self oscillation

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radium98
no manz can test innit
no manz can test innit
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self oscillation

Post by radium98 » Mon Apr 09, 2018 7:10 pm

Hello
this vk200 whent very hot and i cannot have more then 8watts i assume there is self oscillation .can you help me what is wrong even exciter off and when i turn the pot + the power raise funny whith exciter off
Please help me
the amp fitted with original rd15hvf1 13,8v and 2amp
output must be between 12 to 18watts i only have 7-8 with or without exciter and that vk200 is very hot how do i solve the problem
i have found only one new rd15hvf1 in my junk box i have fitted it with a large heatsink.And after power on i have only 7 watts and 1 of the vk200 that come directly from the drain heat and warm could be a self oscillation and what i have to do to increase the power
thanks .
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radium98
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Re: self oscillation

Post by radium98 » Tue Apr 10, 2018 6:49 pm

23 times view and no reply any help ?

Albert H
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Re: self oscillation

Post by Albert H » Tue Apr 10, 2018 8:25 pm

Do you know which stage is taking off?
He who laughs last, thinks slowest! :lol:

radium98
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Re: self oscillation

Post by radium98 » Wed Apr 11, 2018 10:32 am

what do you mean ? by wich stage is taking off .if i only power +vcc the PA the wattmeter raise with rf power ,exciter off or on never mind .

yellowbeard
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Re: self oscillation

Post by yellowbeard » Wed Apr 11, 2018 6:56 pm

Try find the frequency it is oscillating at, or if it is producing RF on a wide spread of frequencies - the first one is parasitic and would require changing component values, the second is feedback and would require backing off the gain and/or screening the stages from each other. Disconnect power to the stages one at a time and see if the problem goes away - that would help home in on the problem, see if any setting of the gain adjust preset makes a difference. I can see a couple of capacitors that are not soldered to the top copper plane, the 1n under the BFR91 and the 10n at the bottom left for instance. I have played with the PKBEE amplifier that is similar, it had inductors on the collectors of all the transistors though... Theres a couple of ideas for you, let us know what happens.
:tup

radium98
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no manz can test innit
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Re: self oscillation

Post by radium98 » Wed Apr 11, 2018 8:31 pm

30m6fdw.jpg
Thank you yellowbeard i will try one by one all the ideas given by you to me to dig and diagnose the problem .but i assume that the problem is around the near drain capacitor of the decoupling and or the electrolityc .and i have seen two same amp that work perfectly given more than 13watts.and one of them had the drain choke removed and replaced by some turns coils and what that differ ?
And indont have there diameter.
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radium98
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Re: self oscillation

Post by radium98 » Sat Apr 14, 2018 8:45 pm

what does make self oscillation ,i have made the coil like this last picture still the same,amplifier output raise to 11 watts without any rf input only when adjusting by the 1k trimmer pot that make variable voltage for the two bfr and the bias of the rd

radium98
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Re: self oscillation

Post by radium98 » Thu Apr 19, 2018 4:19 pm

Any further help what i should do more ? to stop the self oscillation.Only at 1.5 volt at gate of the rd15 i have no oscillation but rf power out is about 300mw and drain voltage is 13 volts

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