On The Inside

On The Inside

Friday, November 20, 2015

Technics 1200s, the Last Repair

If you DJ, and you play with vinyl, then you've probably had to deal with the annoying repair of the RCA cables that are permanently attached. I've repaired these numerous times for friends. Recently, my own 1200's started developing cable problems that can't be fixed by just wiggling the cable a little bit :)

I don't enjoy this type of repair, and I don't think that it should be necessary, so I decided to just modify my decks to not have to deal with this ever again. In the process, I switched my decks to self grounding via the signal connector. This is a potential safety and noise issue, but with how we're using decks these days, I'm not convinced that either are really relevant. That said, I like to live dangerously, "you" shouldn't do this without consulting your lawyer, your city or county building codes, an electrician, your grandmother, a priest, and perhaps a fortune teller just to be on the safe side.

Basically, I'm going to add some jacks to my turntables so that I can just use any set of RCA cables, and in the process, I'm going to eliminate the ground wire.  If you take your decks apart, you will find the tonearm area underneath a steel cover. It's removed here, and I have the new cover with the jacks and wires soldered in place.

The steel cover could be modified, but I think that this is neater.
I used a scrap piece of PC-Board material to make the replacment cover, and drilled a couple of holes to mount the RCA jacks. Pay attention to the positioning, you don't want to bump into the circuit board or any of the tonearm metal. The lines on the plate were to help me position the holes. The circle is drawn with the plate and bottom cover in place.

In the next picture you can see the plate bolted into place. Remember kids, red is right, everything else is left. You want to mark them to avoid strange phasing issues later when you're using two decks. It doesn't really matter much otherwise. 

With the back cover in place, I hope that you can appreciate the wisdom of taking the time to mark the opening.  It's working perfectly, no more dropouts or mysterious hum in the middle of a session.  Cables may still go bad, but now it won't take the better part of an afternoon to repair them, just unplug, toss, and replace.

Thursday, October 8, 2015

Reaktor 6 Blocks - Multi BreakPoint EG

I'm really enjoying a lot about Reaktor 6. The new Blocks format is, on the one hand limiting because they are monophonic, is still a lot of fun and great for rapid instrument prototyping. They do remind me a lot of working with the Nord Modular.

In Reaktor 5 one could do reasonably rapid prototyping with one of two macro collections provided by NI. Both the building blocks collection and the classic modular collection of macros behaved similarly to blocks, they just aren't quite as sexy looking, or sounding.

Since blocks are new, there's still a lot of room for improvement in the diversity of available tools. For example, there were some very flexible and complex tools in the aforementioned collections. One that comes immediately to mind is Erik Weigand's multi breakpoint envelope.

I converted this macro to blocks format, which wasn't too bad since most of the heavy lifting in Erik's macro is done in a core cell. However, it's no small task to get all of the bits of the new user interface elements lined up, something that I wish NI would work on improving.

I've added modulatable time scale and shape factor, added switchable velocity sensitivity, and change the color scheme to be similar to the bento blocks. 

It's free for Reaktor users and you can get it in NI's User Library. Please feel free to leave any comments or feedback, bugs, issues, feature requests, either in a comment here, the user library, or in NI's Reaktor forum.

Monday, September 7, 2015

HeadBox On the Inside

HeadBox is the standalone version of SmartCans that I use for testing and development. It is essentially a stereo audio processor with a built in headphone amplifier. On the back it has inputs for power, stereo in and stereo out.

The inputs and outputs are 1/4" to match the majority of my audio gear. The front has just a headphone jack and volume control. Everything else is controlled over the network. The two clamshell halves separate easily and the boards life out of the bottom for easy access to all parts.

The edison can be seen on the far left on the other side of the pc-board separator. The sound card is on the inside next to the jack board. The headphone amp is mounted to the front panel.

Underneath the jack board is some power supply filtering circuitry. The back panel and support board for the Edison and the soundcard are made by simply soldering two blank pieces of pcboard material.

The construction remains solid because groves are cut into support bosses in the plastic so that when the clamshell is closed, there is very little movement.

