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Post by mark on Jan 17, 2021 1:56:02 GMT
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Post by mark on Jan 17, 2021 2:04:33 GMT
Here's part 2
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Post by Boomer on Jan 17, 2021 16:38:30 GMT
Ah, the back-to-back power transformer method, not too efficient, but it will work and I've seen that idea used on low power tube circuits, like this transmitter or a preamp. 120 volts to the primary, 6.3 volts out, then taking another identical transformer, connecting its 6 v winding to the first's 6 v winding, which steps the voltage back up to 120 again and provides isolation from the line. It also gives a convenient 6.3 volts for the tube filament.
I can see why that was done, 6.3 volt 2 amp filament transformers were SO common at one time in radio stores and surplus for a cheap price.
I used to do tubes all the time, but haven't built or even repaired anything with tubes for a while now.
Booma
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Post by Boomer on Jan 21, 2021 0:32:35 GMT
Thanks OldiesWMRK, tubes are great, you can also do it with ICs and transistors too, like this transmitter I made. The board is the complete transmitter, just add 12 volts DC, audio, antenna with coil and ground. You can see the placement of components. I must have drawn out the schematic, but don't have it on this computer, though it's similar to this one, which I think is from Part 15 Engineer's stuff posted here. I changed the Franklin style VFO over to a 910 khz resonator for better stability, and instead of CD4011, I used a CD4001 IC, for the gates, and used a BS170, a higher power 2N7000. Attachments:
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Post by Admin on Jan 25, 2021 19:19:20 GMT
Suppose the L/C tuning could be replaced with a crystal?
Of course, those are hard to come by nowadays...
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Post by Boomer on Jan 27, 2021 5:42:20 GMT
You can get used broadcast band crystals, but it's hard to find them. I'll agree, an on-channel crystal makes the easiest quartz driven circuit possible, with a J-FET oscillator and crystal, it's about 5 parts for quartz stability. Ah, the days of fundamental frequency crystals.
Replacements are ceramic resonators, PLL and programmable oscillator cans, as well as DDS circuits. My first foray into alternates was a higher frequency crystal with a divider IC, like a 4017, you could get those at Radio Shack.
You could take a very common 6 mhz crystal and divide it to several broadcast band frequencies and it worked well, /4 = 1500, /5 =1200, /6 = 1000, /8 = 750, /10 = 600. All it takes is a 4001 IC as osc and buffer and a 4017 decade counter IC to make a stable oscillator for a few dollars. That could easily replace the oscillator in that transmitter circuit.
I've made many of those circuits over the years, and they're capable as heck. You can drive a bipolar final like a 2N3904 or 2222 for 100 mw, or a power mosfet like a 2N7000 or up to a IRF 510 for 10 watt carrier current stations, yes it's possible to drive the FET directly from the ICs and get that much power at 12-14 volts on the drain.
For $20 you can make a transmitter that can sound just as good as commercial units of its type, if you study up and DIY.
I always have liked the experimenter's crystal radio you have set as the wallpaper for this site. That looks like a nice project, how well does it work? I'd like to hear how it would work with a Q-multiplier coupled to the antenna coil.
Boom
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Post by Admin on Jan 27, 2021 14:36:59 GMT
Somewhere in my box of unfinished projects I have a TTL divide by 16 with a 24 mHz crystal for my 1500 kHz channel. Maybe I'll dig that out again.
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Post by Admin on Jan 27, 2021 15:38:25 GMT
This is the background image for the forum, a stock photo I found out there somewhere.
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Post by Boomer on Jan 29, 2021 2:57:41 GMT
Thanks for posting that crystal radio schematic, it looks nice, and I thought it was possibly one you had built. You don't often see a meter in a crystal set like that. I'd have to think the meter would reduce the sound in the headphones, at least slightly. Maybe the meter is just reading the DC bias from the diode and doesn't affect the audio, those with expertise on crystal sets should check in here.
I've used other divider ICs as well, especially the 4060, in CMOS, with the 74HC higher speed part that runs on 5 volts. They divide by 16 (and other ratios up to 16,384), and have built-in oscillators for a crystal. I've used those too, 10.240 mhz crystal (very common), and the /16 pin will get 640 khz in the broadcast band, just from hooking up that one IC.
Using oscillator and divider ICs is almost as easy as making a VFO, with crystal stability, if you can accept the compromise of having one frequency to use.
Boomer
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