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Junkbox project - 75M RF deck




 
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Author Topic: Junkbox project - 75M RF deck  (Read 1811 times)
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ka1tdq
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« on: October 21, 2018, 10:41:54 AM »

I put this together this morning from parts that I had in my junk box. The thought crossed my mind if I could build a PW RF deck from just stuff that I had laying around, and this is what I've come up with.

A 5763 tube oscillator driving a 7984 for 16 watts output. The loading cap is fixed at 1500pf and plate voltage is 275vdc. If you work backwards, the plate loading impedance is 1675 ohms.

The plate choke and the grid tank inductor are almost the same except for the fact that I ran out of wire winding the plate choke, but it should work ok. I'm not going to neutralize the 7984. At this power level and low frequency, it'll be fine. No metering, but I'll but an LED in the grid circuit to tune for grid current.

The only parts I need to buy are incidental small parts, like hardware and a few caps/resistors. I might put a PNP transistor in there for a transmit switch.

Aside from being a novelty piece for the shelf, a person could hi-fi modulate this and drive a linear.


Jon


* IMG_1414.jpg (2893.89 KB, 4032x3024 - viewed 181 times.)
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PA0NVD
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« Reply #1 on: October 21, 2018, 02:21:01 PM »

That's a fun project Jon. Like it. I made something similar, photos attached. It has an EL84 modulator and an EL84 final. AM modulation by a balanced output transformer for EL95, one side to the PA, other side to the modulator anode and center tap to the +HV. This to prevent iron saturation, so I could use a much smaller iron.
It has also a VFO
Unfortunately the customs in Puerto Limon, Costa Rica did open my container and stole it together with many, many other materials, thousands of dollars lost and things I never can recover. All my VHF stuf stolen, 2 metal storage cases with 120 small drawers and thousands of integrated circuits, my chain saw, a 1790 flintstone rifle inherited from my father, a box with watches of my wife, my antennas, the transmitter and many things more


* EL84 AM-CW zender 002.jpg (1431.58 KB, 2592x1944 - viewed 91 times.)

* EL84 AM-CW zender 001.jpg (1227.01 KB, 2592x1944 - viewed 46 times.)

* EL84 AM-CW zender 004.jpg (1348.35 KB, 2592x1944 - viewed 74 times.)
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ka1tdq
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« Reply #2 on: October 21, 2018, 04:56:14 PM »

Very nice indeed! It's fun to make stuff out of copper. It looks all nice and shiny, at least for a little while.

I did some work on mine today. Here's a few pictures.

For a while, I was asking everybody at work to give me the toilet paper rolls rather than throw them out. They're the nice thick cardboard. I finally put them to use! Just in time too, because after I had collected a bunch the manufacturer switched to a cardboard-less center. My guess is to be more eco-friendly.

Jon


* IMG_1415.jpg (2839.67 KB, 4032x3024 - viewed 87 times.)

* IMG_1416.jpg (2814.23 KB, 4032x3024 - viewed 90 times.)
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #3 on: October 21, 2018, 06:41:11 PM »

Looks nice!! But don't you expect coupling and oscillation if the grid coil and the tank coil are that close? Or do you screen one of those?
Please keep us informed about the progress
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ka1tdq
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« Reply #4 on: October 21, 2018, 06:58:51 PM »

I put the copper shield in between the plate tank and the grid. The coil in behind the grid coil is the plate choke. Iíll see what happens.

Jon
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PA0NVD
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« Reply #5 on: October 21, 2018, 08:26:07 PM »

The plate choke will carry some RF current. It may give some problems. In order to prevent RF current through a choke, I normally test by resonating an LC combination and touch the hot part with one side of the choke and holding the other side. The RF resonance circuit should not de-tune for an ifeal choke at that frequency. You can use the LC circuit of an existing transmitter to test, e.g. the drive circuit. The grid current should not change when touching the grid with the choke under test
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KL7OF
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« Reply #6 on: October 22, 2018, 08:13:26 AM »

Jon...The parts from your junquebox are so nice...no gobs of solder on them...no snark or dust...The TX looks very cool..good luck and keep us informed..
Steve
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KB5MD
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« Reply #7 on: October 22, 2018, 12:00:07 PM »

A little trick I've used when unable to find a cardboard tube is to take brown kraft paper or similar, coat it with varnish and roll it to desired size.  After a day or so it dries to a very hard surface tube suitable for winding on.
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ka1tdq
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« Reply #8 on: October 22, 2018, 06:47:22 PM »

Thanks!

