(2017-05-27 Oscillator, AM BCB


I built this Oscillator to see how far a small loop of wire can radiate the RF.  I’m not using an antenna, just the radiation from the coil.  The coil is about 15 feet of solid insulated telephone wire jumble wound on a pill bottle with a diameter of 50mm or slightly less than 2 inches.  The width of the coil is less than 20 mm or about 3/4 inch.  It measured about 49 uH.  I used a 3.9 nF and 2.2 nF in series, which gives 1407 pF.  This and the 49 uH coil resonate at about 600 kHz, on the lower end of the AM broadcast band.

The 470 ohm emitter load resistor lets a lot of current flow, over 7 mA.  A 9 volt battery won’t last very long.  I thought about using a 1k, but I wanted to try to see if the extra power gives extra range.  I may change it later.

I put it on the interrupter that I built earlier (see blog of May 4) and to a 9 volt power supply.

I’m going to walk around with a portable AM radio to see how far the signal is radiating.  There is so much man-made interference at these frequencies that it’s difficult to receive the AM radio stations.  I usually get a lot better results with FM transmitters.

After a short walk, I found that I could barely hear the signal outside of the house.  The signal is very weak inside the house, and just 5 meters or 16 feet is all it can go before disappearing in the man-made noise.  I may try a larger coil, but the coil is the part of the circuit that takes the most time to build.  I think that the signal might  go a little further if I put an antenna on it.  The problem is the rules allow the antenna to be no longer than 3 meters, so it won’t help much.


2017-05-26 (Not A) Wien Bridge Oscillator 

Oops!  (posted to FB groups)

​A Wien Bridge Oscillator that’s not a bridge.  It’s missing the negative feedback side of the bridge!  That must mean that the amplitude of the sine wave changes with changes  in the temperature, supply voltage, and other conditions.  Not a good design!


2017-05-24 DECWriter LA-36

From a comment I posted to FB Vintage Test Equipment group.

I’m looking for some parts for a DEC DECWriter LA-36.  The model I have has the acoustic modem behind the paper feed.  I’m also interested in other parts.  If there are others who have this big, heavy printer, I also have some manuals for it, such the IPB (illustrated parts breakdown).


2017-05-17 Lamp In Audio Oscillator

This is from a FB comment by me.

Speaking of audio signal generators, I made an audio signal generator, the classic Wien Bridge oscillator with the lamp, in my case it was a 28 volt low current lamp, I believe an 1869.  Well I got the lamps from Radio Shack, and the circuit had an intermittent problem.  I tried more than one lamp before I found a good one.  The end of the tungsten filament is held between the lead which is folded over on itself and then squeezed closed.  There is no weld, probably because tungsten is very difficult to work with.  Apparently the leads were not closed tightly on the filament end, which doesn’t show when the lamp is tested at full brightness.  But at lower temperatures, the leads are loose and make intermittent contact.

Once I got that problem fixed, the oscillator put out a clean sine wave.  My circuit used three transistors.  



At work I had a Fluke 8040(?) that had the LCD tilted up.  Worked great, but after a half dozen years we had so many terminals in (and out of) service that I couldn’t spend as much time troubleshooting, and since Lear Siegler was less than a dozen miles away, we just bought enough spares as replacements for the ones out of service and being repaired.  Things weren’t so bad when there were only a hundred and twenty, but when the administrators saw productivity was much better, they all wanted them for their department.  So a couple hundred grew into 3 or 4 hundred.  And then the Apples and IBM PCs came on the scene…

The original Lear Siegler terminals had a CRT assembly made by Ball Bros, a good reliable CRT.  But to save money, Lear Siegler replaced them with cheaper CRTs from this foreign company that I’d never heard of.  And then when the Lear Siegler terminals went on the fritz, a column of smoke came out of the vents on top.  This was very alarming to the users, but it was only a five cent resistor, in series with the DC power, that burned up.  So instead of a resistor, I replaced it with a 1/4 amp fuse.  It solved the problem, but I wondered why this unknown Korean company, Samsung, didn’t put a fuse in their CRTs.


