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Author Topic: TS-940 Power Supply Shooting 40 vdc (not 28) to Final Unit-Cause of No Output  (Read 1832 times)
N6QWP
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Posts: 228




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« on: November 02, 2017, 07:31:00 PM »

Just discovered the "Primary Cause" for my TS-940 having No Output.  Turns out that the 28 vdc power supply line to the final unit is actually putting out 40 vdc.  Was fooled previously, because the panel meter still shows 28 vdc.  Wondering why?

This would account for no output from the radio.....since there are obviously, parts in both the supply and the final unit that have been exposed to "lethal voltages".  

Obviously, the pass transistor(s) failed and I will have to find out how many other parts in the power supply have also failed.  Assume that the AVR board will have multiple failures.  Most likely?

In addition to the supply, there will most certainly be parts in the final unit that, having been overstressed, will be the most likely to have failed.  Other than the drivers, wondering if there are other "fuse parts" that might have failed?  Read about a zener diode that sometimes might act as a fuse before the drivers go....anyone with experience want to venture whether that is possible or probable?

If anyone can point out the most probable parts to have failed, I would appreciate any direction before blindly jumping in.  Sorry to have missed something so obvious in previous postings :-(
« Last Edit: November 02, 2017, 07:51:46 PM by N6QWP » Logged
K8AXW
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« Reply #1 on: November 03, 2017, 10:21:09 AM »

To say "this or that" probably blew with the high voltage would be very difficult if not impossible.

If this was my rig....and a nice rig it is, rather than "shoot it in the head" at this point I'd do either one of two things.  If I could afford it, I'd get another rig to keep me active (assuming you have only the 940) and then systematically start working on the 940, starting with the power supply.

Since Kenwood no longer supplies power supply boards for this old rig I'd get a PCB from FAR Circuits and stuff it according to information published in a past issue of QST.  (When ordering the board I'd ask Fred Far where this info is published and save a lot of time)

Once you have determined that the new board puts out the 28VDC as called for, I would have determined if the driver and finals are gone. 

From that point on I would have to see what is or isn't going on.  It all might be an exercise in futility but I'm one of those people that finds it very difficult to throw away something that good.

FWIW, every 940 that I've encountered with a failed power supply didn't take out the finals.

I would also contact Clif, (Cannot recall Call Sign at this moment but can be found in this particular forum)  Ask him for suggestions.  While he no longer works on the 940 he usually is willing to supply information/suggestions.

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N6QWP
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« Reply #2 on: November 03, 2017, 11:09:19 AM »

Thanx, I still have a working 930 and a 430 to keep me going.  Wondering if the finals are not usually taken out with a power supply failure....are the drivers usually blown?  Probability?  Wondering about any other parts in the final unit that might act sacrificially to save those drivers.....or are they most certainly DOA?

Since the 940 power supply is no longer working, and I would like to know the present fate of the final unit, wondering if I could hook up an external 28 vdc supply and a rf generator and test the final unit for output or specific problems using a DVM and a homemade rf probe?

If anyone has created a similar "jig" (or can visualize one).....and had good results, I'd like to know how it was (or can be) done.  Since I lack a scope and the expertise to properly realign the bias after replacing drivers, I'd love to know the fate of those drivers and the final unit ahead of time.
« Last Edit: November 03, 2017, 11:31:12 AM by N6QWP » Logged
W9GB
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Posts: 3148




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« Reply #3 on: November 03, 2017, 07:00:51 PM »

Brad -

Power Supply design (AVR boards) for TS-930, TS-940 are known weaknesses.
In addition the Tin/Lead Solder (Asian sourced) was inconsistent quality,
often inferior (creating problems).
http://forums.qrz.com/index.php?threads/pop-goes-the-kenwood-ts-940.262115/
This is covered in the TS-940 Bible and numerous Internet web sites.

Kenwood TS-940 Repairs
David Lowrance, AL5X
http://www.resonantfractals.org/TS-940S/TS-940S-Repair.html

Kenwood TS-940
http://www.beevo.org/TS940%20AVR%20Board.html

Jeff King ZL4AI / DU7
Created the Kenwood TS-940 Page
http://www.jking.netau.net/ZL4AI.htm
==
Japanese designers stuck with discrete and op-amp DC power supply designs
through the 1980s and 1990s.  Over-voltage protection (40 VDC) was not provided.
I can show you the Kenwood KPS-10 power supply, used in their Land Mobile Division,
that suffers similar shortcomings (I repaired it and use it with West Mountain Radio PWRGate).
« Last Edit: November 03, 2017, 07:09:26 PM by W9GB » Logged
N6QWP
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Posts: 228




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« Reply #4 on: November 03, 2017, 07:50:59 PM »

Great referenced sources....thanx!  Can't find anything online regarding the "TS-940 Bible"Huh  Is that a nickname for another source?

Did find another blog on using a switching power supply module to fit inside a 930 and was/am considering such a substitution.  Anyone try that approach?  I am now paranoid of my good working 930.

I previously did buy a replacement power supply (less AVR board) for a blown 930....and am considering using those applicable parts in the 940---but think that if I go for rebuilding, I should use all new parts or a more modern supply.

I love both the 930 and the 940.  Have had three 930's (one failure so far-plus the beginnings of the dreaded dot problem on another).  And, two 940's (one earlier one was plagued by multiple issues).....and this one with the power supply/final board problems.

They were my most desired radios (after my old Collins S-line) when I once again entered the hobby-after a 20 something year long vacation.....and I keep wanting to resurrect them-but my knowledge and most learning of anything new, stopped with the old tube rigs.  I keep trying....even though it is mostly by using "replace and plug in boards" methodology.  With "old timers" memory and eyesight, I appreciate all the pointers that so many have put forth.