Friday, September 4, 2015

An Overview of SmartCans Hardware

The Intel Edison is turned at an angle to allow for the USB connectors to fit while still allowing the earcup to be reassembled. While there is still some room available on the sound card side, unfortunately, there's not enough room there to squeeze in the power supply. In order to solve this problem, the power supply which consists of a battery charger module, a 600 mAH LiON batter and an inverter to supply the necessary 5V for USB power were embedded into the headband.

The yellow spacers around the ear cups were designed to expand the space within the earcup without compromising the mechanical integrity of the ear-cup. They are about 1/4" thick which allows enough room, however, the screw bosses are still able to mate with each other. Of course, longer screws were necessary to attach the two halves. With this method the spacers press solidly against the ear-cup surrounds and do not expand the headphones too much.

The matching yellow inserts in the ear-cups serve not practical purpose other than to fill in the unused space left from removing the volume control and to maintain visual continuity. 

Each of the PC boards were hand etched. Although not strictly necessary for a one off or prototype, the use of PC boards provides for greater reliability. These simple boards were laid out in ExpressPCB and hand etched. 

The cable extending over the headband carries USB, power, and the speaker audio for the speaker on the Edison side of the headphones. The next version of SmartCans will combine the sound card and Edison on one side of the headphones and include a higher quality headphone amplifier on the other side. 

USB really just gets in the way here. The right way to do this is to directly connect the DAC and ADC to the Edison. This would require a new host board for the Edison, or a larger headphone that could accommodate Intel's mini host board. 

Sunday, August 30, 2015

SmartCans at Intel IDF

Along with several of the other Intel Innovators I was invited to Intel's IDF conference in San Francisco to show off the SmartCans project.  I had a great time talking about the project and received a lot of interesting and valuable feedback.

SmartCans Takes First Prize

I've been meaning to post more on the Intel Incubator program at the Sacramento HackerLab that prompted the SmartCans project. The program lasted about eight weeks, give or take, and culminated in a demo night where we were able to show off our projects.

The competition was stiff and there were many great projects.  Consequently, I was somewhat surprised, and of course delighted, when SmartCans was announced as the first prize winner. After the awards, Bob Duffy of Intel interviewed me and I discussed the motivation behind the SmartCans project.

Although there haven't been very many updates regarding the build progress lately, SmartCans is an ongoing project. I'll have more to report soon regarding progress with software as well as some new hardware development with the second revision of the SmartCans prototype. 

Juno VCF/VCA Module

Here's a filter module based on the Juno-106 filter chip. It came from an MKS-30 which had lost a voice. The MKS-30 is being fitted with different filters and will be the subject of a future entry. The Roland IR3109 filter, which is what the 80017 module contains, is a really good sounding filter. It's the same filter that's in the SH-101 and the MC-202. When you put it in a module form it sounds even better.

The 80017 module also contains a BA-662 VCA which is considered to be one of the best sounding chip VCAs ever put into synths. Combine the VCA and the VCF and you get a great sounding module.

This module allows you to use the VCF and the VCA together or separately. The VCF is normaled to the VCA but plugging anything into the VCA input disables the normalization. You can tap the VCF output without removing it from the VCA input in cause you want to further process the sound.

But what about the original MKS-30 sound?

An MKS-30 with 80017s sounds like every other MKS-30 on the planet. It's about as unique as Coca-Cola. Polysynths are a fantastic source of parts and the overwhelming majority of them are mediocre copycats of whatever else was available at the time. So go ahead and cut a few up and do something interesting with them, they won't be missed.

I think that it's useful to think of existing synths as a breadboard with all the hard and boring work like voice assignment, control voltage multiplexing, and patch storage already done. One can then remove some or all of the voice circuits and re-purpose the synthesizer into something unique

Some synths are so bad, cheap, or a fantastic source of parts that I don't even bat an eye about turning functional units into a nice collection of synth parts. There is one caveat, however, you do have to know how to scavenge parts. It's easy to destroy chips trying to remove them.

Circuit Description

If you look at the Juno schematics you can see that the Juno voice circuit is meant to work with CMOS levels and the CMOS is powered off of 5 volts. So for the most part, if you just duplicate the Roland voice circuit you should be able to get the bare chip to play with other gear.

