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Old 11th September 2010, 07:46 PM   #4841
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With the RC compensation change, your diagram suggests removing the 33pF,1k2R between the C and b of Q3 and placing a 100pF 330R onto the base of Q8 to ground.
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Old 11th September 2010, 07:58 PM   #4842
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Yes deleting its original place and values. Nearest force ground point that is.
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Old 11th September 2010, 09:04 PM   #4843
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OK removed 33pF and 1k2R. Installed 100pF 330R between base of Q8 and force ground. Not much change. With Vin decoupling steady state oscillation, without it still oscillates but has more harmonic content. Oscillation sits right at 15MHz.
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Old 11th September 2010, 09:08 PM   #4844
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Swear time! Still at 80mV Pk-Pk?
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Old 11th September 2010, 09:19 PM   #4845
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Lets see if its in the CFP. We got to find its origin. If you make the 9140's gate stopper 60R what happens? If you take the error amp signal connection directly to the 550's base beyond its base stopper what happens?
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Old 11th September 2010, 09:21 PM   #4846
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~25mVrms / 80mVpp. Not swear time, this is interesting. I only swear when magic smoke is let out.

I have switched probes to my better one and verified results. Darn.

How about dropping gate resistance to the irfp9140?
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Old 11th September 2010, 09:22 PM   #4847
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Gues we are thinking in parallel//
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Old 11th September 2010, 09:29 PM   #4848
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Reducing gate stopper to 60R still oscillates at same frequency but Vpp is now 130mV and Vrms is 43mV. To me it means that source is before 9140 gate.

Will move error signal input directly to bc550 base now.
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Old 11th September 2010, 09:35 PM   #4849
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Something is at play in that particular part it shows. Maybe more stopping value.
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Old 11th September 2010, 09:39 PM   #4850
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Left gate stopper at 60R and bypassed base stopper on bc550(Q8) slight gain in magnitude same freq. Now 140mVpp and 45mVrms.

Would like to put gate and base stopper back to original and increase base stopper on Q3 which is the bc546B.

My reasoning is that with the increasing maginitude of oscillation when we reduce the stopper values further up the chain it means the oscillation is before that part. If is was that part we would see a drastic change or no change.
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Last edited by SGregory; 11th September 2010 at 09:43 PM.
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