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 Post subject: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Jan Sun 28, 2018 5:47 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5510
Location: Indianapolis, IN
Hello All,

Starting a new thread for this project since I thought that the BFO itself might be of interest to others... Original thread here: viewtopic.php?f=4&t=282734

What I thought would be a quick & simple external BFO for this radio turned into an all-day project !

Threw together the simple 455 kHz oscillator portion of the "aligner" built recently and installed into a plastic case using a 9V battery thinking it would work fine---well, turned out not to be very stable with the interconnecting wiring, so decided to mount it internally like the factory option. Found an aluminum plate to mount the variable cap and switch that I could attach to a chassis bracket with adequate clearance. I either use an 8-"AA" holder or regulated 12V supply for power, so not concerned with the factory battery box fitting. Also wanted this "module" to be removable...

According to the instructions for factory BFO option, the output was to be attached to the emitter of the 3rd IF transistor. Didn't want to remove chassis again, so examined the location of its socket and saw that wire could be attached without disassembly with a little effort. Placed a small piece of clean, thin cardboard by band drum to protect from heat and held in place with another that I slotted and taped to chassis. The soldering iron wouldn't reach, so bent the tip of soldering gun so it could be used sideways. Used the LED lite-probe I recently made viewtopic.php?f=10&t=334199 for illumination of the work area and propped vacuum cleaner nozzle at edge of opening to pull flux smoke away from cavity. Attached short length of 24 gauge Teflon insulated wire which I routed beneath corner of access cover.

Played with coupling caps and found that 120 pF gave the best results. Had the circuit already assembled onto the mounting plate and realized that circuit was negative ground and radio was positive !!! Pulled apart and subbed a 2N3702 PNP silicon transistor for the NPN 2N4124 and made necessary changes in wiring including accommodation for the change in transistor pin-out from EBC to ECB...

Once operation of the "updated" circuit was verified, installed the "module" and attached the negative lead to the tip connection of the external power jack (since BFO has its own switch). The positive connection is made through the variable cap's mounting.

Works excellent though the tuning is extremely touchy on SSB---enjoyed listening to several QSO's on the 40 & 80 meter bands...

Schematic and pics below...


Attachments:
ARF MODDED SOLDER TIP.jpg
ARF MODDED SOLDER TIP.jpg [ 122.63 KiB | Viewed 873 times ]
ARF 3RD IF E--CONNECTION.jpg
ARF 3RD IF E--CONNECTION.jpg [ 171.83 KiB | Viewed 873 times ]
ARF BFO COMPLT BK.jpg
ARF BFO COMPLT BK.jpg [ 183.34 KiB | Viewed 873 times ]
ARF BFO COMPLT FRT.jpg
ARF BFO COMPLT FRT.jpg [ 175.56 KiB | Viewed 873 times ]
ARF R-3000 BFO INSTLLD.jpg
ARF R-3000 BFO INSTLLD.jpg [ 163.89 KiB | Viewed 873 times ]
455 KhZ BFO schematic.png
455 KhZ BFO schematic.png [ 84.94 KiB | Viewed 873 times ]
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 Post subject: Re: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Jan Sun 28, 2018 6:32 am 
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Joined: Aug Wed 26, 2009 1:50 am
Posts: 1416
Location: Mount Pleasant UT 84647 (from Coventry, UK)
Very creative!


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 Post subject: Re: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Jan Sun 28, 2018 3:20 pm 
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Joined: Nov Wed 14, 2007 11:37 pm
Posts: 1485
Pretty slick!


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 Post subject: Re: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Jan Sun 28, 2018 6:21 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5510
Location: Indianapolis, IN
Thanks Guys !

John


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 Post subject: Re: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Jan Wed 31, 2018 4:03 am 
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Joined: Mar Sun 30, 2008 1:45 am
Posts: 190
Location: Oakland Township, Michigan
Very inventive John! Looks great!

I did something similar to my Bendix Navigator 420 nearly 40 years ago, but I molested the radio a bit more by adding an injection level control to the front of the radio between the volume and DF level controls. This was needed because I lived among what could only be called an army of 75m phone guys. Signals could be quite strong at times. Also, since the the only SSB I could receive on this radio was on 75m, I did not need to tune the frequency of the BFO since 75m is LSB only.

The BFO oscillator is built completely on the injection level pot. Main components are an MPF102 FET, 455 KHz ceramic resonator and 455 KHz ceramic filter. The BFO is extremely stable. I also added a 455 KHz ceramic filter to the radio's IF chain to improve selectivity a bit.

Don't ask me for an exact schematic. I'll have to do a little reverse engineering for that. 40 years is a long time!

Image

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The circuit may have looked something like this (injection level pot and additional capacitor not shown). Not sure if I floated the ground or wired it so it would work with the polarity of the radio.
Image
Sorry for clobbering up your post with my junk. Just thought it was interesting. Great minds think alike! :wink:

-Matt

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http://www.smokypond.com/gallery/main.php


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 Post subject: Re: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Feb Thu 01, 2018 1:25 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5510
Location: Indianapolis, IN
Hey Matt,

Good to see your old project ! Am guessing that you just "attenuated" the output with your "injection level" pot from what I can see in the pic (what I plan to go with "aligner" project eventually). I bought a 455 kHz ceramic filter for my "aligner" circuit, but it worked well without it and so does the BFO, so saved it for another project. Those ceramic resonators seem very stable and easily "excited"---when operating the "prototype" with a 9V battery, it would continue to oscillate with diminishing output for a couple minutes just on the cap charge !

John


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 Post subject: Re: Zenith Royal 3000-1 --- "Home-brew" BFO project...
PostPosted: Feb Thu 01, 2018 4:17 am 
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Joined: Mar Sun 30, 2008 1:45 am
Posts: 190
Location: Oakland Township, Michigan
John, you are correct. The pot is just a simple attenuator with on/off switch; one end is tied to the oscillator output, the other to ground, and the wiper to the radio IF through a cap. I had about 25 each of those ceramic resonators and filters, so I wasn't worried about using them up. I think I still have 10 or 15 of each. I also had a large quantity of odd, low frequency SAW filters. Not sure I ever found a use for those.

You're right about those resonators being easy to excite. I once had one oscillating in a breadboard circuit with a resistor, a couple of diodes and a couple of caps! It was squirrely and had to be tickled with a probe to get started, but it was oscillating!

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Matt - WY8R
http://www.smokypond.com/gallery/main.php


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