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  #1  
Old 01-17-2019, 02:45 AM
vortalexfan vortalexfan is offline
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1970s Kenwood Model KR-4070 issues

Hello everyone recently I picked up a 1970s Kenwood Stereo Receiver Model KR-4070 that when I got it home it still worked pretty well except for extremely noisy controls and a non-functioning FM Stereo light.

I had installed a used but known good bulb in place of the what I thought was a burned out FM Stereo Bulb and the FM Stereo light still refused to light up.
The original bulb was a Stanley rubber grometted 8v .50mA Grain of Wheat bulb that was soldered to a 6" length of solid aluminum wire that was then covered in shrink tubing.

My question is: does the bulb need to be the same specs as the original bulb in order for the FM Stereo light to work or is there something wrong with the Multiplex circuit that's causing the FM Stereo light not to respond to the 38kHz
pilot signal that causes the FM Stereo light to light up when it senses a FM Stereo Broadcast?

Another issue I'm having is that the faceplate for the receiver is held into place with a philips head screw and 2 hex nuts which are attached to 2
Female to male thread standoff style nuts that have female threads that screw onto the knob shaft to hold the knob into place and then on the other end have male threads that the hex nut threads onto that helps hold the faceplate on with, and they are hollow in the middle to allow the knob shaft to go through.

Both of those standoff style nuts which were plastic broke and I need a source for replacements.


I have a picture of the standoff style nuts below.
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Old 01-17-2019, 10:53 AM
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If the voltage, current or type (incandescent vs. led) are different the biasing on the transistor that drives it could get messed up....it could even affect the whole MPX circuit depending on circuit design.
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Old 01-17-2019, 11:07 AM
vortalexfan vortalexfan is offline
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Quote:
Originally Posted by Electronic M View Post
If the voltage, current or type (incandescent vs. led) are different the biasing on the transistor that drives it could get messed up....it could even affect the whole MPX circuit depending on circuit design.
It was a regular incandescent grain of wheat style bulb that was lighting up the FM Stereo light that was rated at 8v .50 mA which like I said I thought it was burned out but then when I tried a different known good bulb in the stereo it still didnt light up.

There was a couple of NOS bulbs like what was in this stereo originally for the FM Stereo Light for sale on eBay for $7 plus $1.25 shipping so i might get those to try.
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Old 01-17-2019, 11:55 AM
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Did you ohm out the original? If the original that you thought was burnt out is good then it confirms that the mpx has an issue. Otherwise make sure you have the right bulb.
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Old 01-17-2019, 12:28 PM
vortalexfan vortalexfan is offline
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Did you ohm out the original? If the original that you thought was burnt out is good then it confirms that the mpx has an issue. Otherwise make sure you have the right bulb.
No, I did not ohm out the original bulb, I actually cut the leads off the original bulb to wire in the new bulb because the new bulb's leads weren't long enough to reach the pins for the bulb leads to go to.

But there was just enough leads to test with and it seems the original bulb was indeed dead as it read infinite on the meter, and it seems that even my bulbs that I thought were good were also bad as they also read infinite so it seems that I just need to get a new bulb and be done with It.

What about those double threaded nuts that I posted a picture of that I need replace on my receiver? Where do I source those?
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Old 01-17-2019, 01:15 PM
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I don't know about the nuts...If I were in your shoes I'd check the threading size and thread pitch of the nut end and the bolt end. Maybe you can find the exact part on the net...If it is custom you may have to make your own or go to a machineist... Might be good to go with aluminum for the replacement.

You may need to start with a solid core stand off and drill the center hole... If you can't find the right thread on the standoff you may need to tap and or die cut the threads you need.
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