History of Reciever Design – 1912 Regeneration and Building the TecTec 1253

Radio Design 1912 Style – Regeneration

Build the TenTec 1253 9 Band Regenerative Receiver.  Listen to shortwave SSB, CW, and AM the old school way.  Plus learn something about the history of radio and receiver design.

Resources for this post

History of Regeneration: Invention to Patent Litigation by Prof. Sungook Hong
regeneration_hong

Principles of Regeneration
principles_regeneration

Radio Receiver Design – Old School vs New School

The TenTec 1253 is “old school” insofar as the receiver is a “Genny” or regenerative from back in the 1920s. Its a radio technology from when dinosaurs ruled the earth – before superheterodyne. The TenTec 1253 can receive AM, SSB, and CW from 1.8Mhz to 22Mhz in 9 bands and its a hoot to operate.

But the TenTec 1253 is also “new school”. It is new school in the sense that it uses modern components to implement the old school regenerative technique. A true old schooler would use tubes and discrete components to build a Regen.  The 1253 is old school in new school clothes.

The “new school” part of the 1253 will let you see how some new techniques and technologies work. This includes: varactor diode tuning, integrated audio amplifiers, voltage regulation, and IC-based band switching – no more wafer switches. None of this was around in the 1920s. So only the technique of Regeneration is old. The implementation is new.

Tools for the Build

So what do you need to build this kit. You can read lots of articles on kit building and see what they recommend. Here the list of tools we recommend if you build this kit

You need the standard stuff: a soldering iron; solder; scissors and knife; screwdrivers; magnifying glasses; high intensity light; wire clippers (or nail clipper); tweezers; and some pens. Don’t forget that you need a large area that will not be disturbed. You will also need a 12v power source for this kit and a VOM to test some voltages. Skipping ahead, we can tell you that the “as tested” assembly time for the TenTec 1253 is about 12 hours.

Some things you might not think of that you may need: de-soldering wick; solder sucker; electrical tape; pins or a needle; a drinking straw; tweezers; and most importantly you need patience and the ability to follow, and sometimes modify instructions, when its obvious that what they tell you is a bad idea tested by experience or foresight.

So what about some of these more unusual things? De-soldering wick and a solder sucker is useful if you make a mistake or want to make a modification. A pin or needle helps in getting components out and restoring the PC board hole pushing out molten solder. Sometimes a good burst of air from a drinking straw will dislodge molten solder – its a simple variation of a solder sucker.

You need tape for this kit to immobilize delicate wires that the instructions tell you to attach early in the assembly process – and here is “do what makes sense” comes in. The wire in this kit is very fragile and a few bends will break it. So solder and tape to a fixed location so it does not move during assembly.

You might also want to use some Styrofoam that you can easily cut to make a cradle or mount for your board as you solder more and more components. The idea here is to make sure you do not crush components on the top of the board when you have it turned over and are soldering on the bottom.

Getting Down to Business

Parts Inventory and Prep

The TenTec 1253 has about 200 parts not counting hardware and two circuit boards. It is very important that you do a parts inventory before you start. I made sure I had all the parts for the kit before starting. Turns out that Iwasshorted a bunch of LEDs and a couple of capacitors. There will be nothing more frustrating than sitting down to build a kit and then find out in the middle of the assembly that parts are missing.

Regarding the missing parts, I called up TenTec and told the story. I was immediately transferred to a technical person who took a “no questions asked” punch list of what was missing. They sent the parts for free and they arrived in a few days. So that was nice of TenTec but the kit should have been packed and QA’ed in the first place before it was shipped.

I recommend that you lay out all the parts on pieces of legal size or larger paper using double-sided tape to secure the parts to the paper. As you take inventory, attach the part to the paper and write on the paper next to the part the value of that component.

Some of the parts are tiny, esp. the inductors, which are color banded. Use a magnifying glass to verify the part value and also make sure you know what the banded colors look like – don’t confuse “orange” with “red” and know what those colors look like on the components.

If you make a mistake you can then practice with the solder sucker, solder wick, and tweezers. Doing this identification work “up front” and then verifying it will make the rhythm of the assembly go smoother. You won’t have to worry about difficult identification once you start board stuffing and soldering.

Parts Stuffing and Soldering

The instructions will tell you to solder each part as you stuff the part on the board. That’s fine, but it slows the assembly and does not really result in uniform soldering. A better process is to stuff the board with about 10 components at a time and then solder the 20+ connections in one shot.

