Pictures from Farhan's Hyderabad High-School Direct Conversion Workshops

 

Farhan came up with the idea of having high school students build their own receivers. We followed his lead -- there are now several such projects underway around the world. 

The simple but effective Colpitts circuit that Farhan recommended. 

The PTO tuning idea came out of  his "Daylight Again" transceiver. 

Oscillation!  

Farhan's High-School Direct Conversion Receiver Workshops in India

Farhan sent this picture yesterday.   If you look closely you can see the students holding their homebrew 40 meter Direct Conversion receivers.  You can even see that they are using the same kind of PTO coil forms that we are using here.  Farhan reports that 11 receivers were built by 33 students.  A few more are being finished and will soon be active in Hyderabad. 

I was really blown away by this picture.   We are doing the same things on different sides of the world.  Our students will like this.  It will be as if they are seeing people of the same age building the same receivers 7,500 miles away.  

In our last session I mentioned to our students that Farhan of Hyderabad had given us the toroidal transformers that they are putting into their mixers.  I told them that in ham radio, when we use parts given to us by a friend we add "soul to the new machine."   And I said that Farhan would be coming to see them in May.  They were really impressed. 

We are starting to see similar efforts in different parts of the world -- Andreas with university students in Germany,   Daniel with high school kids in Canada.   We hope there will be others. 

Fourier Analysis Explained (video) -- Understanding Mixers

Over the years we'vE had a lot of posts about Joseph Fourier: 

https://soldersmoke.blogspot.com/search?q=Fourier

Recently I've found myself mentioning him while explaining how the diode ring mixer in our 

high-school direct conversion receiver project works. 

I think the video above does a good job in explaining how Fourier and his math explain how our mixers work.

My HP8640B Signal Generator Lives Again

I'd really come to like this old signal generator.  The construction is superb.  It was built to be repaired.  As you open it up you find all kinds of useful diagrams and pointers.  It is very solidly built - it looks like something that was built for the Apollo program.  And it was given to me by a friend:  Steve Silverman KB3SII gave it to me in 2017 -- he had it in his New York City shack.  Dave Bamford W2DAB picked it up for me just before Steve moved out of the city.  

I've already done one complex  repair on it -- one of the tines on one of the selection switches fell of and I had to replace the tine.  That was difficult, but it was a very satisfying repair.  

But lately, the HP8640B started acting up again.  It developed an intermittent problem that caused both the signal generator and the frequency counter to just shut down.  

I was thinking that this might be the end of the road for the HP8640B.  I even started looking for alternatives.  But they were all very unappealing.  They come in plastic boxes with names like Feeltech and Kooletron.  The boxes are filled with flaky wiring and boards hot glued to the plastic.  Yuck.  The contrast with the HP8640B could not be stronger.  

So I started to think about the problem.  This was the first part of the troubleshooting process.  I asked myself:  What would cause several different systems (counter, frequency generator, and display) to all shut down?   The power supply was a leading candidate.  

I started reading the power supply section of the HP8640B manual.  There was a line in there that caught my eye:  The power supply boards had on them LEDs that glowed if the board was functioning.  Thank you Hewlett Packard!  I opened the top of the signal generator and found the power supply boards. Sure enough, there were the LEDs.  I turned the generator on, and found that one of the lights was out. Bingo. (Trevor takes a look at the power supply boards in the video above.  I have it cued up to the 12:57 point at which he talks about and shows these boards.) 

Here was the other clue:  The problem was intermittent.  It kind of seemed like a loose connection.  So I just unseated the board and took it out.  I put some De-Oxit on the connector and popped it back in.  Boom:  The LED came on and the HP8640B came pack to life. 

There is a whole bunch of great info and videos on the HP8640B on the internet.  It is almost as if a cult has developed.  This signal generator is worthy of a cult following.  Count me in.  

I especially liked the video below.  Kevin really captures the admiration that many of us feel toward the way this piece of gear was built.  He also kind of hints at the way this sig gen could become a pirate transmitter on the FM broadcast band (at 8:44):  

I know that eventually the problematic plastic gears in this device might fall apart.  I am prepared for this:  I already have the metal replacement gears from India.   

Thanks again to Steve Silverman KB3SII and Dave Bamford W2DAB for bringing me into the HP8640B cult. 

A Couple of New Digital Multimeters: a Fluke and an AstroAI

Our high school direct conversion project made me realize that I really need to upgrade my digital multimeter. I've been using an old Radio Shack DMM that I bought about 25 years ago.  It is OK, but it is not auto-ranging and it is starting to physically deteriorate.  So off I went to Bezos-land. 

First I spotted the Fluke 101.  I was enticed by the brand and the low price.  But when it got here I was a bit disappointed.  It is really small -- smaller than my cell phone. It is auto-ranging, and it does measure capacitance,  but it doesn't measure hFe and the frequency counter only goes up to 100 kHz.  I couldn't use it to measure the frequency of our DC receiver PTO.  So, back to Bezos. (I'll keep the Fluke as a toolbox DMM.)  

Next I found the AstroAI True RMS 6000 DMM.  Obviously not as prestigious as the Fluke, but both the Fluke and the AstroAI are manufactured in China.  The AstroAI was really inexpensive:  Like 34 bucks.  And Amazon would do same day delivery here.  Soon it was on my front porch. 

I've only been playing with it for a day or so, but I really like it.  It is auto-ranging, it has automatic shut-down, the frequency counter goes up to 60 MHz,  it measures hFe and even has temperature sensor.   The frequency counter had no problem measuring the output frequency of our DC RX PTO.  The screen is big and bright. And I think the True RMS feature will be very helpful when I try to measure amplifier gain. 

I like it. And you can't really go wrong for the price.  34 dollars! 

I have the Astro AI DMM in the Amazon ads on the right-hand column of the blog.  I should have bought the package with the additional test probes.   Click over there on the right for more info.