 
« Last Edit: November 03, 2017, 08:01:47 PM by N6QWP » Logged
K8AXW
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« Reply #5 on: November 03, 2017, 08:46:00 PM »

I worked on a 930 (40) memory failing now....anyhow they owner didn't want to roll the dice for $99.00 for a then existing new Kenwood power supply.  Long story short a friend wound up with the rig and he simply obtained a 28VDC power supply, wired it and is now using it.
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KM1H
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« Reply #6 on: November 04, 2017, 06:55:04 AM »

Ive rebuilt dozens of 930 and 940 AVR boards for local hams since the 80's and the POS ELNA electrolytics are a prime cause of failure; Fleabay has PS cap kits listed.
I also use 5W zeners since their current size is about the same as the lower rated ones.

I still have a 940 in the second station and another for backup for it and the TS-950SD at the main station right next to it. Never had any problems with the AVR since rebuilt and the drivers/finals are the originals, both are 8M series.
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WA1RNE
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« Reply #7 on: November 04, 2017, 08:07:19 AM »

Why not just troubleshoot the 28 volt supply?

The parts are all run of the mill - i.e. pass transistors are 2N5885's which can be replaced with 2N5886's.

The circuit is pretty simple. I would pull the pass transistors and check them with an ohmmeter. While you have them out, disconnect the AVR board and check the raw DC supply, including inspection of the 22000 uF electrolytics.

After that check the diodes, zeners and Q1 and Q2 with an ohmmeter. 

 Isolate the PA stage and run a test on the 28 volts supply.

 Once the supply is up and running - and assuming there's no obvious damage to the PA, try reconnecting the 28 volt supply and check the transmitter.


 WA1RNE
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N6QWP
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Posts: 228




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« Reply #8 on: November 04, 2017, 08:34:11 AM »

Thanx again.  This one is an 7,07x,xxxx serial number.  Will try gathering the aforementioned parts and rebuild the power supply and AVR board.

In the meantime, might also attempt hooking up generator and external supply to see if I can find anything obvious in the final units (have purchased two others in the hopes that one might work)?

Receiver still works, so I guess that the sections that supply those voltages are still working.  Have referenced some of the suggested articles on the power supply.  Some are pretty complex rebuilding projects.  Did like the idea of a "crowbar" to save future final units.  Perhaps just replacing the electrolytics, pass transistors and zeners might be enough to get that circuit going to test the final units?

Am still left with the problem of how to properly adjust the bias on the drivers if they are all blown (on one of the 3 final units) without a scope and the required attendant knowledge.  So, that will be the deciding factor.  

Has anyone else had a 930 or 940 that blew up the power supply (resulting in 40 vdc to the final unit) that DID NOT BLOW UP THE DRIVERS Huh  If so, what other part (s) saved them Huh
« Last Edit: November 04, 2017, 08:56:04 AM by N6QWP » Logged
K8AXW
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« Reply #9 on: November 04, 2017, 10:52:56 AM »

Forgot to mention..... I did trouble shoot the power supply.  Several traces were burned off the board completely and I then tried to rewire the power supply bypassing the burned traces.  It finally wound up being a write-off!
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N6QWP
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« Reply #10 on: November 04, 2017, 11:24:14 AM »

K8AXW, you mentioned earlier that you had experiences with 40 vdc to the final units in 940's (and 930's?) where the finals never did blow....how about the drivers Huh  Still wondering what are my chances that they might still be good on at least one of the three final units that I have?

50/50?....0?  Any hope, or are they most always the first thing to go?

Will pull the AVR board and see how it looks.  Was it the just AVR board or the whole radio that ended up being a rightoff?
« Last Edit: November 04, 2017, 11:46:18 AM by N6QWP » Logged
K8AXW
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« Reply #11 on: November 04, 2017, 10:08:51 PM »

OWP:  To clarify, I had experience with a 930 power supply CATASTROPIC failure.  What happened to the output voltage, I have no idea!  It could have gone to zero....or 40VDC.  I have no idea and apologize to you if I led you to believe that the voltage did go UP.

The only thing(s) you can do is fix the power supply and go from there.  You might just wind up parting it out.
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KM1H
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« Reply #12 on: November 05, 2017, 08:31:02 AM »

Quote
Forgot to mention..... I did trouble shoot the power supply.  Several traces were burned off the board completely and I then tried to rewire the power supply bypassing the burned traces.  It finally wound up being a write-off!

That is why I stocked up on various colors and gauges of Teflon wire from the local industrial surplus recycler decades ago.
One of my 940's is wired that way as I didnt want to return the AVR to the customer Grin

I use that wire for many projects.
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N6QWP
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« Reply #13 on: November 05, 2017, 03:06:04 PM »

 Embarrassed Embarrassed OK, HERE'S WHERE MY "OLD TIMERS" REALLY BECOMES EVIDENT.  JUST REALIZED THAT MY DVM WAS READING INCORRECTLY DUE TO LOW VOLTAGE ON THE INTERNAL BATTERY!!!

THIS ENTIRE TREAD IS GREAT INFORMATION ON A 930 OR 940 THAT HAS A BLOWN POWER SUPPLY! (Which my old 930 does indeed have).

After replacing the battery, the 940 output voltage of the power supply and AVR is 29 volts!!  That agrees with the panel meter and explains why it did read 28-29 vdc.

THANK YOU ALL FOR THE ANSWERS TO THE SCENARIO THAT I RAISED HERE.  IT WILL CERTAINLY BE USEFUL INFORMATION TO ALL THAT ENCOUNTER SUCH A PROBLEM.....AND TO MYSELF, WHEN I GET BACK TO MY OLD 930.

« Last Edit: November 05, 2017, 03:11:44 PM by N6QWP » Logged
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