Here is how this works. Stuff the PC board with the components in the order that they tell you. Bend the component wires on the underside along the PC traces at about a 45-degree angle. Stuff as many components as makes sense. When you have enough components stuffed then turn the board over and prepare for soldering. Use a PC board holder or make your own out of Styrofoam in order not to crush the parts on the top when you turn the board over to solder.

Make sure the soldering iron is hot and clean. Look at the task at hand and plot out your strategy – the order that you are going to solder. Start the process. Heat the intersection of the component and the PC board trace and apply the solder to the other side of the component wire. If all goes well, solder will flow onto the wire and the board forming a good electrical bond. If the PC board traces are close be very careful and use the least amount of solder that is necessary. Get the rhythm going to solder all 20+ connections in one shot.

When the joints are cool, clip the excess wire with a wire cutter or a nail clipper. Inspect the solder bonds with a magnifying glass and make sure you did not cross any traces with your solder. Again, a solder sucker and solder wick can come in handy removing excess solder or completely de-soldering a part.

Strategy – Incremental Build and Test

The TenTec 1253 manual is quite good. The assembly is in phases. Each phase is more or less mapped to the functional block diagram of the radio. Each phase has a test procedure. Don’t proceed unless the test works – there is no point in proceeding further if the test fails – the radio won’t work. Debug each section until it works. Patience will pay off. Frequent testing will pay off. Ignore this advice to your doom.

Phase I

Phase I is the audio amp. You should well understand that any radio is the result of a collection of separate functional units or blocks that all must work together. The IC-based audio amp section is a good example of a chunk of the functional block diagram mapped to physical components stuffed into the board along with all the traces that connect this radio section to other sections.

The audio amp is a 9-pin TDA2611A IC. It has 5 watts of audio output that is plenty of volume to dive the speaker included with the radio. The chip cost about $1.50. So that’s the cost of “new school” for this “old school” radio. The TDA2611A only needs a few external parts to work as an audio amplifier section for the radio.

At the end of Phase I you perform the functional block test by connecting power, the speaker, and feeding in some random noise to amplify. If you hear a squeal, well, then you have an oscillator and you really want an amplifier.  You will need to check and fix the amplifier functional block before proceeding. Phase I is the complete audio amp section of the radio.

Phase II

Phase II is the voltage regulator. This is an IC MC7805CT. New school price, about 30 cents. A stable voltage is needed for the varactor tuning and to run the “band switch”. At the end of Phase II you can test the voltage with your VOM for 8 volts. (Down from the 12-15 volts radio power source)

Note that old school is wafer switches – new school is electronic switching.  Big cost savings and reliability.

Phase III

Phase III is the real guts of the radio. Its the RF pre-amp, Regen detector, and varactor tuning. A varactor diode is a neat device that changes capacitance as a function of applied voltage. This varactor diode will form part of a tuned resonant circuit when you “switch in” (band switch) one of 9 fixed inductors.

So your “tuning dial” is really a pot that applies different voltages to the varactor diode changing its capacitance rather than the tuning dial being a mechanical linkage to a air dielectric capacitor as in old school radios.

New school price of the equivalent tuning air dielectric capacitor is about 29 cents. Fronting the tuning circuit is a bare bones RF amp implemented as a FET. The all-important Regeneration that takes part of the tuned detected signal and feeds it back into the circuit is a couple of FET’s with a pot to control the level of regeneration. Too much regeneration and you will get oscillation – a squeal that you will not soon forget. At the back end is an audio preamp into the audio amp section that you built in Phase I.

Phase III is a critical step and you get a couple of circuit options here. I took the standard route but also took the time to install some “stubs” for experimentation. I installed two stubs made from component wire clippings such that I could easily include or exclude the extra components from the circuit at a later point.

Phase IV

In Phase IV you will actually get to hear the radio. Phase IV is really a full-out test for Phase III and everything you have done so far. Phase IV is to jury-rig one of the inductors into the tuned circuit, attach an antenna, add power, and see what happens.

The recommended inductor will get you tuned resonance 6.8Mhz to 8.5Mhz depending on the effective capacitance of the varactor diode which is controlled by the applied voltage (pot) from the voltage regulation section.

