The AM Forum

I've just purchased a nice General Radio mod monitor. I need some advice on the best way to pickup the signal from my GK400B. Thanks in advance.

Mike

http://amfone.net/Amforum/index.php?topic=26093.25

Thank you Steve. Just what I was looking for.

Mike

Here's the one I built recently, Mike. It's based on one of those 'so easy, a caveman can do it' descriptions/articles that Tom/JJ posted here a few years back. The kind that causes you to smack your forehead and say 'duh!' because it's so obvious.

I had a few of those old Alpha-Delta Transi-Traps sitting around unused, one made an excellent donor box. Just ran a piece of RG-213 center conductor between the connectors, and added a BNC on the side with the pickup wire, which I left insulated on the ends out of paranoia. A push cap covers the the hole where the insulated ground stud from the Transi-Trap was mounted. It will come in handy as a wall mounting point eventually.

This replaces the coax wrapped wire ala Johnny Novice that I'd been using previously. Though I was expecting a HUGE difference with the new pickup, it was equal to or a bit less than the wrapped wire. It does clean up the station a bit since the wire setup involved a QIX-style clip lead wire into a PL-259 into a BNC adapter for the scope.  ;D

At some point I want to hook it to the GR-1931A since they're just so cool to watch. Your GK should give it a good workout.

Capacitive couplings don't seem to stay linear with respect to the carrier level on a 1931A.

They take a lot of oomph to drive as well.

Invariable, BC transmitters place a RF inductor across the output connector and ground, with a slider to divide the voltage given to the sampling tap.

I'd try that first, all you need is a high enough inductance to not load the line.  A small roller type is really handy for that job.

73DG

(retired after 45 years of pushing BC rigs)

I was going to post that, Glad you beat me to it.  You need a direct connection to the tank or balanced tuner. I had some luck with wrapping insulated wire around the tank. But in the end, A tapped coil (thanks dennis) is the way to go. 

If you plan to use a coupler box like that, Then you will need an amplifier to boost the signal. 

C

Linear? How so? Do you mean the mod level reading will be incorrect?

Dennis,

Do you have a guess on the specs of the inductor I should try?

Mike

The G/R input network will react with the capacity inline to show carrier shift and peak errors.

I found this out the hard way years ago with a Sparta 1kW TX.

Typical coils were phenolic-wound, about 2' dia. by 6" long, 20 ga, spaced the same as the wire dia.

PS...Sparta broadcast...Greek for sh*t...the total perversion of the Fritz Bauer legacy.

73DG

Well, I guess I'm back to being confused on what to do.  ??? ???

Sorry, but what is confusing you? ???

73DG

A resistive voltage divider ( at least 20 x operating impedance ) will be close to flat with frequency.  Of course you will need some reasonably stout resistors.  Another good way is a toroidal current transformer with enough secondary turns.  If you want that to be flat with frequency you'll need to shield the current carrying wire that passes through the center, leaving it unconnected on one side.


Chris

 

OK, clear now. I guess the input Z on the GR is not 50 Ohms.

That is a good enough way to look at it.  I've never measured it, but it probably pretty low in the impedance department.

It is peculiar how so many are enamored at high source impedance, capacitively coupled samples of RF from their transmitters.  Maybe for O-scopes?

The coil deal is really simple, infinitely adjustable, and works well with a scope, too.

73DG

That's what I built mine for Dennis, and what Tom wrote the original article for. They're quick & easy, better than nothing. Wasn't really intended for something like the 1931A, I just wanna see those meters bounce around.  ;D


Do you have a favorite circuit handy or something you could post? Would be good to add it to the other technical goodies here or on the AMWindow site for future use.

A circuit would be a great help. I'm the new guy and still trying to learn.

Mike

This works.  The spreadsheet gives you the values.  Construct it the way you wish.  My version is the lazy man's version using a UHF tee.

Last time I posted this I got some qrm about capacitive stuff making the circuit innacurate - I have been using this approach for decades - it works - and I'm using Allen Bradley resistors to boot.

No guessing about power needed for resistor - R1 - the dropping resistor.  Make sure you are using a 50 ohm load as shown as that completes the voltage divider. Failure to do so will result in possible maximum voltage as it is developed at the point of sampling.

As of late, I have discontinued using this approach in favor of a Bird sampler that has a variable adjustment and is rated for 5000 watts PEP.  I like the feature of being able to adjust the carrier for 4 divsions, the max being 8 on my scope which allows me an easy method of watching my AM transmissions.

Don't mess with the green fields as those contain the math that makes this spreadsheet work.  Input the yellow fields only.
Al

Chris,

Maybe I'm overthinking this. I'm going to try your method of using a toroidal current transformer. That way I can add or subtract the number of windings that I need by trial and error. Also received a lot of great ideas on O scope samplers.

Thanks,
Mike

I'll be glad to post a schematic, but I am out of town for a week and cannot so it where I am at.  Hang loose and I'll get you something.

73DG