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Author Topic: SB200 "taking off"/self oscillating (I think)  (Read 4487 times)
VK5DO
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« on: July 17, 2012, 07:57:44 PM »

I recently bought a SB200 that I think is going into self oscillation.

On 80m, when I tune up with a CW signal at low power from my exciter, all is ok and the tune is fine until I wind up the exciter power to anything more than about 50w. At that point my swr/pwr meter suddenly goes full scale on both FWD and REV. I button off the exciter immediately and it stops ok. If I wind the power from the exciter down and key on again, all is well until I wind it up again. This all happens ONLY on 80m. 

Radio is an IC7000, Tuner is a homebrew T match that's built to take it and antenna is a horizontal loop fed with open wire. 

Now, I'm fairly new to QRO. Is this typical of an amp "taking off" and how do I stop/fix my SB200?

Yours

Dene

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W9PMZ
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« Reply #1 on: July 18, 2012, 02:55:06 AM »

self oscillation?  if this occured when you turned the amp on the oscillation would start then...

sounds to me that there is a feedback mechanism in the system.

check grounding and shielding etc...

try the exciter and amp into a load, if all is well that will pretty much confirm that there is a feedback mechanism when the exciter goes over 50w.

73,

carl - w9pmz
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KA5N
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Posts: 4380




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« Reply #2 on: July 18, 2012, 03:15:49 AM »

I suppose your shack is right in the middle of the loop.  Just "does" it on 80 meters.  Sounds
like you are feeding the signal back into the amp and the whole system is oscillating.  If you
reduce the drive to below 50 watts and the effect stops, that pretty much confirms it.  So
clean up your setup and once you stop the signal from the antenna feeding back into the amp
all should be rosy.
Good Luck
Allen KA5N
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W8GP
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Posts: 205




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« Reply #3 on: July 18, 2012, 06:47:23 AM »

What band is your loop resonant on? I'm going to guess that you probably have voltage breakdown and arcing in your tuner due to a high impedance at the tuner output. The fact that the problem goes away at reduced power supports this.Add 20 to 30' of ladder line and see how the system behaves then. How does the amp work into a dummy load?
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KE3WD
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« Reply #4 on: July 18, 2012, 12:17:06 PM »

You've got your own RF in the shack. 

73
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W1QJ
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« Reply #5 on: July 18, 2012, 12:51:34 PM »

You've got your own RF in the shack. 

73
I agree this is your problem!
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W8NF
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Posts: 53




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« Reply #6 on: July 20, 2012, 12:20:39 PM »

Start with the Dreaded 33 ohm resistors.  This is classic SB-200 behavior.  I have had several on the bench do this.  They don't oscillate simply by being keyed.  Start raising the RF input level and at some point they go bezerk.  My first clue about oscillation frequency (when I encountered the first unit) was that a nearby AM radio blanked when the power/SWR meter pegged.  But an FM radio was unfazed.  This is not a VHF oscillation, it's low frequency.  My spectrum analyzer showed about 300-500kHz on that unit.  I haven't measured the remainder.

Typically, these resistors will measure 100 ohms or more.  Replace with 33 ohms, as they should be, and this oscillation will cease.  IIRC the originals are 1 Watt, but in routine usage, they dissipate 2.5 watts variable.  I would not go lower than 2 watts, and would prefer 3 watts.  Probably doesn't need to be a bulk resistor.  I happened to have stock of 3W carbon comp 33 ohm resistors that I used for repairs but they're gone.  If I had another on the bench with this problem, I'd use OY series Ohmite ceramic resistors.

Dave W8NF

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WX7G
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« Reply #7 on: July 20, 2012, 04:04:44 PM »

It's the transceiver and not the amplifier.
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K4DPK
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« Reply #8 on: July 20, 2012, 06:10:39 PM »

It is noteworthy that your reflected power is pegging the meter, as well as the forward.

That means you have output on some frequency other than the intended operating frequency, and is indicative of the final oscillating at some frequency where the antenna is non-resonant.

I would change the coax between the exciter and the amp, and add a ground wire between these two devices to see if that has an effect.

Phil C. Sr.
k4dpk
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G3RZP
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Posts: 4543




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« Reply #9 on: July 21, 2012, 01:38:24 AM »

Start with a dummy load and see what happens with that.

If it still does it, you have a problem that good grounding may cure but is likely symptomatic of something wrong - possibly amp input SWR making the transceiver get upset.

If it doesn't, then you have too much RF in the shack. Ferrites on all the leads is then indicated.
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WX7G
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« Reply #10 on: July 23, 2012, 06:04:40 AM »

Here's how to tell if it is the exciter or the amp that's taking off. When it happens does the exciter power jump up? If so it's the exciter.
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KG7AMV
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« Reply #11 on: August 14, 2012, 08:18:43 PM »

Something like this Icom IC-7000 Fire in RX http://youtu.be/bAdDP5v1hVw
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KB4QAA
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Posts: 2367




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« Reply #12 on: August 14, 2012, 08:55:37 PM »

http://www.w8ji.com/hf_stability.htm

Tom has two section on amplifier stability including a test procedure.
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