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Author Topic: TS-940....almost no power out. Can NOT hear TR relay clicking? S meter adjust?  (Read 2067 times)
N6QWP
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« on: October 26, 2017, 06:01:29 PM »

Have a Kenwood TS-940 that has very little output=5 watts showing on external antenna tuner???   First thought it was the finals or drivers.  Purchased another final unit and after changing, same problem.  Notice that I can not hear the TR relay working.  No final amp current or power output showing on meter.

Have proper voltage to finals....Receiver works fine (except for S meter which doesn't register unless I transmit using another rig in shack).  Looking for the S meter sensitivity adjust.

Looking for input from those familiar with the 940 on what are some possible causes for the relay not working?  Also, wondering how I can get some-but very little output-if the relay is really not working???

Is it possible that the relay is working but I can't hear it???  Xmit light does indicate switching.  If so, what are some other causes of very low output-other than the final unit???  I can hear audio in other receiver.  Thanx for any replies.....would like the get this rig back to working again.



« Last Edit: October 26, 2017, 06:22:10 PM by N6QWP » Logged
KA5IPF
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« Reply #1 on: October 27, 2017, 06:51:12 AM »

The 940 uses diode switching for tx. Check the drive from the RF unit to the final. Leave the s-meter adjustment alone. That won't fix a radio that is deaf as a post. On 20m the calibrate signal should be around S-9, if not fix the radio.
« Last Edit: October 27, 2017, 06:53:15 AM by KA5IPF » Logged
N6QWP
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« Reply #2 on: October 27, 2017, 07:58:01 AM »

Thanx for pointing out why I do not hear the TR relay=am I to understand that no relay is used at all for Tx?

For clarification, where am I to get the S9 signal?  On my other recieiver?  If so, I do get a very strong signal on TS-430 with no antenna hooked up to it.  I assume that this indicates that the rf section is working properly.  I also get good audio while listening.

So, if rf section is ok and getting signal to final unit...but no rf being amplified in that final unit, what should I be checking for?  As I previously stated, I have tried another final unit on the 940 with the same lack of power amplification.

Is there any way that rf unit can be good (hear audio and get good signal on closeby rig)....AND final unit can be good (just replaced with another unit).....and something else is the problem???  If so, what???

Is there something on the rf board that is at end of signal path, that can stop short of the final unit?  That would still enable me to hear audio on nearby rig?
« Last Edit: October 27, 2017, 08:22:03 AM by N6QWP » Logged
K4JJL
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« Reply #3 on: October 27, 2017, 08:19:01 AM »

Thanx for pointing out why I do not hear the TR relay=am I to understand that no relay is used at all for Tx?

Some newer radios click on TX due to an aux relay for keying an external amp.  Most modern TR is PIN diode now.

For clarification, where am I to get the S9 signal?  On my other recieiver?  If so, I do get a very strong signal on TS-430 with no antenna hooked up to it.  I assume that this indicates that the rf section is working properly.  I also get good audio while listening.

Try pulling and reseating the boards.  Lots of 940 issues are from corroded contacts on the card edges.
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N6QWP
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« Reply #4 on: October 27, 2017, 08:53:11 AM »

I checked and I do get an S9 reading on 14.000 from the calibrator.....BUT I also notice that the sensitivity seems low as compared to my TS-430.  I can still receive signals, but no indication on meter and definitely not as strong.  

I will pull the plugs and rf board and see if that helps.

Still seeking possible causes and fixes for xmit problem.  Wondering what can be stopping the rf path between the rf board and the final unit.  Each final unit has its own coax to join the two boards....so unless someone comes up with another possible cause, I am leaning towards something at the output of the rf board???
« Last Edit: October 27, 2017, 08:58:04 AM by N6QWP » Logged
K4JJL
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« Reply #5 on: October 27, 2017, 12:37:27 PM »

Still seeking possible causes and fixes for xmit problem.  Wondering what can be stopping the rf path between the rf board and the final unit.  Each final unit has its own coax to join the two boards....so unless someone comes up with another possible cause, I am leaning towards something at the output of the rf board???

