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Author Topic: Kenwood TS-950SDX powersupply problem  (Read 6388 times)
DK3AX
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Posts: 2




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« on: November 15, 2013, 10:49:05 AM »

My TS-950SDX did a fine job for 18 years. No problems at all.

2 years ago I got short interruptions (fractions of a second) in display and audio.
After a while there was a complete breakdown, only the fan of the final amplifier
was running.
I found that there was no longer any 15V supply.
I changed Q1 and Q2 in the AVR-Unit on pc-board X43-307. Then 15V were back again and it worked well for 2 more years.

Some weeks ago the same game started again. I changed Q1 and Q2 2 times.
Every time it worked for some time ( abt. ½ hour) then the 15V were cut off again
and only the final amp. fan run (as it does when switching the 950 off).

Then I bought an completely new ( used from hifissb.com) pc-board.
Again it worked well for about 1 hour , then the 7.5A fuse was blown and I had the same Situation as before.
Does anyone have an idea ?

73

Gerd, DK3AX
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KA4POL
Member

Posts: 1964




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« Reply #1 on: November 15, 2013, 11:30:30 AM »

I guess you got the service manual. It says clearly: If the +15 V line is shorted, F1 blows to protect the circuit. So there seems to be your problem. A prime suspect would be any capacitor, electrolytics in particular like C 10. In the beginning you had an overload condition that did not blow the fuse just yet. It worsened and now it is serious enough for the fuse to blow.
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N4ATS
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Posts: 807




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« Reply #2 on: November 16, 2013, 05:09:32 AM »

Its sad to say but with all the potentials of bad "stuff" that causes the fuse to blow , what I normally do is use an external 15 VDC supply at about 3 amps so you don't burn a trace (however that helps sometimes too in finding hard shorts) and literally "blow" the bad part off the board. Then replace it Works EVERY time. Been doing it for 30 years.
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K8AXW
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Posts: 3725




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« Reply #3 on: November 16, 2013, 09:58:22 AM »

ATS:  Yup! That works!  We had a short in our 4160 3-phase distribution system on dark and stormy night.  Fortunately, our electrical engineer had no intention of asking his men to look for it.

He just kept pulling the breaker in which would immediately trip with a bang that ramped up one's pucker factor off scale. But on the third or forth try the short blew open and his men found it immediately.

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N4ATS
Member

Posts: 807




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« Reply #4 on: November 16, 2013, 12:12:45 PM »

It is amazing what you can do. Typically and it happens quite frequent , hammys will hook up a radio backwards. Usually the diode blows open so the next time you hook it up backwards , it takes out the audio amplifier or some other part will short. A car battery works very well for rapid detection and elimination.....

Simply replace the parts the blew off , good to go...
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WA1RNE
Member

Posts: 825




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« Reply #5 on: November 18, 2013, 05:08:11 PM »

 
It's possible there's another board or module powered by the +15 volt supply that's failing after being on for some period of time.

 I checked a copy of the service manual and noted three 15 volt feeds coming from the AVR unit. Assuming from your description that you can keep the 15 volt supply running for a few minutes, each of those feeds needs to be checked for load current to determine which feed is causing the overload. Then you can trace that feed on the schematic to the module/circuit board, etc.

The instrument of choice for doing this is a DC current probe which can be clipped over each of the 15 volt feeds one at a time (without cutting the wire) to measure load current.

Once you know the module that's drawing too much current you can begin to examine everything from pinched wires to integrated circuits, transistors, etc. which may be failing as they heat up.

 You have to go through this type of logical process to get to root cause. Shot-gunning parts will usually lead to frustration and cost.

WA1RNE
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N4ATS
Member

Posts: 807




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« Reply #6 on: November 19, 2013, 10:34:09 AM »

True  however each "branch" goes to several hundred components that could be creating the issue. Its not easy pulling and resoldering many parts. After years and years of doing this , I find elimination is "usually" my best option , however I repair hundreds of radios and not the typical one radio , one repair kinda guy so I agree , he can spend the days on it and do the component level troubleshooting.

I simply do not have the time when it comes to a hard short , non repairable ,  returned from the factory kinda radio , I blast them and fix them way faster. But then again , thats just my way in a pinch with a "dog" rig.
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DK3AX
Member

Posts: 2




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« Reply #7 on: November 30, 2013, 09:26:14 AM »

Meanwhile persuaded the machine to work again.

I changed Q1 once more, got 15Volts and re-connected all boards again. The next breakdown came, when I
connected the final board, all others worked fine. Next I re-adjusted drive Id and final Id ( which were far away from all what the
service-manual said...) and now it's all fine and working since 4 days without any fault.
I hope, it will do so in future.
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