Re: PA question

J68HZ
 

Well a couple of things… with no voltage on the gate of Q7 it should not be hot at all… it should not be conducting unless you are driving the pants off it.  Measure the voltage at the gate pot fully CCW.  It should be zero.

 

BTW I ALWAYS test new amplifiers with a resistor in series with it on start-up.   That Transistor should be able to handle at least 3 amps at 12 volts, so  placing a 4 ohm resistor in series with the PA will limit it on initial start-up so no catastrophes occur.  When it checks out, you short out the resistor and forge ahead.  I usually have a couple of high-wattage 8 ohm resistors laying in my lab to use as loads for audio amps.  They work great for this (parallel two).

 

 

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

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Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com

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email:  bill@...

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of patrick.nairne@...
Sent: Sunday, February 26, 2017 7:02 PM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] PA question

 

Thanks for your helpful advice. Turns out I wired in the trifilar transformer wrongly. I think a top view picture would be helpful in the construction notes.

After removing the transformer and putting it in correctly I got an output on the PA. Around 1 watt on 24Mhz  and with R1 fully anticlockwise. Heatsink became hot quickly. 

On advancing clockwise the output didn’t increase but the pot started to smoke as did Q7, or at least the insulating pad did! I have a replacement 4.7k pot which I will fit tomorrow but I fear the RD15HVF1 may be cooked.

Any ideas why the bias adjustment didn’t appear to have any effect?

I’ll get there in the end.

 

Patrick M0AFZ

 

From: K9HZ

Sent: Sunday, February 26, 2017 2:19 AM

Subject: Re: [QRPLabs] PA question

 

Post the Drain and Source voltages for all of the semiconductors.  We can help diagnose it from there…

 

 

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com

Like us on Facebook! facebook icon

 

 

email:  bill@...

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of patrick.nairne@...
Sent: Saturday, February 25, 2017 11:52 AM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] PA question

 

Thanks Hans and William. I currently have a problem with no output, so have probably made a construction error! RF from the U3s is good 200mw but nothing from the PA and Q5 and Q7 are stone cold. No voltage seen on Q7 whatever the position of R1. I suspect I have made an error with the trifilar windings. All inter board connections are correct and the shaping set at 05 115. Time to do some checking.

 

Patrick M0AFZ

 

From: K9HZ

Sent: Saturday, February 25, 2017 5:36 PM

Subject: Re: [QRPLabs] PA question

 

I think its a good idea to start with the 10 ohm resistor in place.  You can always bypass it.   We probably need to start keeping track of all the modifications for different bands. Ill report in what i find optimal for 6M once I run all of the studies. 

 

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton - J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com

 

email:  bill@...

 


On Feb 25, 2017, at 4:45 AM, Hans Summers <hans.summers@...> wrote:

Hi Patrick

 

I haven't tried the RD15HVF1 myself and tested it's performance in different scenarios. I did hear yesterday from one PA kit constructor who used the RD15HVF1 for a 6m transmitter and is getting 5W output on 6m. I didn't ask if he did the the 10 ohms mod or not. I would think that it is probably good practice to include it. In testing with the IRF510 I found that the 10 ohm resistor is mostly not necessary and does slightly reduce the power output. But at certain frequencies and supply voltages (or I should say, modulator output voltages) there was some instability, that the 10 ohm resistor cured. Hence my recommendation to include it.

 

73 Hans G0UPL

 

On Feb 24, 2017 23:48, <patrick.nairne@...> wrote:

Hello all

I have completed the PA and have installed the RD15HVF1 instead of the supplied IRF510. Is it necessary to carry out the modification involving the 10 ohm resistor when using the RD15HVF1 ?

Thanks

Patrick M0AFZ

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