There are some things to be careful about though. The filter CV is inverted and has a very restricted range. I use an op-amp to invert the signal and then scale it down to about +- 0.4 volts or so. Even at that level, turning the resonance up full the VCF oscillates from a few hz to 60khz. There is a pot on board to adjust the swing and it's set at about +-0.25 volts or so. This allows it to tune between a few more hz than a few hz and about 25khz or so.

For the res and vca CV inputs I just duplicate the Juno voice circuit using 2n3906 transistors. You don't need the 0.1 mf caps as they are there to smooth out the multiplexed CV. Just a resistor and the transistor is all that you need.

That's really all there is too it. Put the right resistors on the inputs and outputs and away you go.

The important part of the project was getting the smooth juno sound, not making a super accurate filter. If your goals are different you might have a lot more work to do. If you just want the juno filter sound in your modular, very few external parts are necessary.

Getting Started with High Level Plugin Design: Synthmaker/Flowstone, Synthedit, Max for Live, or Reaktor?

A lot of people are interested in getting started with synthesizer programming but are unsure about how to get started. I'm a computer scientist by training and have no problems using low level languages like C or C++. However, even for the experienced, that can be time consuming. For the initial high level prototyping, I tend to still prototype in Reaktor.  I don't think that there's a better choice for this kind of work. If you want to consider some basic idea for a plugin, you can probably find the pieces of that idea already in the library. You can also integrate various testing tools into the prototype environment itself. If you choose to start with Reaktor, then I think that the most direct comparison and simplest transition is to Synthedit. You will find the environments similar on a basic level. An advantage of Synthedit is that building externals is fairly straightforward.  This combination gives you a fairly nice path as follows:

1) Develop ideas using high level prototyping in Reaktor
2) Build prototype of fixed idea in Synthedit
3) Build externals for each of the components in your synthedit prototype.
4) Merge previous externals into a single synthedit "synth/effect" external.
5) Transition to SDK using your synthedit synth/effect external as a starting point.

You could skip right to step 2, but, I find Reaktor so much more immediate and complete, that I find early stage prototyping less annoying in Reaktor. You will also find that Reaktor is fantastic just to have as an effect and synth plugin. Really, for anyone that creates music and wants to go down this path, I think Reaktor is THE starting point. The reason that you don't skip right to step 4 from 2 is that the existing modules in Synthedit allow you to add your C/C++ code one module at a time. For example, if you're building a synth, after you get it working fairly close to your goal, maybe you want to build a specific filter. You can develop this module while keeping all of the standard Synthedit modules in place. You can release this at any time, of course, choosing to incrementally develop more and more of your synth in C/C++. When you have the core developed, you can then transition to a cross platform version of your synth.

Synthmaker: I like synthmaker, it has become Flowstone. Synthmaker/Flowstone will get you to a nice looking plugin much faster than synthedit. You can have something to upload and give away with a weekend's worth of work.  Underneath the hood, however, it's quite a bit more complicated and it doesn't have the nice path that synthedit does to creating a C++ plugin. That said, it has a very nice DSP language that you can use to code up blocks and I've found this helpful for algorithm experimentation. It's often much more immediate at a low level than Reaktor because you don't have to drag a bunch of blocks around in order to add two numbers. Is it worth the price of entry?  For most people, I don't think so. On the other hand, If you don't want to learn C++, you do want to create 3d modeled looking synths with a minimal of effort, and you are willing to accept the stock controls, synthmaker is the fastest way to get to a good looking windows plugin.  If you want to go beyond that, you should measure its utility by asking whether what it does well is worth it to you for some part of your process. If you want to get the basic feel of Synthmaker, then you can get SynthMakerCM from most issues of computer music magazine. It has many demo-ish limitations, but, it's cheap and you can see what it can do at a basic level. IIRC, you can generate a VST with SynthmakerCM as long as you don't mind the UI having a SynthMakerCM logo on the front.

