I wanted to bring this Thread up again mainly to say thanks to everyone that replied to my original post and to let all those that DID reply know what the final conclusion was.

What I found, that was killing the MOSFET on the Soft Key Board, was a bad Band Switch. When I first acquired this amp it was filthy dirty; the result of many years of service in the shack of a SMOKER. It was so bad that when I pulled the top off the RF box the Tar, from the smoke, was running down the inside of the walls. Of course the band switch was also covered with it. I may have actually made the problem worse when I tried to clean the switch's wafers with Tuner Cleaner and Acetone. The smoke was cooked on and Acetone was the only thing I could find that did not mess up the metal and removed the Tar. Anyway, when I finally got the wafer out (the one that switches the inductors in and out) I found that there was a heavy accumulation of crap (smoke, dirt, etc.) under the wires, where they were soldered into the lug on the wafer, and around and in between the last solder lug and the screw / bolt that holds the Band Switch together. This is at the very bottom of the wafer just above the hole where a wire comes through from the underside of the chassis. If you are familiar with this amp you will know the spot. Anyway, it must have been shorting from the last contact over to the screw / bolt because it is grounded. I am not sure why this would mess up the Soft Key but it did. What made this problem so difficult to find was because it was so intermittent and because the sizzling / crackling sound, I was hearing, was coming out through the hole and made it sound like the sizzling was originating from the underside of the Chassis.

At this time I have not reinstalled the Soft Key because I am waiting, and testing, to make sure the short IS actually gone and it is not faking me out. But so far I have tuned up on all the bands and I have had a couple of QSOs with it and have not had any problem with it shorting since I changed the Band Switch Wafer.  So if it holds out I will reinstall the Soft Key soon.

Just wanted to say thanks to all those that responded and to let everyone, that is interested, know what the issue was so that perhaps they can help someone else that is having the same issue.

Dan KI4AX

Allen: Well, I might agree with you, that the sizzling is coming from the cap before it explodes, except the sizzling occurs, and I experience the loss of Idle Plate Current, even when I take the Soft Key Board out of the amp all together and wire it back to original. What ever the problem is it is with the amp and not the Soft Key Board. I have checked all of the Diodes (3 total on the Soft Key Board) for correct installation and they are all in the correct way. That is unless Harbach's Assembly Instructions are incorrect. And I have measured the Filament voltage at the location where I am pulling it from, for the Soft Key Board, which is the Transformer side of the Filament Choke. It is a little high at 3.5 volts but not outside the capacity of the 10v cap on the Soft Key Board. However, I can not attest to the voltage it receives during the sizzle. So far I have not been able to catch the thing sizzling while I have my DVM hooked up to it. Its almost like it knows I'm watching but I know it is just an inanimate object.

The sizzle, as best i can describe it, sounds like the sound made when you hook the jumper cables up to your car battery and it sparks a little. Its sounds like a DC short / sizzle to me but what do I know.

W1QJ: There are no dumb questions... I am open to any suggestions that you may have. That's why I am here... maybe someone can think of something I have not.

Thanks again....

Dan KI4AX

Allen:

Well, this is what I have been trying to do is to increase my skills and knowledge by working on stuff like this amp.

Yes, both 20uf and 2uf electrolytic caps have been replaced. I suppose it is possible that the 2uf might be installed with the polarity reversed but it has not swelled or exploded and the Soft Key has worked at times until the MOSFET goes bad. Also, I have replaced the relay and the parallel resistor with the Harbach relay and resistor. This did NOT help the situation. The problem existed with the old relay and resistor and the problem still exists with the new relay and resistor. So I do not think the relay and / or resistor is the problem. I suppose stranger things have happened but I just don't think this is the case. The relay is not buzzing and even after the resistance of the MOSFET rises to the point that it affects the Bias Voltage and conduction of the tubes... it still pulls the relay in and there is no buzz. I even sent the bad Soft key back to Harbach, who has a test jig that they used to test my Soft Key, and they said there was nothing wrong with it... but that's because all their test does is see if it pulls the relay in. It does not tell them anything about the Bias Circuit.

