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Author Topic: knwd ts- 850s problem with alc display  (Read 2601 times)
8P6HG
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Posts: 6




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« on: August 13, 2001, 03:05:38 PM »

i own a kenwood ts-850s and have never had a problem with it, until a couple of days ago.when i key the the transmitter, the alc show a full scale reading. on fm mode, when i key the transmitter the alc goes way up the scale and the power drops. this will last about a couple of seconds,then the alc drops off and the power goes back up.on ssb it will pop up full scale on the peaks althoug i have the alc set at desirable  setting and i also notice that the output power drops off during this behavior.when this problem happens the receving stations do not hear any changes etc.can anyone help.
Thanks & 73
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WB2WIK
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Posts: 20611




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« Reply #1 on: August 13, 2001, 05:12:02 PM »

I have a TS850S/AT also and don't understand what you're saying...

You said, "on the FM mode, the transmitter ALC goes way up the scale and the power drops.  This will last for a couple of seconds, then the ALC drops off and the power goes back up."  This doesn't make much sense, since in the FM mode, the ALC is normally inactive and the ALC bar-graph reading _should_ go all the way up, and stay there, until you've stopped transmitting.  In CW, RTTY and FM, there is no ALC.  ALC functions only when using SSB.

Also, you report that on SSB, when the ALC "pops up full scale," the power actually drops off, but then comes back to normal when the ALC reading comes back down.  Although this is very unusual behavior and I've never seen this happen, it sounds like possibly an oscillation in the ALC loop (maybe!), which is probably visible on an oscilloscope but not necessarily anything that another station would hear on the receiving end.

Unusual problem, indeed!  If I had the rig in front of me, I could probably diagnose it, as I have the service manual for the TS850S(AT) and have owned one since they first came out more than ten years ago.  But "remotely," it's difficult.

Still, re-check that ALC indication on FM issue.  My TS850S indicates ALC "full bars" (full scale reading) any time I transmit on FM, and that would be normal, since the ALC is disabled on FM.

73 de Steve WB2WIK/6

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8P6HG
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Posts: 6




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« Reply #2 on: August 13, 2001, 06:21:40 PM »

tnxs steve for your response. when i key my 850 on fm mode the alc bar graph peaks and stay peaked for about 4 to 5 seconds, while the power drops out completely and then the reverse occurs.i have observed  that in the FM mode, sometimes it would not transmit  full power and the alc graph stays half way while both the power and alc graph seem to show some type of forward pulsating motion.
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NB1B
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Posts: 4




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« Reply #3 on: August 13, 2001, 07:41:27 PM »

There is a problem with the ALC circuit in some TS850S.  I know because I had one.  The problem shows up exactly as you stated- the ALC swings from zero to max, and the power out goes from max to zero in sync with the ALC.  It starts out affecting just one mode, and eventually gets worse and starts to affect all modes.  Before I had mine fixed, the power would swing from 0 out to 100 watts out every 5 seconds or so.

The bad part is the "circuit" that contains the multivibrator that creates the ALC voltage that is then rectified and amplified to control the power output of the rig.  That "circuit"- I say that because it is one of those multi-part circuits that look like one big awkward component- oscillates, and when it does the ALC output swings wildly.  Replace that part- which you can only attain from Kenwood- and the problem is solved.  

You may only have the problem on one mode and certain power levels now, but eventually it will make the xmitter useless (in my experience, it took about 9 months from the time I first saw the ALC swing until the xmitter was totally unusable due to the power swing).

73 and good luck.
Dennis NB1B
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K0RS
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Posts: 739




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« Reply #4 on: August 14, 2001, 01:34:10 AM »

Dennis is correct, this is a fairly well documented problem in 850's.  Check out KØBX's web page:

http://www.qsl.net/k0bx/

Follow the link on the left side entitled "TS850 TX ALC problem."  You can have Kenwood repair this for about $150 (US) or pay Pacific Coast Parts $10.30 for the part and install it yourself.  You can find detailed instructions on how to do the repair on the web, but it's not for the faint hearted.

73.  Larry
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« Reply #5 on: August 14, 2001, 09:52:18 PM »

www.kenwoodparts.com  is another
source for this part, same price but
this source is on east coast NY.

73 walt
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