2019-11-16 Fixing a Driver Transformer With An Open Primary

Also FB group Building Transistor Radios


I was working on an AM radio that had a driver transformer with an open primary. I slit open the wrap and found the tiny fine wire on the right was loose so I soldered it to the heavy wire. But it was still open. ☹️😱 So I checked the left wire and found it wasn’t connected, so I soldered it to the heavy wire. Checked with the DMM and it’s *still* open! I’m so sad! Buah-wuh-wah. 😭😭😭 Why is this so fouled up?? It’s crazy! Like it’s been hit by lightning! Now I’m just bummed out!

Update a few hours later – I tack soldered into the circuit a transformer with four 600 ohm windings. I connected two of the windings in series for the primary, and the other two in series with the ‘center tap’ to drive the bases of the two output transistors. The radio now works just fine – there was *nothing* wrong that could cause the driver transformer to have excessive current and burn out. What the hell caused the driver to go bad??


2019-11-15 Build A LED Tester

Big Clive’s YouTube video

Ohhh nooo! After all the trouble Big Clive went to show us how a Joule Thief works, he *digressed* to using a 9V battery! Horrors! 😱😱😱

I built a Joule Thief with a socket like he used for the LED. But the JT must not be left open circuited without the LED because the voltage will rise very high and can damage the transistor. So I connected a 6.8V zener in parallel with the socket so the voltage can’t rise above 6.8V.

One point: the typical 5mm LED and most others, too have a reverse voltage rating of 5V maximum, so if you insert the LED backwards, you are putting over 9V in reverse across the LED. I have never found that the LEDs I’ve tested have been damaged by this, however it is excessive voltage and should not be done.


2019-11-12 Kicad and ExpressPCB Printed Circuit Board Software

From FB group, 2019-11-12

I hope Kicad has improved in the few years since I tried it. Then it really wasn’t a single app, it was several programs that kinda sorta did something. I didn’t expect much – it was about what I paid for it. I came to the conclusion that if I wanted something decent, I would have to pay for it. Up until then I used ExpressPCB, and it did a decent job, but it was proprietary and locked me into a single vendor. But I don’t need a large quantity of PCBs, so I think that ExpressPCB was my best choice.


2019-11-07 Audio Impedance Matching Box

I’ve often had to kloodge together an audio transformer, jacks and other stuff to make a medium impedance output match a low impedance headphone. But it turns into a mess of wires and parts dangling from a transformer. I thought about what I would do to make a box with the necessary stuff, like inputs, jacks, switches and outputs and jacks. But I just never got around to putting it onto paper or other form. I found something that gets much of what I was thinking to build, except I would add a few more stereo (TRS) jacks and RCA jacks, along with switches to put the stereo to mono and to put the headphone elements in series. So there is more to the box that I want to build than is shown here, but this is a very good start. This uses the popular Bogen T725 line matching transformer, available online or one similar to it.


2019-11-05 Einstein 100 Years Ago

Einstein became famous 100 years ago.


2019-11-01 Microwave Motion Detector (BigClive)

Here is a good explanation of a microwave motion detector with a schematic.

A few important things:

This gadget detects *anything* that moves that interferes with microwaves. It isn’t limited to living bodies.

The microwaves will go through drywall or other insulating surfaces. It will be attenuated by glass, presumably because glass has lead and other RF absorbing materials in it.

The ones I bought are supposed to be 5.8 GHz, but I have measured them much lower, about 1.1 GHz. That makes them possibly illegal.

These can be used in a vehicle to trigger someone else’s Radar detector. Just watch out for quick braking cars! 😅😅😂😂

The more recent ones I got have a metal cover over the back, presumably to shield the rear areas and stop rear movement from triggering it.


2019-10-21 Testing Transistor In Circuit

From FB group Vintage Transistor Radios

James Ault

Adam Bernhardt
Maurice Wright is right. The tester puts DC through the device and measures its current gain. But when it’s in circuit it is amplifying AC such as audio or RF, and that’s not what a typical tester does.

Any experienced tech will tell you that the best way to find out if a device is working properly is when it’s in the circuit doing its job. You can measure the DC voltages and currents with a DMM or use an oscilloscope to see the signal as it is amplified by the device.


2019-10-12 RF Oscillator Won’t Make A Sine Wave, Only Sawtooth

I wanted to make an RF Oscillator that puts out a sinewave at about 734 kHz, which uses two convenient values, 100 uH coil and 470 pF capacitor. I used the toroid core with 16 turns of 30 AWG wire, with a tap at 4 turns then later 8 turns. I built the circuit in following photo and experimented with it a lot, changing values for the various parts and every time it would not oscillate or it would put out a sawtooth wave. I was monitoring the waveform at the emitter with my oscilloscope. The supply voltage was the same 4.5 volts throughout the experiment.