Here is the real kicker about this whole problem.... the Cap that explodes is in the input circuit on one side of an opto-coupler device and the MOSFET that keeps going out is on the other side of the opto-coupler and are not connected together electrically. Anyone that would like to see the drawing for the Soft Key I would be happy to furnish the file. I can also send the Manual for the amp which, of course, includes the drawing for the amp.

As always... I appreciate any and all comments.

Dan KI4AX

Thanks for all the replies. What ever is causing the problem it is NOT the Harbach Soft Key. The Soft Key going out is a result of the problem. I have repaired the Soft Key several times only to see the Plate Idle Current taken out by the problem taking the Soft Key out. I have actually watched the Idle Plate Current fall to nothing while hearing the sizzle. As long as the sizzle does not occur the Soft Key will remain working. Once I hear the sizzle it is only a few seconds and the the Soft Key fails and takes the Idle Plate Current with it. When the MOSFET, on the Soft Key Board, is subjected to the extra voltage (or what ever is going on) its internal resistance rises and introduces that resistance into the Grid Bias Circuit because that is where the MOSFET is in the circuit. Also, this problem persists even without the Soft Key Board even installed. And when it happens, without the Soft Key Board installed, the result is the same.... no Idle Plate Current along with reduced or no output; it affects both Grid and Plate Current during TX. I have tried looking for it in the dark, I have replaced most of the wires in the amp (actually found a couple that were melting together), I have been through the Grid Bias Circuit forward and backward, I have replaced all the resistors, wires, relay, and anything else that I could find in THAT circuit to no avail. I have cleaned the band switch, I have cleaned both Air Variable Caps, I have replaced the Coax Connectors, I have done everything I can possibly think of except replace the tubes, transformer, and band switch. What would really help me the most is if it would start doing it and not stop. That's the real crux of the situation here... its intermittent and only happens when I am not looking and stops when I look. If it would just start smoking or some component would go out or something.

With a B+ voltage, for the plate, of about 2350 volts wouldn't a short in that circuit be pretty explosive and show up pretty quick?

I appreciate the comments and suggestions.

Dan KI4AX

Yes, I have installed the Harbach Power Supply Module and the Harbach Soft Start Kit. I had installed the Harbach Soft Key Module, also, but had to remove it from the amp because this short (if that's what it is) keeps damaging it. Or at least I think that is what is damaging it. When the short is NOT occurring every thing is fine and the Soft Key works fine. But, as soon as the short occurs the Soft Key stop working correctly. It continues to key the amp but the MOSFET, on the Harbach Board, begins to introduce resistance into the Bias Circuit, of the amp, and kills the Idle Plate Current. But the problem also affects the Idle Plate Current with out the Soft Key in the circuit. It is very intermittent and so I have not been able to locate the source. I can hear it but I can't see it (so far).

Thanks for your reply.

Dan KI4AX

I have an SB-200 that I have been refurbishing. When trying to tune up on various bands into a dummy load I am experiencing a loss of Plate Current and Grid Current. The loss of Plate and Grid Currents are always accompanied by a faint sizzle or frying sound. This also occurs, sometimes, when checking Idle Plate Current and causes a loss of Idle Plate Current when it occurs. I am not overdriving because I am, when I drive it, only driving with 20 to 30 watts; when checking Idle Plate Current I am not driving the amp at all.  I have replaced almost all the wires in the harness thinking that there was a short in the harness but that did not fix the problem. I am thinking that I am getting a short between the plates of the LOAD capacitor.... would this short account for the loss of Plate and Grid Currents? Anyone have any ideas on where to look? I have been watching the underside of the chassis for the short but can not see anything.... even in the dark (hi,hi). Oh, by the way, this short is very intermittent which makes finding this problem even more difficult.

