2019-06-11 Selenium Rectifier Burned Up, How To Replace

from FB group Vintage Transistor Radios Jun 12

Bob Kirsch has the right idea, but the single replacement diode *must* be rated for 400 V peak inverse voltage or more. A 1N4007 is cheap and readily available. I assume you’re in a country where the AC line voltage is 120 VAC, because the rectified DC is 135VDC according to the schematic. The tubes/valves do not draw current for 11 seconds when first turned on so the DC across the 80 uF capacitor can be as high as 170 volts, so it’s best to use a capacitor rated for 200V or more.

As he said, increase or add to the 22 ohm resistor to obtain 135VDC when the set is fully warmed up. The resistor(s) wattage rating(s) must be at least the voltage across the resistor squared divided by the resistance. So if the voltage across it is 30 volts, and the resistor(s) total 100 ohms, then 30 times 30 divided by 100 is 900 divided by 100 equals 9 watts. A 10 watt resistor would work but get very hot, it would be better to use a higher wattage or two 10 wat resistors in series. And make sure they don’t overheat other components.


2019-06-09 Strange 3 Conductor Crossconnect Wire

from FB group Bell Telephone… Jun 11

I’ve seen this 3 wire crossconnect wire, but I can’t remember where. Long ago we had racks of datacomm with a patch panel with patch cords. We used 3 wires of four wire IW, one wire for common, one for transmit and one for receive. These were for RS-232 unbalanced signal levels.

I often removed the old 3 wire IW from the apartments: green, red and yellow just like the old 3 wire cords used before modular connectors. So there were some uses for 3 wire crossconnect wire in the field.

The thought just occurred to me that I may have seen it used for the 1A2 systems, for connecting the power supply up. One wire for common, one for the -DC and one for some other voltage, maybe 10VAC or ringer? Or maybe it was for the boxes that were on the backboards, for the interface card for the Telco to administer the four wire, full duplex analog data circuits. The tech would use 3 wire to connect the AC adapter to the box – 2 wires for the 24 VAC and one for ground.

Shane Young
The Telco techs who installed datacomm were “innovative” in that they would not always adhere to standard practices. They might use two pair and twist both wires of one pair together, making it a 3 conductor. They might take a roll of this 3 wire and screw it to the backboard using a mushroom, because they had extra rolls on their truck that weren’t used, and they figured why waste a whole roll of good crossconnect wire in a customer’s MPOE when they can use this junky wire that no one wants to pilfer (steal). 😁



from FB group Regenerative Receiver… Jun 13

Cyril Hodson
It seems to me that these plug-in coil forms would be easy to make with a 3D printer. They would be made of thermoplastic so I don’t know if there would be problems with soldering pins or with being close to tubes/valves. Your idea of using plastic pipe with super glue may have the same problems with high temperatures. It would be interesting to try it and see. One could print 3 or 4 coils using different colors of plastic to identify them. Or it might be possible to print ribs on a length of plastic pipe. It seems that 3D printing would save a lot of work fabricating these coil forms manually. What do you think? 👍 or 👎 ?


2019-06-06 1/2 Price Offer For Lot Of NOS Capacitors

Posted to FB group Building Transistor Radios Jun 15

Months ago I started watching an eBay item of a lot of twenty Hammarlund 3 to 15 pF air variable silver plated ceramic trimmer capacitors. They were made in 1956, NOS / NIB, sealed in their original packaging and individually boxed.

For months the seller wanted $150 for them, no budging, no price cuts. Last week I offered seventy for them, the guy counter offered, and I replied with $75. The seller accepted, so I got the lot for under four dollars each, not bad.


2019-06-03 Biomass? Be Concerned About Other More Important Things!

From YouTube video Denmark is Riding the Winds of Change, June 5

It amuses me how these dilettantes (with 1st names starting with K) argue about the pros and cons of bullshit (biomass) while the world is going to hell! 😡😡😣😣😲

Why are you concerned about trivialities when we should be concerned about the future? Of overpopulation. Or methane – a far worse greenhouse gas than CO2. Of saving the planet by eating mostly no meat. By putting pressure on utilities to decommission coal power plants ASAP. And become educated about what’s happening worldwide, such as sea level rise forcing depopulation of large areas due to flooding. You may not like the facts but they will determine your future.