Max: I love MFL (max for live) primarily because it allows me to build user interfaces that work in the live environment. If you already own and use live and want to upgrade to suite, then this is probably worth the investment. There are quite a few practical elements in the MFL library so it's not bad for that first prototype, but, it doesn't translate as well to synthedit as Reaktor does as there are quite a few work/thought-flow differences.  If you have full blown Max there is a feature similar to synthmaker called GEN that allows you to write modules in a terse DSP like language. I tend to prefer Max+Gen, as opposed to SynthMaker, for this kind of experimentation now, but this doesn't come with MFL and I don't know if you can buy just the Gen license to add to MFL. In any case, it's an expensive route and I think Max has quirks that are in some ways more difficult to master than Reaktor/Synthedit; granted, some things are decidedly easier.  Max does allow you to use externals so it can be a bridge to coding your own fully C/C++ plugins, although for me, it's not quite as straightforward as Synthedit.

Pd: Pd is just open source max with a lot less of some stuff and some more of other stuff. If you think that you might get on with Max or MFL, download Pd first, it's free.

tl;dr: Reaktor is the place to start, Synthedit provides the most straightforward natural path from "Reaktor idea" to "C++ plugin." SynthMaker provides the shortest path to released plugin that looks good, but it doesn't provide a natural path to C/C++. Max/MFL/Pd are cut from the same cloth, they have a different thought/work flow, some say higher learning curve, MFL is a natural fit for Live users and these tools provide some path to C++ plugin development, but it's not as straightforward as Reaktor+Synthedit.

The Mul-Ten

Huh? What's a mul-ten? Simple, it's a multiple-attenuator. Yeah, I like the name, goofy as it is and it didn't cost me an arm and a leg to make.
  1. Panel from blacet - $4
  2. Knob $0.50
  3. Jacks 5 * $0.75 = $3.75
  4. Pot $0.50
  5. Labels/Solder $1.25
  6. Grand Total $10.00
I had to put some labor into it too, of course, and, if I was selling these, I'd want to make some profit. Nonetheless, I think you should make your own multiples. There's really nothing to them and if it takes you more than fifteen minutes to make then you're doing something wrong.

The panels are rejects from Blacet and come pre-drilled for one of their modules. So there was no effort or time spent drilling the panel. I just had to decide what to do with the holes that were already there. As it turns out, this module is very useful and the combination is often used together. But the idea came from thinking "how do I use this big round hole at the bottom of my mult." Ok, it's no flash of genius, I agree. But it's as simple an example of the process that I can think of.

If I had been  paying attention to the Blacet site, however, then I could have made a pretty good guess which module these panels were for. Blacet makes a multiple attenuator that uses the same configuration that mine does. Their's is better, doh! They normal the mult to the attenuator so that you don't waste a mult jack when you want to use them together. I like my name better, but, I failed. I didn't do two things that you should always do when building modules.
  1. Look at what others are doing to incorporate the best ideas that are out there into your own work.
  2. After you think that your module is as good as it can be, ask yourself, can I do better?

The Do Over

Since my module is all about simple DIY, it won't cost much to correct my mistake. Sure my labels don't look all that hot but they're easy to change. If I had made a custom panel for this then I would have to do the entire panel over just to accommodate the change. I use a cheap Brother label printer that prints white ink onto clear tape so that it's easy to make changes when I'm forced to do a redesign. They are functional, cheap, and very ghetto! By the way, the labels and solder didn't really cost $1.25, but hey, I need to round up to ten bucks cause, well, ten bucks!

So what about that fancy normaling? There's no additional cost, just wire the input jack's normally closed connector to the mult common. In my case I also want to swap the wires going to the input and output jacks so that the panel has a better flow. That will force me to change the labels.

The Real Thing

Compared to the CMS masterpiece of a mult at $217, Blacet's module is not all that expensive. At $30 for the kit you won't break the bank. Besides, they use better pots than I did and since their module uses a pc-board you can probably assemble it in less time, mabye ten minutes instead of fifteen. On the other hand you can have three diy mul-tens for the price of one Blacet multiple-attenuator kit.

Thursday, July 23, 2015

The Amateur Radio Parity Act of 2015

You're looking at KL7R's $10 version of a popular portable antenna. It's great if you want to use something like this while you're out camping or sitting at at a picnic table, but it's another thing altogether if this is your only antenna choice.

