Thursday, October 8, 2015

Simpleceiver ~ Part 3

Secrets of Homebrewing Revealed

How to build a project

 
Heathkit had it right and much to their credit they had a very high success rate with their kit projects. Let us examine the difference between the Heathkit approach and today's kits. Much thought went into a Heathkit project so that there was a logical and progressive build concept that frequently is missing in today's kits.
 
Typically a kit today, as received, is a bag of parts with an internet address and perhaps an link to a you tube video. Ben KK6FUT has called this solder smoke --you solder all of the parts and then watch it smoke when power is applied. The Heathkit approach was to chunk the overall build into logical and manageable small sized tasks and to test as you go. In effect the portions of completed work enable the builder to verify a circuit is working before proceeding to the next phase.  In essence the portions completed become a part of the overall test system. We strongly believe in this approach and Ben and I in the many articles we co-authored have adopted that principle.

 A friend in the UK Nick (G8INE) sent in the following which adds a bit to the decode about today's kits and kits building practices.

·         A lot of kits arrive as either a single bag of bits, or the components divided by type, not section so there’s a tendency to press on once the bag is open.

·         The received knowledge in many instances is to build from the lowest components up, its easy to start putting ALL the Rs in, then ALL the Cs, and that is encouraged in many places.

·         Some “kit” building starts with the bare board, and then building once you get all the parts – it needs a lot of self-discipline to break that sort of build up into testable stages, especially as by the time that you get to being confident working that way, one tends to be confident that it’ll work, or that you’ll be able to debug it … 

·         The whole kit building culture has shrunk, so the pool of wisdom extolling the virtues of a staged approach isn’t there as a part of the landscape, just Pete and Ben shouting the message !!

·         There’s a sort of instant gratification culture where people don’t expect to embark on long projects anymore, so the expectation is that you can just settle down and blast through the process in a single sitting.

·         There’s a lack of building culture, so people tend not to have all the right tools which includes patience and the attitude that encourages you sit and look at what you’re doing  then think it through as well as the hard tools.

[Thanks Nick --many really excellent points about the why of today. Pete N6QW]

 
Another piece to the puzzle is something Bill, N2CQR and I frequently hear arising from the SolderSmoke podcast is: "How do I know something is working?" We are frequently amazed that many new to homebrewing just don't know. By chunking circuit elements into small pieces this enables the homebrewer to really delve into each circuit element and to understand what is occurring in that part of the circuit. No collector voltage is a sure sign of two things: 1) the circuit is wired improperly and 2) no collector voltage, it follows no output. Pretty simple but often overlooked.
 
In the LBS* project a person emailed me with a photo of the audio amp circuit he built from the project and had included voltage measurements. The subject of the email was of course "Your Circuit Doesn't Work!" At one point the measurement was 3.0 Volts and another point it was "0" volts and yet these two points were supposed to be connected. Thus both should  read either 3.0 Volts or "0" volts. Close inspection of the photo showed the points were not connected. The individual had the answer in hand but never looked at the why of the data. Connecting the points made the circuit perform as it should. Homebrewing is more than Solder and Smoke. Now if this person forged ahead simply built the whole radio and had the same result (no audio) where do you start looking for the problem?
 

Where to Start?

 
The first step is to start from the back end and work forward. Below is the block diagram of the Simpleceiver. This is slightly different than the original post as the audio amplifier has been changed to a 2N3904 driving an LM386. Appropriately this is the back end and the starting place. Build the audio amp and get that working. A simple go no go test after making sure of the following is to simply connect power (see caution) and a speaker. Then placing your finger on the input should result in a large hum from the speaker --you will know if it is working. If there is no output then you will only have to deal with a small portion of the overall project to find out the why.

Steps before "Power On"

  1. Check all wiring to insure the wiring matches the schematics.
  2. Look for poor solder (cold) solder joints --this is soldering not welding!
  3. Look for any shorts or solder bridges.
  4. Insure all parts are the parts to be installed
  5. Insure that transistors and diodes have been installed with the correct polarity.
  6. Install a 1N4002 in the Plus power lead with the arrow pointing toward the circuit. (This prevents the circuit from going up in smoke when you hook up the power leads backwards.)
  7. Avoid the use of Wall Warts!
Once you have gone through this list apply power and hope there is no smoke.
 

 

Simpleceiver Audio Amplifier Schematic Diagram

 



This is the schematic that will be used for the receiver. In an earlier post we mentioned that the free simulation software was a handy tool. The 2N3904 pre-amp stage was simulated in LT Spice and the addition of the 1000 Ufd bypass cap was a result of that simulation. In case you are wondering this circuit has been used and reused many times in our projects. It works --so keep things simple and just use it!
 
 
* LBS s shorthand for Let's Build Something which was a two part article in the January and April 2015 Issues of QRP Quarterly co-authored by Ben, KK6FUT and Pete, N6QW. This project started with a direct conversion receiver and by utilizing most of the block modules from the direct conversion receiver ended up with a 40 Meter SSB transceiver. The Simpleceiver and companion Simpleciter is like the LBS in that it ends up being a working transceiver but uses a different approach.
 
73's
Pete N6QW

7 comments:

  1. I am closely following this project. Another helpful trouble-shooting idea is to print the expected voltage levels at certain key points on the schematic! Thanks for your time & effort on this blog! Thank you! WA7JTU---Steve

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  2. Hi Steve,

    Thanks for your email. Yes that is a really great idea and another closely following is to sketch the waveforms that might be expected.

    Stay tuned.

    73’s
    Pete N6QW

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  4. Hi Ed,

    Thank you so much for your post. There has been much written about audio distortion with the LM-386 and a whole host of cures have been developed some as simple as running the chip at 20X. I am not sure your comment about being leery is based on your personal application of this device or you are simply repeating what others have said.

    There is a fine line between simplicity with low part count and the performance to be expected. I can say that this same circuit has been used (and reused) in many of the radios I have built. (Check my youtube channel for the Shirt Pocket Transceiver and the JABOM and you can hear this circuit. Search for N6QW). Now if you are an audiophile and want low distortion then you may find the LM386 offensive.

    The original Simpleceiver prototype used a discrete part audio amplifier and I would rate that circuit as about equal with the LM386 BUT it has a lot more parts. It was used because it happened to be handy and so it was installed in the prototype. If you want to build that amplifier instead of what is shown the go to http://www.n6qw.com/ and find the link to the LBS and all of the information about that circuit is in that link.

    This brings up another major factor in our modular build approach and that is flexibility. Many times I will substitute my favorite circuit for one that is in a particular radio. The Simpleceiver has been designed with that in mind --don't like the LM-386 then sub a LM380. But keep in mind the LM-386 is a low part count application and once you have the Simpleceiver working you may want to experiment with other devices. The TDA7052 has been touted as a favorite sub. I bought six of them and every one of them was smoked in the recommended circuit. So that is why I tend to stick with things I know work --warts and all.

    Thanks again for your post -- homebrewing is as much about experimentation as it is building a specific radio.

    73's
    Pete N6QW

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  5. Never ask an LM386 question or mention phase noise. Any reasonable explainable is impossible. It takes experimenting. This blog says it all:

    http://qrp-popcorn.blogspot.com/2015/10/qrp-workbench-line-in-audio-amplifier_29.html?m=1

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    1. Thank you for the link --great info and it certainly addresses how to fix the LM-386 warts. Next time I get asked I can simply direct any questions to this link.

      73's
      Pete N6QW

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