2017-05-05 27 Mhz Signal Generator Continued

 This continues the updates from my blog of 2017-04-21.

Last night was the fifth of May, and I made several important changes to the circuit.  I was finding that after adjusting the 8 to 50 pF N150 ceramic variable capacitor, the frequency would drift for hours, then overnight the drift would settle down to a low level, until I adjusted the capacitor again.  I think this capacitor may be wearing out, so I removed it.

I put two capacitors in series to get the frequency back down but these were only 18 pF, so I removed one turn from the core, leaving 9 turns.  I then changed the parallel capacitors to get the frequency down to 27.145 MHz.  I also added a 0.2 to 5 pF variable capacitor in parallel to give a small amount of adjustment.  Once I adjusted it to the correct frequency, the drift was much less than before, the oscillator stayed on the correct frequency overnight.  Now this Vackar oscillator is doing what Vackar oscillators are supposed to do: be very stable and not drift and stay on  frequency.

It’s May 6 and I left the oscillator running on the shelf all day.  The frequency has been very stable as long as I don’t move it or put something near it.  I think it has drifted only a few hundred hertz since yesterday.  Once I got rid of that 8 to 50 pF ceramic variable capacitor, it has been solid as a rock.  It’s hard to believe how much difference the right capacitors make.  

I wound another T37-7 core with 9 turns of solid PVC insulated telephone wire and measured it at 350 nH, but cores may vary by as much as 20% from core to core.  I can say that the one in the oscillator is under 400 nH.  

The circuit needs to be built on a PC board with a ground plane to provide shielding.  Also it should have a varicap and potentiometer to adjust the frequency.  The whole circuit should be in a metal case.  It might be okay to put it inside an Altoids tin.  Also a buffer stage should be added so the oscillator will be isolated from external influences.  The buffer stage should be Amplitude modulated so it can be heard on the receiver.

This afternoon I was reading about Vackar oscillators and I found different documents with somewhat different parts values and ratios.  The circuit is a type of Colpitts oscillator, it’s just the capacitors and their values that are different.  All I have to say is the circuit is tolerant of my experimenting and is well behaved.  Now if only I could get my regenerative receivers to work, so I can receive what the oscillator is putting out.

Update May 13 – I left the circuit running on the shelf for most of the week, and it has drifted upwards a few kHz during that time.  Not any fast changes, just a slow movement upwards, possibly due to temperature change.  This is long term, so if I used it over a period of a few hours, the drift would be less than a hundred Hz.

One problem that this has is it’s sensitive to handling, so making changes to the trimmer capacitors takes a lot of trial and error.  I could put the varicap back on it, but I’ve read that it can cause some problems.  But it has the advantage that I can run a few feet of wire from the circuit to a pot, so I can adjust the frequency without coming near the circuit.  I also thought about adding a thermistor to the pot to correct for temperature changes.  But I don’t think it really needs it.  What I need is to amplitude modulate the carrier with a tone so I can use it for experimenting on the regen receivers.

Update May 20 – I made several important changes to the circuit.

I changed the capacitor connected to the 33 ohm resistor from 470 pF to 1000 pF.  This is a DC blocking capacitor and has very little effect on the frequency.

Removed the 30 pF N150 and 5 pF variable from across the coil.  There are no more capacitors in parallel with the toroid coil. 

I changed the 68 pF SM to 100 C0G.  

I put the 30 pF N150 in parallel with the 470 pF on right side of the coil.

The frequency was still too high so I added two of the 0.2 to 5 pF from the left side of the coil to ground.  One is at max 5 pF, the other is part way.

I calculated the effective capacitance of the parallel tuned circuit and found the left side of the coil has appx 118 pF.  The right side of the coil has appx 502 pF.  The equivalent capacitance across the coil is 94.4 pF, the frequency is 27.145 MHz, and the XL = XC = 62 ohms.  The coil calculated to be 364 nH, which agrees with measurements.


