[AMPS] Blown TL922A... What to do?

[Tom Rauch]

> Has anyone been able to measure the frequency of oscillation in, for
> example, TL922? What dip frequencies are found with a GDO in a 922? In
> other amps?


I'm missing something in the above Steve.

Tell me how a GDO can be used to measure the mode of 
resonance? How do you know the impedance is high, instead of 
low? How do you know what you are measuring in a complex 
system, since it has no directional ports?

I can dip my amplifier tank on 1.8 MHz. Does that mean it 
oscillates there also?

These are serious questions, don't take them as flip remarks. I'd 
like to know those answers.
 
> >Of course you nee a real amp to measure. What amplifier do you
> >have available? If you have one that I have, we can go through the
> >measurements and compare results on here.
> 
> 
> I don't. I've never mastered a straight key, so I've not been able to pass
> the code tests to get on hf over here. A degree and 25 years designing rf
> for a living doesn't count. I'm hoping to sidestep the system by taking
> Extra class exams next month.

OK, so you can't measure a PA...and haven't measured one. 
Without doing that, on a device that measures impedance of 
coupling between ports, how do you know all this stability 
handwaving is true?
 

> For sure, if you have a low impedance at a given point and frequency, it
> is difficult to generate high voltage there, and if you mistune the output
> network you can get a high impedance and lots of volts at the tune C.
> That's simple fact.

You can get MANY times the normal AC voltage.

As a matter of fact, underloaded tanks are commonly used in HV 
supplies. TV sets use a "flyback" transformer that steps up voltage 
far beyond the actual turns ratio, because it is an "underloaded 
tank". 

Your car uses a similar principle to generate the ignition pulse.

Remove the capacitor from an ignition coil, or the capacitance 
resonating a flyback transformer, and the voltage will drop like a 
rock to the normal voltage expected using the turns (impedance) 
ratio. The same thing happens in a tank, and the TUBE is 
subjected to those peak voltages just like the bandswitch.

I can make virtually any HF amplifier arc and damage something by 
mistuning or having the correct load fault/exciter transient.

I can't make many of them arc by removing parasitic suppression, 
and I've never been able to make a bandswitch arc that way unless 
the PA oscillated at the fundamental frequency, or near it. 
 
> There's a few types of amp that crop up regularly here with problems, such
> as TL922. Anyone near Newbury got one I can poke around inside?
> 
> Steve

Try this test. Excite the anode with a VHF signal source, and move 
downstream through the anode and tank system measuring 
voltages.

I'll send you $100 if you find a commercial HF amplifier that steps 
the anode voltage UP as you move through the tank.

I'm sure we will all agree that if, when the anode is excited by a 
signal source at the frequency where the alleged "parasitic" 
occurs, in order to cause an arc downstream, the voltage would 
have to be stepped UP.

It would take about 500 dollars to buy brand new equipment that 
would measure this effect, most people already have the equipment 
sitting in there room.

Why, when such a measurement is so easy, haven't more people 
made it?

That one simple measurement conclusively proves (I've made it) the 
claims tank components arc from VHF parasitics on frequencies 
far removed from the tank resonance is wrong.


 


73, Tom W8JI
w8ji@contesting.com

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