Help understanding TX stage

General discussion and support for the Peaberry SDR V2.

Re: Help understanding TX stage

Postby mrwizardly » Tue May 19, 2015 3:43 am

Replaced the FETs and still low output.
I sure wish there were some sort of level measurements to help find out where I'm losing the drive.
I'm afraid that without a bit more to go on this may be an elephant radio (all ears, no mouth)

Has anyone gone through and made some sort of list of what should appear where as far as the input to the PA stage?
I'm getting all the correct biases, and Q4, Q5, Q6 & Q9 are switching correctly.
Pins 7 & 8 of U8 are outputting the proper frequency at I think 2v p-p.
What should appear at the collector of Q9?

This is a case where measuring something changes it. I have a 100MHz scope with very high input but don't know what it should look like (collector Q9)

Any help or suggestions from a working model would be greatly appreciated.

From the previous thread you can see I thought I blew the FETs. Now I'm not so sure.
Mark
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Re: Help understanding TX stage

Postby pilotcfii » Tue May 19, 2015 2:45 pm

I had a similar problem sometime back. David's suggestion to me was to download his CW app, install a key and see what a power meter says. I did that while the PB was connected to a dummy load and voila! I had power. Then it's figuring out how to produce that power using your favorite modulation method. For me it was getting WSJTX connected.
73
Dan Malcolm
K4SHQ CFI/II
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Re: Help understanding TX stage

Postby ks1h » Tue May 19, 2015 4:53 pm

Make sure your tune freq is different from the LO freq ... You will get no output if they are the same.

73, Ken KS1H
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Re: Help understanding TX stage

Postby mrwizardly » Tue May 19, 2015 6:35 pm

Thanks for the suggestions.
Doesn't the CW mode with pressing the "ENTER" key produce the same result?
I'll give it a whirl.

The issue seems to be some drive problem. It 'was' working on FM because I could see 1.3 Watts on the power meter which was suggested earlier in this thread.
Then the board slipped, made contact with my metal rack and PFFFFT!

I replaced the FETS. Only get a signal into my Icom rcvr as if it's only transmitting with the exciter. No output with my 50ohm dummy load. It should be about 20v as stated earlier in this thread.

So, I've enlisted some help from a fellow ham to give it a second pair of eyes. My brother who used to work in the satellite/microwave business with SMT parts and very low power.
We'll see.
Sure would be nice to have some 'expected' measurements either in a list or screen dumps from a scope. That would get me in the ballpark pretty quick.
Mark
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Re: Help understanding TX stage

Postby mrwizardly » Tue May 19, 2015 7:45 pm

What should be the P-P drive voltage at the collector of Q9?
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Re: Help understanding TX stage

Postby W4MMP » Tue May 19, 2015 9:09 pm

Mark,
I will get back to you soon with some of the details you are looking for as I need to trouble shoot one of my Peaberrys.

But to begin to answer your question, I need to know what bands your rig is built for. The values are different for every band. The gain is not linear when moving across bands. The two lower bands of the Peaberry will have more gain than the upper two bands.

I have 80/75/60/40 and 30/20/17/15 rigs. What is yours?
73
Ron / W4MMP
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Re: Help understanding TX stage

Postby mrwizardly » Tue May 19, 2015 9:19 pm

Thanks, Ron
I built mine for 60/40 30/20
I have checked one of the FETs that I swapped out and it indeed had punctured the gate. Had about 3M ohm from S->D
The other two seemed to be OK.
Having swapped all three (as suggested) you would think the output would return.
Anyway, thanks...I await your response.
Mark
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Re: Help understanding TX stage

Postby mrwizardly » Tue May 19, 2015 9:27 pm

With my 5watt dummy load (50.2 ohms) on 7.076Mhz I get 4V p-p CW mode. It is the correct frequency on the scope also.
14.076Mhz = 1.5V p-p
This is for reference for when you start.
Mark
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Re: Help understanding TX stage

Postby W4MMP » Tue May 19, 2015 10:18 pm

Mark,

With my 30/20/17/15 on 20Meters (14.076):

Q9: Base -> 4V P-P
Q9: Collector -> 12V P-P

Your absolute voltage levels may be different but the important point is that you see about a gain factor of 3 to 4 across Q9

Q7 or Q8: Drain -> 20V P-P

You mileage may vary due to a different band build.
73
Ron / W4MMP
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Re: Help understanding TX stage

Postby mrwizardly » Wed May 20, 2015 12:41 am

I'm definitely not getting your measured on Q9 pins.
Go back one step further:
What should I be seeing out of U8: pins: 7, 9 ?
I see the proper DC bias of 2v on the center tap between 4,5 of T3.

At this pint I think I'm trying to find what failed. The resisters/capacitors are probably bullet proof.

I could have 'hurt' U8, if it's output doesn't come close to yours.
Thanks, Ron
Mark
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