2019-06-01 Elevator Telephones Were Scary

from FB group Bell Telephone of.. June 3

If the elevator phone wasn’t working, I had to hustle over and find out why. If the phone was not working, the security was supposed to tag it as out of service. That was definitely my scariest job. Most of the time the POTS line came into the big box with all the relays that controlled the pump, each relay having 208 V three phase AC. It was tough trying to clip my butt set on to the pair without getting zapped. And if someone used the elevator while I was working there, KA-CHUNK! the relays actuated and the big pump started up and the first few times it scared the crap out of me! I finally got to the point where I wasn’t surprised, I just expected it to go off anytime.


2019-05-31 DIY Plastic Variable Capacitor

I hear others complain about the high cost of variable capacitors, especially the older air variables with metal plates. So I decided to DIY and make one. This is my first attempt so forgive the lack of elegance and unfinished look. I’m sure it could be improved on and made better with some effort. The supplies and tools I needed were:

Two metal food cans, cleaned and labels removed

One 1 liter soda bottle, cleaned and label removed

Wire, solder

Tin snips or heavy duty scissors or hacksaw with fine tooth metal blade

Utility knife and ruler for straight cuts

Patience 😉

See the photos.

One can will be used like it is, with a length of wire soldered to it. I cut the rim of the other can off with the tin snips and slit it along its length so it can be compressed and fit inside of the other can. It’s a good idea to be careful with the sharp metal edges. For the most part they will be covered when it is assembled.

The liter bottle must be cut so that there is a smooth sheet of curved plastic that fits around the inner can, with 1/8″ to 1/4″ or 3 to 6mm of overlap around the outside so the metal cans don’t touch and short. I used the tin snips to rough cut and the ruler and utility knife to get straight edges.

Before assembly the two wires should be soldered on. I arbitrarily chose locations, they should be close to the loopstick coil.

It assembled without any glue or tape, but it could be improved with a bit of thought and elbow grease. Also it could be improved by mounting the assembly on a flat piece of wood. I guess some way of sliding the cans along could be added. Maybe cutting holes and putting a wood dowel through the assembly would make it more stable. But as it is, the snug fit does the job.

I measured the minimum capacitance at 52 pF and maximum at 185 pF. That’s not bad. If this is used with a 225 microhenry loopstick, it will tune from 780 kHz to 1470 kHz. The capacitance depends on the size of the cans, the size of the plastic sheet, the dielectric constant of the plastic, and how far the cans are overlapped. So, YMMV. 👍

Instead of cutting the inner can, I thought about putting it on a deep groove in a heavy piece of metal and hammering the can into the groove. If it’s deep enough the can’s diameter would be smaller until it would fit inside if the other can. Another way to do this would be to get two pieces of plastic pipe that would fit inside each other.

Tuck Choy
A dual gang could be made by mounting 2 of the outer ‘stator’ cans to a flat surface, end to end. The 2 inner ‘rotor’ cans would have a dowel through both centers so they move in unison. All this could have been made more compact by cutting the cans and laying the sheets flat, possibly stacking more to get the full 365 pF or more. The soda bottle dielectric is very thick, reducing the capacitance – it could be thinner.


2019-05-30 Does Fiber Cable Last Forever

from FB group Bell Telephone of.. May 30

Alan H. Immerman
Well, underground, in a duct, the fiber and the plastic conduit looks like it was installed yesterday when it’s dug up a dozen or more years later. There is no weathering or UV deterioration underground. But then there are vermin that can chew up the fiber cable, but I’ve never seen that happen with the black underground cable. They make it tough, and the duct protects it more. I guess the guy was right. He said that fiber cable was easier to cut than copper, but it also takes much less time to repair. The networks are only as weak as their weakest link, so it’s important to make sure the fiber network has redundancy – it should be designed in a ring so a single break anywhere will not take it out of service. And it’s common for fiber cables to get upgraded to higher capacity before they get old. Just pull out the old and put in new.