I started with the coil of 16 turns with a tap at 4 turns. Later I changed to 16 turns tapped at 8 turns. I started with the 2N4401 for the transistor and then changed it to a 2N3904 to see if it would change anything – it didn’t. The waveform stayed a sawtooth, with only changes to the frequency and amplitude. I varied the base bias resistor, starting at 150k, then I connected a 100k in parallel for about 68k, then I replaced those with two 22k in series for 44k.

I started out with a 10 nF or .01 uF DC blocking capacitor between the top of the coil and the base. When I reduced it to 1 nF or .001 uF, the sawtooth frequency went up. I put a 100 to 10k variable resistor in series with this capacitor and it varied the amplitude and frequency of the sawtooth, or else the sawtooth fell to nothing. I did a few other things that I haven’t mentioned. The only tendency I saw was the sawtooth got a slight wiggle in it somewhat like a damped sinewave.

This was supposed to be a Hartley oscillator. I’ve come to the conclusion that this circuit makes a very reliable sawtooth generator.

Update Oct 12 – I moved the .01 uF from emitter to +4.5V to emitter to – across the 1k resistor. I moved the scope probe from the emitter to the collector. Now the scope shows a sine wave on the sawtooth wave. Then while I was messing with it the sawtooth stopped and I saw for the first time just a sine wave! Yes! But the sawtooth came back, it just reappeared. It still is an unstable circuit. At this point I had set the 100 to 10k variable resistor to 100, so I increased it to 270 ohms. The sine wave was more stable, and it didn’t want to go into the sawtooth mode. I disconnected and reconnected the +4.5V several times, and most but not all of the times it put out only the sine wave. So it is still not stable and wants to go into sawtooth oscillation even though to sine waves are at the collector.

Update Oct 13 – I experimented with the values a lot and I have gotten a better feel for what puts the circuit into the sawtooth oscillation mode. I’m currently using the following circuit. It seems to be more stable and less prone to make sawtooth waves. The .01 uF from the top of the coil to the base has a lot to do with the sawtooth. I put a larger capacitor in parallel and it brought the sawtooth frequency down. Apparently this capacitor is charging and discharging. The 1k in series with the .01 uF reduces the gain enough to stop the sawtooth waves.

The waveform for the following circuit is sine wave but the bottom is skewed because it’s being clipped. Another odd thing is the L and C values should give a frequency of 734 kHz. But I measure 638 kHz with the DMM. This is too low and far off what it should be. I don’t understand what is causing the big difference. Normally the circuit is off no more than ten kHz.u


2019-10-09 Not Enough Ports, Too Many Wall Jacks!

from FB group Bell Telephone…

This reminds me of one of the netheads in our dept. I get a request to connect an additional jack on the patch panel up to the switch. I go over to the IDF and find that all of the ports on all 3 of the 48 port switches were being used. So I forward the trouble ticket with the “all ports in use” back to the help desk, and they forward it to the netheads. The nethead gets it and looks at the switch stats and sees a port has no activity since the last powerup, 30 days ago. So nethead forwards it to me to use port 37. I go back over and disconnect the patch cord for port 37 from the patch panel and plug it into the jack that needs to be live. So far, everything is okay.

Two weeks later the help desk sends me a trouble ticket that some printer is not working, no one can print to it. You guessed it, that nethead had me disconnect a jack used for a working printer! Okay, I forward it back to netheads for them to resolve because there are too many piglets and not enough teats on mama hog!! 😫😫😣


2019-10-07 Zenith Royal 500H Troubleshooting

I got this from eBay, it doesn’t make any sound at all. It has a small crack in one corner but the case is otherwise okay. The screw on the bottom is corroded but the battery holder is in halfway decent shape. It needs a bit of cleaning, the metal is a bit tarnished but it’s all there – pretty good for more than 57 years old. Made in ’62, it’s S/N 811910.

The first thing I found was the earphone jack switch contacts were not closed so the audio output couldn’t get to the speaker. So I scraped the contacts and bent them so they make contact as long as the earphone plug is not plugged in. But I think that after a plug is plugged in, the contacts may no longer make contact. But for now it’s okay.

The next problem was there was still no sound from the speaker. I stripped it down to get to the PC board. I tested the speaker with the DMM and it measured a dozen or so ohms so it seemed okay. I pulled the speaker out of the cabinet to give it a visual and it’s really weird. The voice coil is not in the center, it’s off to one end of the oval cone. I found that there were iron filings all over the cone and voice coil, so I removed them and some dust and put the speaker back.

I still wasn’t hearing anything from the speaker. The electrolytic capacitors had the black plastic cases and those are the ones that usually go bad, so I replaced some and the radio was now making a bit of noise in the speaker when turned on but still no audio. But when I held the PC board in a certain place I heard audio. So I probed around on the board for an intermittent connection and resoldered a few joints, and I think I got rid of the bad connection.

The volume control was noisy so I gave it a shot of Deoxit and it quieted down. It’s now acting halfway like it should, but it needs an alignment to make it more sensitive. The schematic I downloaded has alignment instructions so I’ll try that soon. I’ll have to replace the earphone jack to get it to be reliable.

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