The detected signal goes into the audio amp section. So, at this point, you can see how all these radio sections work together. On 6.8 MHz to 8.5 MHz should be able to hear some broadcast AM and amateur radio SSB and CW. We did, with a wire antenna about 15 feet long. Pretty exciting.

If all this works its time to take a rest and enjoy your accomplishment. The rest is all downhill from here. If your radio does not work at this poin then you have some diagnostics to do.  Again, patience and block/functional testing pays off.  Functional block testing is something that was definitely missing in the Heath kit builds from the 1960’s.

Phase V

Phase Phase V is the band switch control board. Ok, in old school this would be a multi-position wafer switch that would switch in different inductors to form a tuned circuit with the varactor diode. In new school this is CD74HC4017 decade counter. New school price – about 36 cents.

The idea is simple. You push a button ( simulate a clock pulse) and the CD74HC4017 counts. As it counts a different pin on the IC goes “high” to power an indicator LED and to “switch in” one particular inductor of the 9 inductors.

So there you go. 9 bands implemented as 9 fixed inductors switched in one at a time into a resonant circuit with the varactor diode providing the variable capacitance. When you press the button nine times the counter rolls over and you are back to the first fixed inductor and the lowest of the 9 bands – if you got the color-codes right when you stuffed the inductors.

The band switch control board is a separate PC board with components on both sides of the board. In the picture above the push button is on the other side of the board along with the LEDs. The white wires will eventually go to the main board and the bank of 9 fixed inductors. The LEDs press against and protrude through the face of the radio cabinet and correspond to the band markings. The test for Phase V is to push the button and watch the 9 LEDs light in sequence.

Phase VI

Phase VI is to install the bank of fixed inductors on the main board that get switched in one at a time to form the resonant circuit with varactor diode. At this point, you pretty much have a complete multi-band radio.

Testing at the end of this phase is to ensure that each time you press (clock pulse) the band button, the next LED lights in sequence and the proper inductor is “switched in” to form the resonant circuit for the indicated shortwave band. The white wires you see below are from the band switch control board.

Note that the board has unused positions for additional capacitance to tweak the resonance frequency for a particular band. Using these and some simple math you can move the bands from the factory spec within limits.

Phase VII

Phase VII is mechanical assembly where you place the circuit boards into the metal case and make other physical connections. Mechanical assembly is difficult. It is difficult or nearly impossible to fit all the tiny LEDs into the holes provided in the metal face. But this is really not necessary as you can see the lights through the holes. The fitting and alignment of the push button used to advance the electronic “band switch” will try your patience to get it “just right” so it does not stick and is properly aligned.

We opted not to attach the headphones jack as we have no intention of using the radio in this way. We opted to not install the battery tray as this makes the internals very crowded. Our radio is powered through the supplied external power jack on the back of the radio. We opted to not install the tiny power-on LED as there is no good way to mechanically attach this diode without the risk of shorting to the metal case. The final assembly and the mechanical design of the case leaves much to be desired.

Operation

The TenTec 1253 is fun and a challenge to operate. You need to understand the process of Regeneration in order to have any quick success in tuning SSB and CW. AM broadcast is easy to tune across all the bands. SSB and CW tuning is a result of a complex interaction of the RF gain control, Regeneration feedback, and fine-tuning. You need to experiment with these controls and their interaction to get the “hang” of this. After some experimentation you should be able to tune an SSB signal quickly.

Be aware that you are using technology of the 1920s and the fiddling with Regen control is just what the hams and shortwave listeners did nearly 90 years ago. You can build this radio as an experience is kit building, understanding how signals were tuned and detected in the 1920s, and also how some of the new technology is so much more cost effective than what was used only 30 years ago.

To replace a mechanical wafer band switch with a 36 cent IC Decade Counter… To replace an air dielectric tuning capacitor with a 29 cent varactor diode… well, there is something lost for sure; but the economic advantage is obvious.

Preserving Radio History

I listen to shortwave radio on a number of old radios.  The oldest I have to date is a floor standing model from the 1930’s – a piece of furniture.  I also have an 11-tube German Nordmende Tannhauser radio from the 1950s.

So, I listen to radio as God intended in the spirit of the history of radio. So the past will be preserved.  To get a historical perspective, check out our resources below.

Read more

Principles of Regeneration
principles_regeneration
History of Regeneration: Invention to Patent Litigation by Prof. Sungook Hong
regeneration_hong

Click the image below to see the full schematic

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