If you've been a ham anytime in the last couple of decades you've probably heard of PRB-1, which requires reasonable accommodation of amateur radio antennas from government agencies responsible for zoning. PRB-1 has had some positive effect, however, it does not apply to private entities responsible for CC&Rs. This has become something that many new hams have to deal with as it becomes increasingly difficult to purchase a home that is not affected by such restrictions.

The Amateur Radio Parity Act of 2015 is designed to help close this gap requiring reasonable accommodation of amateur radio antennas. In simple terms, the bill is to" direct the Federal Communications Commission to extend to private land use restrictions its rule relating to reasonable accommodation of amateur service communications."

But look, don't take my word for it, you can read the bill yourself, learn more about it, and directly contact your members of congress by heading over to killingradio.com.

Kurt, KU0O has set up killingradio.com, to make it virtually painless learn about the bill, read the bill, and to contact your representatives to voice your support for the bill.

So, please, head on over to killingradio.com and voice your support for the Amateur Radio Parity Act,  and do it today!

Tuesday, May 26, 2015

Crystals in Plastic

Just crystals, in plastic bags. I call it "Crystals in Plastic"

Their resonance is trapped, constrained by man's excessive need to organize, they are left insulated from the electronic world at large. Although free to connect with each other, they find the excitement wanting.

On the shoulders of giants...
(Available in a print)

Brothers - A Bunnel 6B and a Mizuho QP-21 pcboard.

Wednesday, May 20, 2015

On Filter Response

In a previous installment I showed a filter pulled from a CB radio. I've been battling with the filter impedance thing for some time now and can't seem to get a good method to reliably determine filter impedance. Here's my simple test set with two series pots and a couple of fifty ohm shunts.

So, you're supposed to adjust input and output for the best filter shape. Yeah, that's easier said than done.

I have found the process of trying to determine filter impedance empirically without the aid of expensive test equipment frustrating to say the least. This little diversion started because I wanted to measure the impedance of several filters that are in my junkbox, in particular, this nice little filter that came out of a Sears Roadtalker 40.

The way that this is supposed to work is that you put the filter into the jig, attach the input side to your sweep generator or noise source, the output side goes to your scope or spectrum analyzer, and you adjust the input and output knobs alternately until the ripple in the passband is minimized.

Great, that sounds easy-peasy!

Well, the devil is in the details. First, if we use the noise method, at least with the gear that I have, it is challenging to see the passband ripple in the noise, Ok, no problem, I'll use the sweep method. I have a couple of different sweepable generators, one of them should get the job done, right?

Generator #1

It's a BK-Precision 4040A. I'll say this, choosing the word "precision" as a part of your brand name is a stroke of marketing genius. Unfortunately, I don't think that the engineering department got the memo.  It's not useless, but I'm not really sure for which application the sweep function is intended. There is no sweep output so you cannot use the sweep generator to drive the X input of your scope in XY mode. If you do use the sweep function, it's very touchy and extremely wide, too wide, in fact, for adjusting a crystal filter. It does have a voltage control input which would be awesome if the start and end controls scaled an external ramp input, but  no, it's an unscaled input that is virtually useless without taking the time to put together a scaling jig.  The manual states that the input takes 0-10V ± 1V for a 100:1 frequency change. Great, plus or minus ten percent of the input range, that must be some of that precision.  Ok, enough bagging on the BK, mine was a craigslist find, it still gets some use. Speaking of bagging on the BK, you can order a handy tote! Ok, for real, enough bagging on the BK.

I also have an old HP signal generator, the ubiquitous and somewhat infamous 8601A. I say infamous because they are so well known for blowing their output amplifiers, which are made of unobtanium, that someone has re-engineered a replacement. Unfortunately for me, I think that 1) my output amplifier is blown, and 2) that the $90 cost is too high for me to justify repairing this old girl.
This lovely example is much cleaner looking than mine and also has the RF and aux outs routed to the front like any sane person would expect. Ok, I can see the value of having the cables out of the way for fixed applications, but that wasn't going to work for me.