The incidentals are turning out to be more than I expected. My Digikey order is now $50!

I'm going to use an LED to indicate plate current (actually, cathode current) as well.

Also, the 6-32 nuts were too large for the 5763 tube socket so I had to grind down the edges.

Jon
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ka1tdq
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« Reply #9 on: October 26, 2018, 08:10:32 AM »

I decided not to put a PNP transistor in for keying. I've put a switch on the front to key the rig with a separate set of contacts going to an RCA jack on the back to key up auxiliaries.

I put adhesive heat shrink one the coils to hide the electrical tape and made the output coil more stout with beefier wire.

I've been pondering modulators and everything costs money. But I do have an Elmac AF67 sitting unused. Elmac put pins on the back connected to the plate modulation transformer secondary (someone throw some flowers on that engineer's grave). I'll run the same 275vdc to the 6L6's plate and screens so I won't need an additional power supply. The data sheet says to run 16 volts bias. Even with the low voltage, it still should be plenty for 16 watts carrier.

Jon


* IMG_1443.jpg (2915.11 KB, 4032x3024 - viewed 55 times.)

* IMG_1444.jpg (2904.65 KB, 4032x3024 - viewed 67 times.)
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ka1tdq
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« Reply #10 on: October 27, 2018, 08:34:45 PM »

This project keeps evolving. I did wind up putting a shield in behind the grid coil (thank you for the suggestion). It also adds a little aesthetically. 

I put the transmitter on a piece of wood for a nostalgic look. I countersunk fishing weights in the wood to give it some stability when adjusting the controls.

The box next to it will be the modulator. All that will be inside is an audio output transformer (8 ohm to a few thousand).  I'll reverse feed it with an audio amp, and I'll incorporate a negative peak limiter too.  The meter will read plate current rather than using the LED idea.

Jon


* IMG_1456.jpg (2466.78 KB, 4032x3024 - viewed 85 times.)
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KB2WIG
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« Reply #11 on: October 27, 2018, 09:27:19 PM »


J,


Putting the device on wood adds to its gravitas.

Yer meter box looks really nice too.


klc

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ka1tdq
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« Reply #12 on: November 03, 2018, 06:20:03 PM »

I finished the power supply today (toolbox picture).

The audio amp came in today, and this is how the rig's lineup will look. I'm waiting on my Digikey order to finish everything up.

Jon


* IMG_1532.jpg (2361.97 KB, 4032x3024 - viewed 47 times.)

* IMG_1530.jpg (2120.05 KB, 3670x2752 - viewed 54 times.)
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ka1tdq
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« Reply #13 on: November 03, 2018, 08:15:15 PM »

Here's the inside of the modulator. I haven't finished it yet, but it'll also have a negative peak limiter powered from a 13.8vdc power supply.

Jon


* IMG_1533.jpg (2596.87 KB, 4032x3024 - viewed 46 times.)
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ka1tdq
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« Reply #14 on: November 04, 2018, 04:56:22 AM »

Completed modulator

Jon


* IMG_1535.jpg (3085.38 KB, 4032x3024 - viewed 57 times.)
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #15 on: November 04, 2018, 10:28:50 AM »

Jon

Iím wondering if this particular transformer can accommodate the DC plate current (without saturating the core) flowing through the output (high impedance) winding.

The information that I found on line shows it as designed for ďpush pullĒ amplifier applications... which implies that there would be essentially no unbalanced DC in the high impedance winding.