2017-05-04 Interrupting The Supply V to the Oscillator

I was working on the 27 MHz oscillator for testing the regens, and I thought it would be a good idea to turn the carrier off for a small fraction of a second every 1 or so seconds.  So I built a two transistor flasher with a third transistor to invert the pulses.  Instead of mostly off with an on pulse every second, the output is mostly on with a very short off interruption every second.

I built the circuit and I put a LED on the output and found that it was always off unless I touched the base of the first transistor.  I found that the two transistors had too much gain, so I added the 22k resistor as a collector load resistor for the first transistor.  The circuit started working properly.  The interruption rate is 1.4 per second.

I got the Suntech AM-FM-SW Radio for $20.  It’s so cheap and the performance is seriously lacking.  The FM band has images that seem as strong as the stations, so I put the extendable antenna down, and the performance wasn’t as bad.  But then I can’t hear the difference between the quiet spots on the dial and the unmodulated carrier of my oscillator.   So now I have my oscillator powered by the interruptor circuit, and the short interruptions make it easy to tell the difference between the carrier and other signals.


2017-05-03 Cheap Frequency Counter

I got these today in the mail.  Notice the temporary wire I poked into the connector.  I ordered some adapters but I haven’t received them yet.  I bought a frequency counter with the rubber ducky antenna but it doesn’t have the sensitivity that this cheap one has.  I got two of these for $28, free shipping.

These have made it a lot easier to adjust the oscillators to the right frequency.  I no longer have to tune the receiver to the carrier to find its frequency.  I just hold the wire close to the oscillator and I get the frequency to within a hundred Hz.  I no longer have to find the oscillator’s carrier among other strong carriers, hoping I got the right one.

When I received the package, each counter was sealed in a plastic package.  But the corner of the package was cut off, and the battery connector had been pulled out.  This leads me to believe that the counters were opened up to test and/or calibrate them to indicate the correct frequency.  I had read that some consumers had received these and they indicated the wrong frequency.  Apparently the makers retested them to find any problems.

I also bought a Gooit frequency counter.  This wasn’t very much, about fifteen dollars from an eBay seller.  This is also sensitive enough to pick up a small low power transmitter.  It has a short pull up antenna but no input jack.  


2017-04-27 Superregen FM BCB Receiver

I built this superregenerative FM broadcast band receiver today.  This circuit is similar to the one I posted on January 22.

Here is a list of parts values I used:

ANT – 12 cm insulated wire connected through 10 pF capacitor to FET source.

C4 – two 2.2 nF MLCCs from + to ground plane ( I built this on a small piece of copper circuit board.)

C5 – 18 pF silver mica capacitor 

C6 – not used – replaced with a jumper.

C7 (A and B) – two 0.2 to 5 pF plastic variable capacitors 

C3 – 10 pF silver mica capacitor

R1 – 10k 5% carbon film resistor (May 7 I changed this to 3.3k, then 2.2k.)

R3 – 2.7 M 5% carbon film resistor 

R4 – 22k 5% carbon film resistor, later reduced to 11k to help make earphone louder. 

Q1 – 2N3819 VHF JFET, later BF256 and BF244.

Q2 – C9018 high gain low noise NPN BJT similar to MPSA18.

L1 – 6 turns 20 AWG bare copper wire wound on drill bit shank, appx 6mm ID.  Coil stretched so it measured appx 120 nH.

L2 – 6 turns 24 AWG solid insulated wire on a FT37-43 toroid core, measured 14 uH, later rewound to 33 uH, cut back to 25 uH.

(Some of following copied from FB post)

My Superregen won’t quench.  I built the circuit found here, with a few changes.

I used a 2N3819 for Q1, and a FT37-43 for L2, measuring 14 uH.  At the end in his notes he gave some limits to the values of some parts, and the values I used seem to be within the limits.