First things first, a bit of testing. Sweep output hooked up to the X input, scope in XY mode, blanking out put hooked up to the blanking input, check!  I forgot to snap a picture, but, the blanking is reversed, the retrace is bright, and the sweep is dim.

I wired up a quick inverter using a 741 and connected it in line with the blanking signal.

This gave me a relatively nice looking sweep signal and I could get good blanking just by tweaking the intensity control of the scope. 

It's not quite that bright in reality, I used a long exposure on my camera to get a good trace picture. However, you can clearly see that the retrace (the falling edge of the ramp) is much dimmer than the sweep trace. The flat portions at the ends of the sweep cause the sweep to be fixed for a period of time at the start and end of each sweep. This makes it easier to setup the scope and signal generator.

Although the op amp worked well, there is no bipolar power supply that's easy to use in the 8601. I could make a simple zener supply from the +26 and -6 volt supply rails, but there isn't a lot of room in this thing so I decided to simplify the design. The op amp does a proper inversion around the ground point, but strictly speaking, that wasn't necessary, I just needed a change in the polarity of the blanking signal. So I replaced the op amp with a simple inverter made from a PNP transistor so that I could power it directly with the negative supply rail referenced to ground.  While I was at it, I put the generator on the repair bench to move the connectors around to the front. That way I could use the holes in the back for a new blanking output and a switch to disable it when I didn't want to use the sweep generator with my scope. It's a lot less hassle to reach back and flip a switch than it is to move both the scope and the generator just to connect a cable.

Here's the little inverter, built on a bit of perfboard, ready to be tucked into  the back.  The green wire is the inverted blanking output and the other three wireds are input, ground, and negative power. On the right you can see it tuked into the top of the 8601; it's the yellow blob just to the left of the transformer.

The original outputs on the back came with a nice label plate that just flipped over and added my own labels.

 Here's the output of the ramp, the original blanking signal, and the inverted blanking signal. You can see that the blanking signal is now inverted.  Yes, I should have kept the PNP in its linear region so that I preserved the nice ramps on the blanking signal, but it's not that critical and I was able to just throw the switch together with resistors that were just lying on the bench, literally. I have no idea what's causing the small spurs, I thought that it might have been my circuit, so I took it out, but they didn't go away. In any case, it's a blanking signal, it's hardly critical.

With blanking fixed and the jacks moved around to the front of the generator, I installed it back onto my bench ready to do some more testing. The output of the generator isn't quite according to hoyle. It looks reasonably clean on the scope, but, it appears to be low and the highest output position doesn't increase the signal at all. I suspect, as I stated earlier, my output amp has problems, but, for now, it's feeding a fairly clean and adjustable signal to the output, so, I'll proceed anyway.

My first attempt used my Boonton microwattmeter as a detector. It's old and crusty, but it works. I have no idea how accurate it is, however, as I don't have anything that is precise enough and reliable enough with which to check its calibration.  In any case, it's too slow to be useful in this context. To get a reasonable trace you have to run the sweep speed faster than the Boonton can reasonably follow.

I switched gears and grabbed an RF probe that I hacked together some time back for some reason. This is not a work of art by any means and was constructed without much thought, somewhat blindly, from an old handbook.  
The resistor tacked onto the 1/2 watt was not in the original build. After reading more detail I learned that the value of the resistor was based on the 10 megaohm input impedance of most VTVMs of the day. My scope has a one megaohm input impedance so I tacked a resistor in parallel to create the approximately correct voltage divider circuit. It helped, but I also read that these are really designed for a relatively low impedance drive circuit and I suspected that it was loading down the filter and my scope was already at its highest sensitivity. Further, it did not give a log output which would give me a better picture of the filter's response.  Finally, I never liked the housing choice for a probe, it was made to be quick and I would prefer to have an RF probe in a more probe like enclosure. So, I removed the simple circuit. Upon doing so I found a cold solder connection from the diode to ground, that might have been the cause of some of the nose, no matter, I was sticking to my plan.

I built a log detector using an MC13136, which is a receiver on a chip. Yes, it's a bit of a waste, but it's a through hole part that I have a few of which meant that I could whip it up quickly on proto-board.

