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Author Topic: Kenwood TS-940S PLL unlock problem?  (Read 1744 times)
W4AJF
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Posts: 259




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« on: October 08, 2006, 06:17:03 AM »

Hi, I have a Kenwood TS-940S and when you turn it on for a second, the display reads the frequency then it blanks out and you just get dots. No receive or xmit. If you rotate the VFO or change bands the display comes back for a second but then blanks out again. This happens on ALL bands, all modes. One of the web sites said that this problem may be resolved by adjusting some of the coils on the PLL board with an RF milivolt meter but that did not help. Also, if someone has a TS-940S maybe they can help me with this:  On my TS-940S under the top cover, there are 2 grey coax wires together that come out from a hole (sort of) behind a small transformer near the back of the radio, they terminate in a 6 pin connector but the 6 pin connector is not attached to anything. Can someone  tell me if theirs  has those 2 cables attached to a connector and where they go, if it does plug in to something?  That would be a huge help. I can send a picture if you need it.
  Anyone with any help or familiar with the symptons I have, I sure would appreciate any help.
Thanks,
Adam Farber W4AJF
adamf@eeco.com
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N3OX
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Posts: 8854


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« Reply #1 on: October 08, 2006, 07:53:05 AM »

This happens all the time in the TS-440S and it is a symptom of a problem with some VCO on the PLL board.  I don't know if it's the same in the TS-940S, but for the '440 the VCO was potted in some hygroscopic rubber and the dielectric constant of the water that eventually gets absorbed throws off the tuning of that oscillator.  Mine works after a period of warmup, but what really needs to happen is that all the potting stuff has to come out and a couple of components are replaced for good measure.  

I don't know how different the '940 circuitry is, but the 440's dots problem is covered in detail at the TS-440 yahoo group:

groups.yahoo.com/group/TS-440

Maybe that will help by analogy... I don't know.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KB4QAA
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Posts: 2327




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« Reply #2 on: October 08, 2006, 09:13:02 PM »

I have had a TS-450 for 16 years and it has the same problem.  It has been back to the shop twice, where they replace two components that are Kenwood propietary and quite expensive.   I don't recall what they are.

The problem has returned a third time, and at this point I'm living with it.  There is no pattern to it, but it comes back to life after leaving it on to bake for a while.   I can go months with no difficulty then have problems every day for a week, then it disappears.  

I am aware of the TS440 potting problem but have never bothered to take my rig apart to check this out.

good luck.  Bill
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W4AJF
Member

Posts: 259




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« Reply #3 on: October 13, 2006, 05:25:31 PM »

OK, I put in a new PLL board and that solved the PLL problem, the display reads out steady and the radio does seem to receive and xmit. It has full output and decent receive but when I transmit I hear a sort of buzz from the speaker and the frequency on the radio moves up! In SSB when I modulate I hear the buzz and the frequency moves up each time I modulate. When I throw an FM carrier it buzzes and the frequency moves up. Any help out there?

Thanks,

Adam Farber W4AJF
Cooper City, FL
adamf@eeco.com
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