For an antenna I connected a short 12 cm length of wire to the FET’s source, as he recommended.  The coil is 6 turns of 20 AWG bare copper wire, wound on a drill bit shank, and spread out untl it measured about 120 nanohenrys.  I used an 18 pF silver mica capacitor and two 0.2 to 5 pF variable capacitors in parallel with the coil.

 I used a dip meter to tune the coil and capacitors to a point within the FM broadcast band.  I measure a little over 2 volts at the source, so the JFET is conducting in the linear region.

I used a scope to check the R1 – C1 point and I see no oscillations.  I’m using 9VDC regulated for the supply.  I connected the Q2 output to an amplifier and I get noise like a hiss but no stations.  I can hear my cell phone burps loudly, and hum if I touch points in the circuit.  So I’m fairly confident that I have the circuit set up properly  – no major malfunctions.

I just thought of one thing I can do.  I can put the circuit on a variable supply and try various supply voltages.  I’ll try that and update this.

Update evening until 1:30 AM – I experimented with several different parts.  I put it on a variable supply and it seemed to do a little bit better at 6 volts.  At the beginning of this session I could receive only a single FM station, one that’s very close and powerful.  This was with the 12 cm wire connected through a 10 pF capacitor to the source.  I couldn’t hear any other stations.

I removed the L2 and rewound it, so it has 9 turns on the FT37-43 core and measured 33 uH.  The audio seemed to be louder.  But as I tune, the audio is garbled, seems that the level is too high for the audio amplifier I’m using.  Still only one station.  

I moved ‘center tap’ one turn closer to the positive end of the tank coil.  I also spread the turns a bit, and adjusted the variable capacitors a lot.  I think the coil could be longer and lower inductance so it will cover the high end of the FM band.

I searched online for more information about this regen design.  I found another builder’s story of the problems he had with his regen.  He used too much bypassing capacitance, so he reduced it and the regen started to quench.  So I also reduced mine.  I changed C4A and B to a single 1 nF capacitor as shown in the original schematic.  But it still doesn’t work.

Update Evening Apr 28 – I was looking at a tiny .jpg I had saved of the pinouts for the 2N3819, and I noticed they were backwards.  Instead of S  G  D, the pic showed D  G  S.  So I checked three datasheets to see.  The ON Semi showed S  G  D, the Temic showed S  G  D, and the Central Semi showed D  G  S (!!!)   So what brand of JFET was in my circuit?  Oh, no!  It was Central Semi!  So I removed the 2N3819 and flipped it over, and soldered ot back in.  Now I tuned for the strong station but I couldn’t hear anything on the earphone.  The one thing I was certan of was that there was uncertainty on what the true pinout was for the 2N3819 that I was using, so I decided to use another JFET.  I chose a BF244, which I have plenty of, and it’s rated for VHF or better.  After I installed the BF244, I still couldn’t hear the strong station.  I was looking for a possible short, and I looked inside of the coil and found a blob of solder shorting turns.  Once I got that out, the strong station tuned in, but I heard no change, just a single strong station.  The voltage at the source was about the same as before, a bit over 2 volts.

So I got rid of a questionable JFET, but the replacement didn’t perform any better.  I can hypothesize that the second JFET in this circuit with no change implies the poor performance problem is most likely not being caused by the JFET itself.

There’s an old saying that says “Amplifiers will oscillate, oscillators won’t.”  Well this oscillator sure doesn’t want to oscillate, it just won’t quench.

Update May 2 – I spent a lot of time this evening trying to get a sound out of the circuit, but it was dead silent.  I cut out the JFET and relaced it with a BF256B, and it started picking up a very weak signal.  Apparently the JFET had died when I was unsoldering or soldering the various parts.