The TO-92 part to the right is an LM78L05 which provides a regulated five volts for the log amp. To the right you can see a 1N4004 in series with the positive power terminal. This serves to drop the input voltage a small amount and to provide reverse polarity protection, something that might happen with such a casual power input. The two resistors at the top right are 100 ohm resistors in parallel, so, yes, this has a 50 ohm input impedance, but, it also has a wide range.

Does this yield an improvement? Yes, it does, but, it's still fiddly. At least now I can see something of a filter shape and adjusting the input and output pots does not completely kill my signal. It is not really effective at coming close to the impedance of higher impedance filters. I have one with a known impedance of 3.9k and as best as I can get with this setup is, larger than a few hundred ohms. 

My best result was with a Yaesu filter believed to have a 200 ohm impedance, my adjustment showed about 150 ohms. As for the filter that is the subject of this post, it remains a source of frustration.

What I've concluded however, is that old analaog equipment with voltage controlled sweep is not ideal for this kind of measurement. The controls themselves must interact; to get a wider image on the scope you either have to adjust the sweep, or, the X gain of the scope, either choice moves the bandpass in the image on the scope.  The precision required here is really only in two dimensions; first, you need precision in frequency, and second, you need precision in measurement of the resistors. The latter is independent of the test fixture, while the former is much more easily controlled in the digital domain.

The goal is to be able to repeat these measurements easily;  in the next installation I will move forward with a DIY simple scalar network analyzer based on the AD9850.

SmartCans Final Hardware Assembly

In getting the headphones buttoned up for presentation I ran into many small issues. In the end, what I expected would take just a couple of days took almost the entire week.

Here we can see the left side buttoned up. The small holeplug fills what was originally to be the exit point for the wiring. This presented two major issues. First, it was just ugly. Yeah, there isn't any way to say that other than it was ugly. I thought that it might give something of a borg look, I even thought about using clear heat shrink tubing so that you could see the wires. Ok, in another life I might have been an artist, but not this one. So, given that, I opted for the artistically challenged version of artistic, minimal and simple. I would stick to black heat shrink tubing where exposed and try to constrain the profile as much as possible as the spacers were already making the phones larger than they were originally.

Second, bringing the wires over the top of the circuit boards took up a significant portion of the vertical space and put too much pressure on the pc boards. Although it took a bit more effort, running them out of the top of the cups not only looked better, but allowed for much more breathing room inside the cups.

Here's the inside of the left can. The spacer is made of wood and colored with yellow stain. Yellow looks cool with black, and well, it was the only color of stain that I had in the garage. The wood is just plywood from a model shop. The wood is about 1/4" thick and serves to lift the rest of the earpiece away from the bottom half of the cup by that amount. Additionally, it serves the same role as the top half of the cup by presenting a flat surface that helps to bind the ear cushion in place. 

You can see how the cable is routed through the pcboard out of the cup and through a slot cut into the wood. The cables are connected on both ends with a 9 pin 0.2mm header. Even these are really too large, but, they are what I had on hand. The three pin 0.1 inch header to the right is the line level input which will connect to the 1/8" jack on the top half of the can. The cup gouged in the wood at the bottom is there to make space for the jack to fit.

If you look closely you can see some bright yellow felt underneath the pcboard. This is to prevent rattles.

Here it is, (almost) ready to be buttoned up. I didn't apply the heat shrink tubing until later because I needed to space it from the headband's point of view.  I say almost because this picture was taken before I realized that the cables really needed to go underneath the pcboard as shown above.

The original screws that held the two halves together were too small and I had to hunt down something that was long enough and about the right size. Fortunately, plastic is forgiving so some standard machine screws did the trick. After both sides were buttoned up.  I focused on getting the harness together for both sides. One of the things that I did early on in the assembly was to identify the pin numbers for each wire. I used a sharpie to mark each wire with the number of dots related to the pin number.

Here was an opportunity to use the "Western Union Splice" as taught to me by my seventh grade electronic's instructor, Mr Crandell. Of course, there's not really any loading stress, but you want the wires to be as small as possible. Don't forget to put on all of the heat shrink tubing first.

