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Junkbox project - 75M RF deck




 
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Author Topic: Junkbox project - 75M RF deck  (Read 1670 times)
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ka1tdq
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« Reply #25 on: November 06, 2018, 11:44:04 PM »

I finished wiring the oscillator and the grid tank. I'm waiting on DigiKey order #2 to finish the final and the modulator. I'm going to mount the Heising choke on the back of the modulator box so it won't be seen.  Lots of iron for 16-ish watts!

Jon


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AB2EZ
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« Reply #26 on: November 07, 2018, 09:21:30 AM »

Jon

I recommend a Heising capacitor value that produces a reactance that is equal to the reactance of the Heising choke ... at the frequency where the reactance of the Heising choke is equal to the modulation resistance (3667 ohms).  I.e.

A) Modulation resistance = 3667 ohms

B) 2 x pi x 30Hz x H = 3667 ohms

H = 20 henries

C) 2 x pi x 30Hz x C = 1/(3667 ohms)

C = 1.5 uF

Stu

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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ka1tdq
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« Reply #27 on: November 07, 2018, 01:58:14 PM »

Good formulas to note. Iíll copy and stash them.

I took a stab in the dark when I placed my DigiKey order and went with 3uF. Itíll work. I can tweak the bass and treble controls if I need to compensate.

Itís QRP AM anyway, so nobody will hear it.  Smiley

Jon
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AB2EZ
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« Reply #28 on: November 07, 2018, 06:20:58 PM »

Jon

3uF will do.

Remember:

The Heising capacitor wil have a DC voltage across it of 275V.

If the modulation has a lot of low frequency content... e.g. a 30Hz sine wave... and if the modulation index is high ... e.g. 100% negative peaks... then the peak voltage across the capacitor can be as high as 2 x 275V.

I usually use two or more capacitors in series (with a 100k ohm voltage equalizing resistor across each)... to divide the voltage among them.

For example, a pair of 3uF 450WVDC capacitors in series (with voltage equalizing resistors across each of them) will have a value of 1.5uF, and a 900WVDC rating.

You definitely donít want the Heising capacitor to fail in a way that it becomes a short circuit. If it did, it would possibly damage the modulation transformer and/or the Heising choke. A 0.15A fuse in series with the Heising capacitor would be good insurance.

Stu
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ka1tdq
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« Reply #29 on: November 08, 2018, 02:33:07 AM »

The 3uF capacitor has a rating of 400vdc, so I'll get another one. Then I'll be at 1.5uF anyway.

All the iron is now mounted and the weight/watts ratio is pretty high! 


Jon


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ka1tdq
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« Reply #30 on: November 09, 2018, 07:35:54 AM »

The RF deck is done.  I'll start testing once I finish the modulator.  I'm just waiting on that 2nd 3uF cap to arrive.

In the meantime, here's a link to pass the time:

https://www.youtube.com/watch?v=CdXesX6mYUE&start_radio=1&list=RDCdXesX6mYUE

Jon


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ka1tdq
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« Reply #31 on: November 10, 2018, 07:27:39 PM »

I have it completed, but it's going to take a little more fine tuning.

I'm getting 5 watts output right now.  The grid tank cap is fully unmeshed and the LED isn't glowing as strongly as I think it should. I need to pull a few turns off the coil to try to bring up grid current.  This won't be easy because I used heat shrink with glue.

Also, the plate tuning cap is fully unmeshed.  I did disconnect the 47pf doorknob cap in parallel, but I'm still fully unmeshed.  There is another fixed cap in parallel, so I'll need to install a smaller value one.

Jon



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ka1tdq
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« Reply #32 on: Yesterday at 12:02:03 PM »

I kept cutting the inductor coil down for the grid tank until the capacitor came off being fully unmeshed for the highest glow on the LED.  I ended up going too far and had to add a red toroid on top to increase inductance a little bit.  It turned out to be a good thing since the big red toroid adds cosmetically to the thing.

I'm now getting 10 watts carrier.  I just need to get some wire to connect the 8 ohm output of the PA amp to the modulator to see how it sounds.  

Jon


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AB2EZ
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« Reply #33 on: Yesterday at 12:21:14 PM »

Jon

Looks great!

Stu
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ka1tdq
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« Reply #34 on: Yesterday at 07:41:39 PM »

Thanks, Stu!

I've been puttering with the modulator and the combination that works is connecting the high impedance secondary's center tap to the heising capacitor, and one end to ground.  The other end is left dangling. By doing this I get a low percentage of modulation and the audio is distorted.  I tried connecting the other end of the secondary to ground via a 2k resistor, but it wasn't happy with that.  I was thinking that I needed to balance the secondary.

...if I had only bought a single ended transformer. I should've bought a lot of other BS over the years, and life would be so much simpler now.  Oh well, no rest for the weary.  Enough complaining...

Again, this is a push-pull secondary transformer feeding a heising circuit.  Any thoughts?

Jon
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ka1tdq
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« Reply #35 on: Yesterday at 10:53:36 PM »

Ok, my bad. I didn't have it tuned properly. Once I readjusted both caps, the audio cleared right up. Monitoring the rectified RF output through my headphones, the audio sounds very good but weak. The amp I chose doesn't seem to have enough drive, but that's ok!  I'm glad that everything works.

So, another nice shelf radio to add to the collection!   Smiley

Jon


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PA0NVD
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« Reply #36 on: Today at 08:29:11 AM »

Make at least a few QSO' with it Jon!!  Fun
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ka1tdq
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« Reply #37 on: Today at 09:42:03 AM »

Iíll go to Las Vegas, drop it at the entrance to Carsarís Palace and run like hell!

Jon
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AB2EZ
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« Reply #38 on: Today at 10:49:05 AM »

Jon

Use the entire secondary of the mod transformer. One end should be connected to ground, and the other end to the negative side of the Heising capacitor. Make sure the pair of capacitors is connected properly in series: at the junction beteen them... + to - . Also make sure that you have a 100kohm 1/2 watt (or larger) voltage equalizing resistor in parallel with each capacitor.

The center tap of the secondary should be floating (end insulated to prevent an accidental short to something).

Stu
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ka1tdq
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« Reply #39 on: Today at 10:59:16 AM »

I didnít want to use equalizing resistors across the caps because I didnít want to place B+ (even at low current) across the secondary. It seems to do ok without it.

Iíll go back to using the full secondary now that I discovered my tuning error.

Jon
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AB2EZ
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« Reply #40 on: Today at 11:46:01 AM »

Jon

If you measure the DC voltage (no modulation) across each capacitor, you may be surprised at how uneven the split is if you rely on the leakage resistances of the two capacitors to be close to equal.

With a pair of 100kohm balancing resistors, the dc through the transformer secondary winding will only be 275V / 200,000 ohms = 1.375 mA.

Stu
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ka1tdq
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« Reply #41 on: Today at 11:58:23 AM »

Ok, Iíll make that change.

On the other hand though, I was just testing and the plate choke for the oscillator went ďpuff!Ē I only applied power, so a short mustíve developed on the load side of it somewhere. Itís a real bother to get to, so Iím going to shelve this project for a while.

It does look cool though sitting on the shelf. 🙂

Jon


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