If it were my radio, I'd follow the schematic/block diagram through the stages with an RF sniffer (stripped piece of coax plugged into a spectrum analyzer).  You might be able to do it with your other radio with a couple of 6db attenuators in-line.  See which stage the signal falls off.
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N6QWP
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« Reply #6 on: October 27, 2017, 12:55:26 PM »

Now wondering if another voltage source-other than the measured 29 vdc line to the finals-bringing other voltages to the final unit, could cause a good final unit to be unable to amplify the signal coming from the rf unit and on through to the antenna jack?  

Hopefully, someone with a schematic can seek this out and provide an answer to this line of inquiry.  Specifically, what voltages and where to check ?

Of course, any other ideas from brainstorming the 940, will be greatly appreciated.  I am just hoping it will be something obvious and simple to check out with just a meter.
« Last Edit: October 27, 2017, 01:07:17 PM by N6QWP » Logged
ZS5WC
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« Reply #7 on: October 28, 2017, 04:36:27 AM »

 ???Is the Internal Tuner switched out?.
If you TX, what is the ALC meter showing?.
What does it do on CW / AM?.
are the Carrier and Drive knobs correctly set?.

No final bias is a bit of a concern-maybe no signal +TX signal to bias the finals.

73 de William
ZS4L / ZS5WC
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N6QWP
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« Reply #8 on: October 28, 2017, 08:09:24 AM »

No difference if the internal tuner is in or out.  NO meter readings except for final voltage.  EITHER known working pull final unit OR original final unit installed=NO difference.  Mode makes NO difference.  Power and mic controls both set to maximum to barely get 5 watts output.

Bias and other adjustments are all on the final unit (I believe), so if working in another rig, how would those voltages be different?......unless fed from another board (source?).

Trying to find something NOT on the final unit that can cause the problem???

« Last Edit: October 28, 2017, 08:15:41 AM by N6QWP » Logged
KC4ZGP
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« Reply #9 on: October 29, 2017, 12:17:20 PM »


An amplifier really doesn't amplify. It simply controls a power supply's output current flow.

It tells the power supply how much current to release and when. Voltage stays the same.

Kraus



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N6QWP
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« Reply #10 on: October 29, 2017, 05:10:04 PM »

So......if the amplifier is not getting a signal passing through it.....what does that indicate Huh  Profound words of wisdom are fine, but do not really help me, at this point, to isolate the problem and fix it.

How about some simple definitive trouble shooting tips with a minimum of test equipment?  Grasping at straws here, and would love simple, direct hints on where to go next.  Service manual is in the mail.....that should help with any pointers from "Elmers" who actually have or have had experience with the TS-940.
  
My last surmising of this problem makes me wonder if perhaps the "new" final unit might actually be at fault?

Without the Service manual and schematic, I am seriously handicapped in figuring out possibilities.

Would that scenario play out to the symptoms that I am describing?  If the "fragile" drivers were somehow faulty (even though this unit performed well in another 940) would that create the situation that I have with this TS-940 Huh
« Last Edit: October 29, 2017, 05:30:15 PM by N6QWP » Logged
ZS5WC
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« Reply #11 on: October 29, 2017, 10:36:17 PM »

 :)The Service Manual and Schematic are both available on the internet.
WRT the finals, I would ensure all the voltages are correct, secondly, when you transmit, there will be a TX enable line that will be high to enable bias regulators, which will in turn bias the transistors.
Without finals bias standing current on transmit, there will be no or very low output.

73 de William
ZS4L / ZS5WC
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N6QWP
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« Reply #12 on: October 30, 2017, 08:33:37 AM »

Thanx William--Found the manuals on internet.....in the mail.  When they arrive, I will follow your instructions and see what is really up with the final unit.
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KM1H
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« Reply #13 on: October 31, 2017, 05:02:25 PM »

Quote
The 940 uses diode switching for tx. Check the drive from the RF unit to the final. Leave the s-meter adjustment alone. That won't fix a radio that is deaf as a post. On 20m the calibrate signal should be around S-9, if not fix the radio.

Actually the diode simply enables the relay.
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KA5IPF
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« Reply #14 on: October 31, 2017, 06:03:32 PM »

Look again. The TX is routed thru 2 parallel switching diodes. The reed relay grounds the RX input.
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