Although you can see the battery in this picture, the battery does not connect into the harness. The harness supplies five volts to each earcup and the battery only generates about four volts. You can see the power leads spliced into the harness, however, they go to the power supply modules visible on the headband.

 Here's a better shot of the power supply modules on the top of the headband. On the right is the charger module which takes USB power in and supplies power to charge the battery. On the right is a USB power module which takes battery voltage and converts it to five volts for USB. This module originally came with a USB connector attached. I removed the connector as it was unneeded and excessively large.

Before I put the headband on, they look like this taped up with a bit too much electrical tape, we'll work on that.  The headband is an aftermarket product that is designed as a replacement band for headphones that have a worn band. It serves a larger purpose here in that it covers the power supply components.

Here they are, buttoned up both with a cable plugged in so that they can be used like regular headphones, and without. Since they can also be wireless, it was important to make the cable removable.

The Death of a CB

A trip to the K6IS hamfest this past weekend landed me this beauty for the princely sum of $5.  I probably should have worked him on the price a bit, but, sometimes I'm just not in the mood to haggle and this was one of those moments.

Yep, that's a Sears RoadTalker 40 complete with a CB radio call sign of BDX-1676.  Now, that might be an official callsign from the FCC, or it might be some other CB related number, I really don't know. It does have the correct format for an original call sign, but I have no idea whether those numbers and letters were used by the FCC. By the late seventies, CBs came with a form that you filled out that basically had you self assign your CB call, not that anyone bothered.

More often than not with these old rigs someone has made a small addition, note the toggle switch at the top of next picture.

The switch goes down to the PLL area underneath so I assume that it was some sort of frequency mod. The biggest disappointment here was that the switch was totally dead, the handle was just flopping around. That's one switch that won't make it into my junkbox. 

Ok, but that's not why I gave up a fiver for this sweet sweet radio, nope, the reason is a bit simpler. It's because there's gold inside, sweet sweet filter gold. Ok, some of the other parts will make it into my junkbox. I always pull the driver and final transistors, the transformers, and the pots with switches. Sometimes I pull the toko cans, sometimes I don't bother. Any other crystals also get pulled, you never know when you might need som odd frequency, most of the rest goes to the recycler. 

There she is, ready to be plucked. We can see right away that she's got a useful center frequency of 11.2735 Mhz. Now to find the matching crystals, they're in there somewhere. This could be one of them right here.

As it turns out, that crystal was for 11.730. Now, that's not useless, it's perfect for a dual conversion receiver as it is 455kHz away from the filter. However, it can't be all there is because we should have USB and LSB crystals somewhere in the radio that are just a few kHz away from 11.2735. I assure you that the surgery was swift and painless.

Could this be them? As it turns out, no, these are 12.230 and 12.319 Mhz crystals that don't appear to bear any direct relationship to the 11 Mhz IF. I'm not really sure what they do in this radio either although I suspect that they have something to do with the PLL.

 A little digging and we've found the rest of the treasure hiding in the corner under some wiring

There you have it, the filter, it's LSB and USB crystals, and a bonus crystal 455 kHz away for building a dual conversion reciever with a 455 kHz IF. 

Now,  here's the catch. I don't have any impedance specs for the filter. Sometimes you get luckier and the filter either has the data printed on it, or, you can find specs online. The only reference that I have for the filter is another ham trying to sell a set just like this online. Having the schematic of the radio would allow me to at least make an educated guess, but that's not so easy either. The actual radio model number is 934.38260700, and, as near as I can tell, there are no schematics online. 

So, feel free to comment below if you have any insight about the impedance, data catalogs for TWE-FEC crystals, or a schematic for the Road Talker 934.38260700.


With an ongoing discussion on QRZ, more data has come out about this filter, however, it's still not clear what the impedance is precisely. It is the same filter used in the radio shack TRC48 as well as several other road talker, and probably radio shack, radios of the time. 

I took the time to trace out a partial schematic from the pc-board. I can't get much more than this because jumpers start to take the circuit to other unknown places.  This is not a low impedance filter.