Thanks
Dan KI4AX

Thanks to everyone for your comments. I realize I have made a mistake which may have some bearing on the situation. I purchased a piece of 1-1/2" Electrical Conduit to use as a mast where as I should have purchased 1-3/4" Galvanized Water Pipe. The reason I chose the Conduit is because it was lighter (thinner wall) and I did not want to load up the rotor with 40 or 50 LBS of dead weight. But I guess if the Water Pipe fits better the downward amount of weight is not as critical. I am not sure what the MAX down load is on the CD 45-II rotor... I could not seem to find that info in the Manual. But the main thing is to reduce the lateral load on the rotor so that I can get up to the 8.5 Sq. Ft. rating on the rotor. Not being able to achieve the higher rating on the rotor will cause me to have to use a bigger rotor. So I think I will take the conduit back and purchase a piece of water pipe and see how that fits. Perhaps it will fit sufficiently and I can use that as my mast.

Thanks again for all the comments.....

Dan KI4AX

I am sure many of you have answered this question in the past either for your self or someone else but I have to ask again because I don't know.

I am getting ready to go up with 40' of Rohn Tower, a Wilson System 33 Tri-Bander (similar to Mosley's TA-33), and a 22 Element 2 Meter Beam (don't know the exact specs). I plan to use a 10' piece of 1-3/4 Galvanized Electrical Conduit for the mast. The rotor is a CD 45-II. According to the specs on the rotor I can have a maximum of 8.5 sq. ft. of antenna with a Thrust Bearing. The Wilson Tri-Bander is 5.7 sq. ft. and I am sure the 2m beam is not more than 2.8 sq. ft. But without a Thrust Bearing the rotor is only good for 5 sq. ft. I plan to put about 6' of mast above the tower and about 4' down into the tower with the rotor at the bottom.

The Rohn 25g tower that I have includes a tapered top section with a 2" O.D. tube at the top. My question is really about the top section. Does the tapered top and the tube take the place of a thrust bearing and bearing plate? Can I depend on the tube at the top to take the lateral forces? Or should I get rid of the tapered top section and start looking for a Thrust Bearing Plate and Bearing?

Thanks

Dan KI4AX

My internet security software has been detecting a lot of port scans on this web site. I think it has been hacked.

Good luck and keep posting the results as we are all interested in finding out the cause of your RFI. I experience a very similar RFI at my QTH and I believe it is being propagated here via the ionosphere. I can usually hear it best on 15 and 20 meters and appears at the same time as the propagation does on those bands. Is the RFI at your QTH present all the time? Or does it come and go with the propagation and/ or day and night and/ or different weather conditions (high/low humidity or wet/ dry conditions)? Does anyone else experience interference that seems to come and go with the propagation at your QTH?

I have a similar interference on HF. Mine is loudest on 20 meters. It shows up on 15 meters also but is not loud enough to be a bother. The RFI noise I am experiencing seems to be propagated to my QTH since it comes and goes with the propagation and its strength varies like a propagated signal. It almost sounds like 60Hz a.c. Like a buzzing or grinding noise about every 15KHz. It is not always present but is always present, to some extent, when the band is open. I have my doubts as to whether I will ever find a solution. Any one with any suggestions? Anyone know if RFI can be propagated by the ionosphere?

Apparently there is no one here familiar with Kenwood Paddle Switches or the mod. that I would like to perform. Thanks for all the answers to the questions that I did not ask and also for all the opinions that I also did not ask for.

73 Dan KI4AX

Beam me up Mr. Scott!

KE3WD: Well, I have the top and bottom covers off and I have my flashlight trying to "see" the back of the switch. There is a meter above, vox controls and frame to the right, circuit boards, wires, and frame, to the rear, microphone socket, mode switch, and JJY/WWV switch below, and the VFO housing to the right. Not to mention all the wires. Perhaps you are the Great Oz and you can pull some magic lever, there behind the curtain, that will just make all this stuff disappear. There is so much stuff in the way that I can barely see the back of the switch, let alone, get my probes in there to check the operation of the switch. But for the Great Oz it is not difficult.

