2019-03-22 Tuned Circuit For 702 kHz

from FB group Building Transistor Radios

Stoffel van Aswegen
I used the free Android app ElectroDroid to do these calculations. For 702 kHz, a 220 pF fixed capacitor (NP0 or C0G) and a 230 microhenrys coil are needed. The standard ferrite loopstick is 230 uH, but you can wind your own on a coil form such as a toilet paper roll. For a TP roll 44 mm diameter and coil 70 mm length, it takes 92 turns. But beginning at the 85th turn you make a tap every 2-1/8 turns, up to about 100 turns. Then you can connect the capacitor to one of the taps to tune the circuit to the right frequency.


2019-03-19 Radio Alignment Questions

From FB group Building Transistor Radios Mar 20

Bob Kirsch
Any recommendation for doing a complete alignment? Documents?

I used a HP sweep gen decades ago but I don’t have one now, just an old signal gen. I thought about building a sweep gen but I need to get a few parts. I have bought a few radios on eBay and when the caps started going bad and volume got weak, the owner diddled with the IFs, not realizing it wouldn’t help, only make it worse. So the only thing I’ve seen have been simple instructions in the Sam’s schematics.



From FB group Building Transistor Radios 2019-03-19

John Ferguson
Yeah, I used to use the Sylvania ECG and the NTE catalogs to cross-reference transistors. But then I got some better product manuals from the makers, especially Motorola and National Semi. And I got hold of the professional industry documents. ECG and NTE give a ‘one size fits all’ cross-reference, and that’s just too iffy for a good substitute.


2019-03-15 The Tape Recorder 6V6 Making Crackling Noise

from FB group Building Transistor Radios

Tuck Choy
I was a reel to reel tape recorder fanatic when I was a kid, and had a recorder with a 6V6 output to the spkr. The first thing I did was modify the recorder to add an external spkr. One time I plugged in a clip leads with no spkr. When I played back my recording I heard crackling noise inside. Well, that wasn’t supposed to happen, so I looked inside. The unloaded output transformer’s counter EMF was causing the plate pin of the 6V6 socket to arc to the chassis as the audio got louder! 😱😱😱


2019-03-14 Pi Day!

3.14 is Pi day.


2019-03-13 Regen Receiver Schematic

from FB group Building Transistor Radios post by Tuck Choy. I sharpened and lightened it to make it more readable. From EPE Mag. Sep 2005 by Thomas Scarborough.

Since only four of the 6 inverters of the CD4069 are used, the CD4001 or CD4011 quadruple NOR or NAND gates should substitute, if the other unused gate is grounded or tied to positive. I have a few CD4049 hex inverters, I should try one of those and see if it will work.

Update Mar 13 – Tuck Choy posted the complete article as a .PDF file.


2019-03-12 Dipper, Wavemeter, Receiver Added Pt. 2

Update 2019 Mar 12 – I continued to add capacitors and variable resistors along with switches to change them similar to the VK2ZAY video on YouTube. By switching the capacitor and trimmer resistor, it can now act as a receiver. I clipped a 2 meter long piece of wire to the tuning capacitor and with the trimmer set to a few hundred k and with it switched to only the 4.7 nF capacitor, it is receiving a strong local station. It’s getting a bit crowded, both the schematic and the circuit with all the switches.


2019-03-11 Dipper, Wavemeter PT. 1

from FB group Building Transistor Radios Mar 11

I watched a YouTube video from Alan Yates VK2ZAY, he built a combination Dipper, wavemeter, receiver and RF generator. I built his schematic ‘right side up,’ he admitted he built it upside down with positive ground and drew the schematic, see:

I didn’t attempt to build it with the switch, I chose only the wavemeter function. I’ve been trying to get it to work but it’s acting funny, probably like superregenerative. I’ll have to adjust some resistors to see how it works.

Update Mar 12 – I found one problem, I connected the two BF199 transistors base to base and collector to collector; they should be bases to collectors. It wouldn’t have happened if they hadn’t been BF199s, which have the emitter on the center lead. Now that I straightened that out I have a superregenerative oscillator that squeggs at audio frequencies, not above (ultrasonic). It makes a loud squeal at any and all tuning capacitor settings. The tone varies in frequency from low growl at low voltage to high pitch at 9 volts. I put a loopstick with a tuning capacitor next to the coil of the Dipper/wavemeter and tried to get them to interact but I had no success. I need to investigate with a frequency counter to find out what its frequency limits are.

Update Mar 12 – I changed some resistor values to change the tone. It works great for generating a tone. I ran my signal gen to a few loops of wire around the coil. When I tune it or the signal gen to the same frequency the tone gets much higher until it peaks where both are the same. So it’s working as an audible wavemeter, sensing the external RF.

I am making more modifications. They will be on the next blog, Pt. 2.


2019-03-10 Phones Work During Power Outages?

From FB group Orange Talk Mar 11

The old fashion ‘non electric’ phone depends on the phone line for its power. The old phone lines used to have backup power at the central office, so they worked if the power went out. But nowadays the power comes from a local box which has batteries but they don’t last for long, maybe an hour. If you have cable it’s worse: the phone is run by a battery that’s inside or outside of your home, so if you lose power the battery may last only a short time. So having a ‘non electric’ phone is no guarantee your phone will work during a power outage.


2019-03-08 Winding a Ferrite Loopstick

also links to this in FB groups Building Transistor Radios and Ferrite Loopstick Exper.

I got some of these ferrite loopstick bars from Goldmine Electronics while the were on sale. They are the small kind that fit into pocket radios. Materials were the bar, a piece of paper for insulation, a roll of 32 AWG enameled copper wire, and heat from a soldering iron. See the attached photo.

I first cut a piece of paper to wrap around the bar and overlap a little. I used my soldering iron to melt some candle wax on the paper to hold it in place while I wound the wire. I used a short length of adhesive tape to hold the wire while I wrapped about ten turns. After that I melted some candle wax on the ten turns. I proceeded with the winding, periodically melting wax on the turns to hold them in place.

The finished coil measured a bit high so I removed turns to bring it to 680 microhenrys, which is what is used with the 140 pF ‘polyvaricon’ capacitors found in the cheap pocket radios.

I’m not sure if this FB group link works for everyone.

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