NTE801, is this a generic chip or is an exact N version required? It appears that it pins out the same.
Follow Mooly's instructions in post #2. Note the current position of the pot and then turn it slowly from end to end until the stereo indicator lights. If this is successful, tune to a weaker FM stereo signal and adjust the pot again to the middle of the setting that causes the indicator to light.
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Old 11th August 2013, 10:25 PM   #8
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Pin 6 of the SN76115N is connected to the beacon (stereo indicator light). The schematic shows 8.2V and 1.2V on this pin for mono and stereo respectively. Could it be that the indicator lamp is faulty?

Please measure the voltage on pin 8 of the decoder IC. The datasheet explains that the circuit may be maintained in mono mode by connecting pin 8 negative. Qg12 has the ability to pull this pin low and appears to be used to enforce mono mode during low signal strength conditions. If pin 8 measures less than about 2V, try temporarily disconnecting one end of diode Dg16 to see if this brings back the stereo mode.
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Old 11th August 2013, 11:32 PM   #9
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I'm afraid I'm not going to be able to get to this again until Tuesday. By the way, I remembered a few things. The beacon worked perfectly last week and I was locking into stereo mode just a few days ago, so I don't think it is a value having drifted over time. I think I'm not going to mess with the pot for now.

I did measure the voltage across the bulb. It is zero volts when on a strong station and putting out mono mode (even though it is suppose to be stereo). Once I move off the station and get only static, the voltage starts kicking in and out at a low level and the bulb flickers. That's why I think the bulb is ok, it does react to voltage, but I guess it could also just be acting funny.

The only other measurement I took (because it was easy and accessible from the top), was measurement point 15 at the power supply and I was getting about 8v there.
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Old 12th August 2013, 06:32 AM   #10
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Try the pot first. Its a bit unusual having an input level pot too, might be worth doing the same procedure with that one as well. Mark it and give it a whizz to and fro. They are notorious.

The IC would be the very last suspect but you mention 8 volts and the diagram (and they are not always correct) shows 10 volts on the L and R outputs which indicates the chip itself should be running on more than that. Check pin 1 voltage and trace it back to the PSU and make sure its correct.

Pots are favourite though.
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