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   Home   Help Search  
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Author Topic: SB-200 Need Help  (Read 10757 times)

Posts: 113

« Reply #15 on: September 18, 2012, 02:04:00 PM »

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....

« Last Edit: September 18, 2012, 02:07:55 PM by KI4AX » Logged

Posts: 4380

« Reply #16 on: September 18, 2012, 03:30:16 PM »

My last thought is that you are hearing an arc inside one or more of the tubes.

I have had one 572B fail this way.  If you have a known good set try them.
If not try keying with the plates disconnected. 

As someone suggested earlier.  Look at the tubes in a darkened room (of course the
filaments will light things up some.

Good Luck

Posts: 960

« Reply #17 on: September 18, 2012, 05:09:21 PM »

Would the amp be going into VHF oscillations?
Have you replaced any diodes for the relay circuit? The Relay power supply?? There's a cap in there that has positive going to ground (33@460V)
If "The cap that blows" is the 1000uf@10 then that diode might be installed backwards.
Be careful how you key this amp. I'm guessing you are grounding the 135 volts bias circuit to key the amp.
I see diagrams with and without the optical device
Here is one with:

This is an explanation of an SB200 without the optical device and the warning of the ANT RELAY high voltage.


Posts: 113

« Reply #18 on: October 12, 2012, 05:00:00 AM »

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.


Posts: 6760

« Reply #19 on: October 12, 2012, 09:20:42 AM »

Dan:  STOP right there!  Heath Co. made a modification to this area.  On the original design C26 was mounted with the "hot" side of the cap connected to the bandswitch right next to the bandswitch wafers mounting stud. 

Since this is a high voltage point the possibility for arcing here is very high.  Later Heath came out with C26 mounted in a different area whereas the "cold" side of capacitor is next to this wafer mounting stud.

If you send me an email I'll send you complete information on this including photos.

Al - K8AXW

A Pessimist is Never Disappointed!
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