2019-05-29 Another Oscillator For AM Band

Also posted part to FB group Building Transistor Radios May 30

I built this coil by winding 20 turns 28 AWG enamel coated wire on a soda bottle cap. It has an inductance of about 17 microhenrys. I wound a tap for the antenna, but it would be more stable if a separate coil with 5 turns of wire was used for coupling the antenna. I used a soda bottle cap for the coil because it’s easy to obtain and the air core inductor has the lowest temperature coefficient so the frequency is very stable. A coil form could be made from a pill bottle, a piece of plastic pipe or piece of wood. I used a push pin to make holes in the plastic for the wires. After I wound it I coated the wires with clear nail polish to make it stable.

I replaced the 27k with a 1k. With the two 3300 pF capacitors, it oscillates at 995 kHz. I haven’t applied modulation to it yet. I’m still experimenting with the values for the 100k resistor and capacitor I drew in series with the coil lead.

My Earlier blog about this is dated May 26. Here is the YouTube video about this with the schematic at the end.


2019-05-26 Low Power AM Transmitter

I Can’t Find One

I’ve seen several AM broadcast band transmitters on websites and on YouTube, but all of them require a 1 MHz oscillator module, which is difficult to obtain. I see modules for every other frequency not in the 550 to 1600 kHz AM band, but it seems all of the ones (and crystals too) for the AM band are sold, and not available. The modules put out a square wave, so they will transmit on multiples of the module’s frequency. Also, the crystals and modules are not adjustable, they are just a single frequency. So I want to build an AM transmitter that will allow me to choose the frequency it is on. In the metro area where I live, there are so many AM stations that it is difficult to find a frequency that is available.

I Finally Found One I Like

I found this ‘build a beginner’s AM transmitter‘ project on YouTube, so I decided I would build it and modify it to get it to put out a bit more power.

With the 27k resistor severely limiting current, it can be heard within a few feet of the 3 meter antenna wire, but not in the next room. The way it is it could be used to align a radio if the radio is close to the antenna wire.

The schematic at the end of the video called for a wire wound around a toilet paper roll to give 12 microhenrys, but I used a half inch toroid with 13 turns, giving 12 uH. The added orange winding is a few turns for driving the antenna. The schematic called for two sets of three 1000 pF (102) ceramic capacitors in parallel, I replaced them with two 3300 pF (332) capacitors. These gave a frequency of 1204 kHz.

I used a 2N3904 for the transistor. The 27k between the +9VDC and the circuit restricted the circuit to very low current so I reduced that resistor value until it seemed to be loudest. I measured the resistance and it was 5.1k. I used a 0.047 uF instead of the 104 ceramic capacitor on the audio input, for better high frequency audio response. Even so, the input should be driven by a device’s speaker or earphone output.

Photo shows how small this is. The dark green wire in left is modulation input. The short orange wire from the toroid is for the 3 meter antenna wire.

Update May 28 – Paul Burnside
I replaced the 27k with the 10k trimpot and a 2.2k resistor in series. The higher current increased the output, and I adjusted it to see where it seemed to be optimum. It measured 5.1k. There was no change in the frequency, 1204 kHz. The transistor was a 2N3904, so the optimum point may be different depending on the transistor. So far it has very stable frequency, not the FM I’ve experienced in the past.

Parts Values For Other Frequencies

It looks like this is a variant of a Vackar Oscillator. I’ve built this kind of oscillator before and it can be very stable. It is a modification of a Colpitts Oscillator. You can find out more by searching for Vackar oscillator.

I calculated that for a frequency of 600 kHz, the coil should be 30 microhenrys and the three capacitors in parallel should be replaced by a “472” 4700 pF ceramic capacitor. If you use the 12 uH coil with a 4700 capacitor, the frequency should be 948 kHz. For the 12 uH coil with 2200 pF capacitors the frequency should be 1385 kHz. For 1600 kHz, the coil should be 9 microhenrys and the three capacitors in parallel should be replaced by a “222” 2200 pF ceramic capacitor.

Other values will give frequencies in between. Two variable capacitors could be used, but the values would have to be 500 pF or more for each, and the usual variable capacitor is expensive, as much as several tens of dollars US. It’s just easier to pick combinations of ceramic capacitors for the three in parallel until the right frequency is reached. Just start with low capacitors and add until the frequency comes down to what you want. Also, if the coil is on a toilet paper roll or plastic pill bottle, it’s best to wind the coil with taps every few turns. Just use an alligator clip to clip on to one of the taps to change the frequency.

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