Update May 7 evening – I had been working on the 27 MHz Vackar ‘signal generator’ for a few days, among other things.  Now I’m back to try and get this FM regen to quench or to squegg as some others have called it.  One thing I had not experimted with was the JFET current.  This is limited by R1, which has been 10k.  The JFET’s current is less than a half milliamp, so I decided to reduce the resistor to get about a milliamp.  I removed the 10k and replaced it with a 3.3k.  Wow, this made a big difference!  All of a sudden the earphone started hissing like a superregen is supposed to when it’s not tuned to a station.  I checked the signal at the top of this 3.3k with the scope and it was quenching at about 88 kHz.  The DC voltage was 2.4 volts, so the source current is still under 1 mA, about 0.74 mA. 

I can tune it and hear only one strong station, but I hear quieting in the hiss at some other points, so it’s trying to pick up some not so strong stations.

The quenching starts at just under 9 volts supply voltage.  If I drop below that, the quench and hiss stops.  I think the circuit would work better if I reduced the 3.3k to let more current through the JFET.

I put a 6.8k in parallel with the 3.3k, so it’s equal to 2.22 k.  I heard the strong signal, but it was garbled.  I turned up the supply voltage, and as I passed 11 volts and got to 12V the audio cleared up.   It sounded much better.   The voltage across the resistors was 2.5 volts DC, so the JFET current was a little more than 1 mA.  And I’m happy to say that this is the first superregen receiver that I’ve been able to hear a station clearly.  And after many hours of tinkering I can say I’ve accomplished something.


2017-04-23 Linear Power Supply Renovation 

In the past I’ve built several of these small power supplies to use around the house, and they come in handy.  But those others used a 12 VAC transformer instead of an AC adapter, so I had to add a fuse and other stuff to prevent shock.  Using the AC adapter is much more convenient because all of that is included in the wall wart.  All I have to deal with is low voltage DC.  Those power supplies used old designs that didn’t have good regulation.  I wanted to replace them with a linear regulator chip which gives excellent performance.
Years ago I had built a 9 VDC shunt regulated power supply, low current, only 40 mA max.  A shunt regulator wastes the power that isn’t being used by the load, in this case it was built for powering a pocket transistor radio.  A Zener diode is a good example of a shunt regulator.  If none of the Zener’s current is used, then the current is wasted heating the Zener diode.  So I dissembled the shunt regulator and used some of its parts to build a new regulator using a LM317.  I also scrapped another power supply for the circuit board and some parts.  Both of these used an old transformer type wall wart with outputs about 13 volts no load.  So I used the adapter rated 9VDC at 600 mA.

I’ve been building these small RF circuits and I’ve been using a bunch of bench supplies to power them.  The circuits need very little power, but the voltage needs to be stable, so that’s why I decided to build a few of these with  adjustable regulator chips.  A few other parts and I’ll have a 1.25 to 9 VDC well regulated supply, that can put out about 200 mA to a load, up to 8 volts.  The wall wart output drops as the current goes above 150 mA and the regulated output drops below 9 volts.  If I had a better AC adapter that wouldn’t happen.  But I can live with the lower current.

This new power supply uses a LM317 with a 270 ohm resistor from Output to Adjust pins, and a 2.5 k wirewound pot from the Adjust pin to negative.  This would allow me to adjust the voltage up to 12V, but the adapter can’t put out that much under load, so I put a resistor in parallel with the pot to limit the voltage to 9 volts maximum.  But even at 9 volts, if I put a load of more than 150 mA on it, the voltage drops below 9 volts.

I added some capacitors and diodes to the basic design to filter ripple, and help protect the circuit from overvoltage or reverse voltage.  This is pretty much standard practice with the adjustable VRs.

I used several EMI / RFI suppressor sleeves on the input and output wires to prevent the RF from the circuits from radiating from the supply.  I added a 2200 uF, 16VDC capacitor on the circuit board to help the capacitor in the wall wart.  

The circuit fit on a perfboard the size of a large postage stamp.  I got the heatsink from an old PC power supply, but I should mount the heatsink on the aluminum lid of a project box for better heat sinking.

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