Stu
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ka1tdq
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« Reply #16 on: November 04, 2018, 11:49:48 AM »

Right, push-pull... I shouldíve bought single ended. I got this one in particular because it fit nicely in the box for the power handling capability. Could I use the center tap to one side of the winding?

I guess alternatively I could use a Heising circuit on the output of the high impedance winding. The current would just flow in one direction through the Heising choke. I donít want to do this though because I want everything to be compact and fit in the box. For once, I want to build something that isnít the size of a Sherman tank!

Jon
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PA0NVD
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« Reply #17 on: November 04, 2018, 12:24:10 PM »

Just a simple 6L6, KT88 or so single ended instead of the audio amp, one side of the balanced winding to the 6L6/KT88 plate, other side of the balanced winding to the transmitter, centertap to +HV. That cancels the DC magnetization for the most and you have sufficient Iron. A 12AX7 is sufficient to drive and gives you microphone sensitivity. All that goes in the space of a mod choke and less weight
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ka1tdq
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« Reply #18 on: November 04, 2018, 12:47:37 PM »

I could do that, and might, but I was still thinking about using my existing setup.

Feeding the B+ into the center tap and the transmitter from one side. The other side would need a resistor to balance the load. My plate impedance is calculated at 1650 ohms. Iíd need to put that in with a sufficient size to dissipate the heat. Iím clearly not worried about efficiency here! The whole setup would be about 10% efficient.

Jon

** On second thought, Iím pulling 85mA through the final. I would need a resistor of about 3300 ohms to balance out the other side... at a minimum of 25 watts to handle the heat.
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #19 on: November 04, 2018, 12:59:09 PM »

Jon

You need a Heising choke that can accommodate around 100mA of DC, and having an inductance value of 20H.

With 275V on the plate and 75mA of DC plate current, you would have the following:

DC input power at carrier: 21W
RF output power at carrier (assuming 80% efficiency): 16.5W
Modulation resistance:  3667ohms

Set 2 x pi x H x 30Hz = 3667 ohms => H= 19.5 henries

Stu
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« Reply #20 on: November 04, 2018, 01:05:33 PM »

Another great resource for boxes like your using for the modulator are cigar boxes from an upscale cigar store. If a box gets beat up over time, an exact copy of a well known cigar box will be available. They also make nice jewelry boxes for kids and great places to put spare parts in. A cigar box is man's second best friend! Please make a note of it Grin Grin Grin

Most cigar stores will give the boxes to you for free but I have seen them for sale. Even if they are for sale, a good story will usually net you one or so for free Cheesy Cheesy Cheesy
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #21 on: November 04, 2018, 01:14:32 PM »

With a balancing resistor you indeed generate a lot of heat, and require double modulation power and DC power. A power tube and a 12AX7 seems simple to me and you don't have to look for a Heising choke and cap, which is difficult for a lot of people that like to copy your project. I even used a 2 x 115 V small power transformer as modulation iron with good results. At 350VDC they run down to 150Hz . which is not bad at all. At the high side 5 kHz and beyond was no problem. That makes it even more a junkbox project
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ka1tdq
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« Reply #22 on: November 04, 2018, 01:21:25 PM »

Yeah, thereís lots of options here. Itís turning into a big ordeal just to make my originally intended 16 watts carrier.

The other question is, am I actually going to use this on the air? I have my big class E rig for 75 meters. I might just do the resistor trick to check the audio. Make sure it works, and then put it on the shelf.

I do like the looks of the whole setup, including the audio amp. If I actually use this as a working rig, Iíll probably use Stuís idea.

Jon
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AB2EZ
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"Season's Greetings" looks okay to me...


« Reply #23 on: November 04, 2018, 01:26:54 PM »

Jon

Check out the Hammond 193C. I used one to build an external Heising modulator for my Johnson Ranger.

Stu
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ka1tdq
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« Reply #24 on: November 04, 2018, 01:58:07 PM »

Yeah. I need to order two more ceramic standoffs anyway. Thanks!

Jon
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