The relay idea is probably the best solution for what I would like to do provided I could locate one for three bucks, and locate a place to put it. A better solution would be to find a thermistor temperature control and have the fan come on only when high temperature calls for it. You know...like the modern rigs.

Just for giggles guys... go see if you can find a replacement fan for a 40 year old TS-520 and see how much you pay (make sure you include your shipping). Then go see if you can find 6146 tubes and how much they cost. There is a crap load of tubes out there but almost no fans. Do you think I want to preserve my tubes, which are very easy to replace, or do you think I should try to preserve the fan which is very difficult to replace because there are no new ones and very few used ones? In another 20 or 30 years how many fans will there be and how much do you think they will cost? Fans are mechanical with bearings and windings and moving parts. Some day no matter how clean or how well lubed you keep the fan it WILL wear out. There are guys now with fans that are wearing out. But shame on me for trying to perform a modification that might preserve my 40 year old fan. So I must now ask... Who is the Elmer here?

Dan KI4AX
 

KE3WD: Well if I wanted to take the rig apart and just test the switch with my ohm meter I would have just done it. I've tested switches dozens of times and have no problem using my ohm meter. The reason I am here asking people, that supposedly know more than me, what they know about the switch is because it is very difficult to access the back of the switch to test it. As I said in one of my previous posts I am trying to avoid removing all the knobs, windows, front panel, and finally the switch just to "test" it.

Also, I do not see the "sin" in asking questions about the switch or the mod for that matter. Is this not how humans "learn?" By asking questions? And where would this hobby be if it were not for people that asked questions and performed mods? I was under the impression, from the knowledgeable people I have talked to, that a good portion of the features in our modern rigs were ideas that came from people that asked questions and made modifications. Some people begin to mod their rigs the first day they get them. That's why there are mods for rigs, sometimes, before they are even available on the market. Many of these mods make what the engineers did better and makes the rigs more user friendly. As humans, in general, this is what we do.

As far as "this is a bad idea" and that "I am barking up the wrong tree"... I am not trying to "convince" my self of anything. I am trying to gather information about the mod and the switch before I just go off "half-cocked" and take everything apart and go at it. I have had that "valuable learning experience" you talk about...that's why I am here asking questions. So that I don't do the "half-cocked" thing. Been there and done that. I suppose that when you get ready to put up a new antenna or even buy a new rig that you don't ask questions? I suppose if I have to ask a question about my new antenna that I should not even attempt to put it up... I should just box it up and send it back? Neither am I trying defend a bad idea...I am trying to gather more information. My questions about why there is no warning in the manuals for hybrid rigs is a valid one. If residual heat was detrimental to the rig don't you think Kenwood would have warned people about it?

Perhaps you should make a post here, on e-ham, warning people about the residual heat in hybrid rigs and suggest to the hundreds of thousands of hams, with hybrid rigs (who have had no problem with residual heat), that they start running their fans for five or ten minutes after a qso before shutting down. Perhaps all us hybrid users should post a warning sign above our rigs?

This is exactly what I don't like about e-ham's forum... you ask a question and you get all kinds of answers except the answer to the question that you asked.

Dan KI4AX

The reason for my post was to find someone that is familiar with Kenwood Paddle Switches and that could give me more information about the switch. Other than a picture of a paddle switch in the TS-520 service manual there is no information about the switch out there that I can find. The schematic only shows part of the switch and does not show the unused contacts. My assumption would be that the switch is a triple pole single throw... but that is an assumption and you know what they say about assuming things. I would just like to confirm what I am doing before I just tear into it and do it. What sense would it make to take all the knobs off, take the front panel off, and take the switch out so I can get at the terminals only to find out that the switch will not do what I need it to do?

Dan KI4AX