MePADS

Description: If you are considering building the Desert Ratt 2, it is advisable to read all the accompanying text, as it contains relevant construction details. Engaging with the comments and replies can also provide additional insights, as previous blog posts have shown that readers often ask pertinent questions. Notably, Paul NA5N's comment included useful information regarding the 2 x 1,000pF (0.001uF) capacitors in the regenerative stage. There has been a long-standing interest in building NA5N's Desert Ratt regenerative receiver since discovering the attractively drawn schematic online. An updated version, known as Desert Ratt 2, along with a comprehensive description of its operation, is available on Paul's website. Late last year, an episode of "Chat With The Designers" dedicated to the Desert Ratt further intensified interest in constructing the DR2.

The WBR was a successful regenerative receiver, performing well on SSB/CW but lacking gain with AM stations. This behavior is expected, as a regenerative detector must be set below the oscillation point for AM reception, resulting in less gain than when oscillating, which is optimal for SSB/CW. Bipolar transistors are known to perform better in regenerative stages for AM due to their higher gain when not oscillating. This characteristic was a significant factor in the decision to build the DR2. Paul has noted that the Desert Ratt is less effective with SSB/CW compared to AM, which aligns with personal experiences, confirming its suitability for AM reception.

The design features a phase splitter transistor that converts the single-ended output of the detector to a balanced output, allowing the LM386 to operate in differential mode. This design choice addresses the high RF presence within regenerative receivers, where the common-mode rejection of the LM386 is advantageous. The detector utilizes two germanium diodes, evoking nostalgia for earlier crystal-set building experiences. The schematic indicates a design change from the original Desert Ratt, replacing a variable capacitor with 1N4004 diodes used as varicaps.

A preference for a high-quality air-spaced variable capacitor led to the use of a Millen 50pF capacitor combined with a 6:1 reduction drive, ensuring an effective tuning rate for AM stations. Modifications made to the original schematic include the addition of an RF attenuation potentiometer at the antenna input, addressing common-mode hum issues when using a short indoor antenna. This adjustment allows the use of an outdoor antenna without overloading the receiver and significantly reduces hum.

Earlier versions of the Desert Ratt provided instructions for winding the coil on various forms, while the DR2 schematic does not specify this. A toroidal inductor was used instead, utilizing a T68-6 former. A two-position center-off switch was implemented for band switching, replacing the SPST switch in the original design. Powering the DR2 from a shack power supply initially introduced processor noise, which was mitigated by using a separate SLA battery, although additional filtering was added to the power line for further noise reduction.

The inputs to the LM386 were initially connected incorrectly, resulting in screeching sounds, which were resolved by swapping the inputs. The connection for pin 7 was left unconnected, as its configuration in the original schematic was unclear. The construction utilized a simple PCB chassis directly built upon, with a variable capacitor mounting bracket created from double-sided copper-clad laminate. The variable capacitor chosen was a high-quality, silver-plated component, which had not been previously soldered and was likely over 35 years old.

The assembly process began with the audio amplifier, providing immediate feedback through audible signals when touching the input. The layout included the LM386 amplifier, a 2N3904 phase splitter, and a preamplifier. The power supply line filtering included a 1mH choke and a 1,000uF electrolytic capacitor, effectively reducing interference. The power indicator LED was chosen for its forward voltage drop, aligning with the design specifications.

The construction of the detector and impedance converter stages followed, with initial testing yielding positive results as proximity to the diodes produced audible signals. The number of turns for the toroidal inductor was estimated using online calculators, ensuring optimal performance for the winding configuration.If you`re thinking of building the Desert Ratt 2, although the pictures in this post are numerous and quite large, I do recommend reading all the text too, as I have included what I thought were relevant details on the construction as part of my narrative. Also make sure to read the comments and replies. Previous blog-posts have taught me that rea ders often ask pertinent questions, so you may be able to glean a little more information from them too. In fact, just before I wrote this, Paul NA5N made a comment which includes a usefiul piece of information about the 2 x 1, 000pF (0.

001uF) capacitors in the regen stage. I`ve been wanting to build NA5N`s Desert Ratt regen ever since I first found his veryattractivelydrawn schematic for it online. I then found the updated version, called the Desert Ratt 2, and a very good description of how the circuit works all of these documents available on Paul`s website.

What more could an avid regen builder want Not much, it turned out. Late last year, when N2CX and N2APB dedicated an episode of Chat With The Designers to theDesertRatt (and to the subject of regens in general), I just had to listen and of course, it fueled my interest in building the DR2 even more. The whiteboard for this particular episode of CWTD is here, and the podcast audio is here. The WBR was a successful regen for me and while it worked well on SSB/CW, it didn`t seem to quite have the gain with AM stations.

This makes sense, as a regenerative detector has to be set below the point of oscillation for AM reception, at which point it has less gain than when it is oscillating (which is where you set it for SSB/CW reception. ) Even so, I had read that bipolar transistors tend to work better as regen stages for AM, as they have higher gain when not oscillating.

The search was on for such a receiver, and this was one of the key deciding factors in building the DR2 for me. In fact, Paul has mentioned (I forget where I saw it, as I have done so much reading on this receiver) that the Desert Ratt doesn`t do so well with SSB/CW as it does with AM.

My experience with it backs up this assertion, thought it`s a pretty neat receiver for AM. In particular, I wanted a receiver for covering the 49M SW BC band as although my Elecraft K2 covers a few of the BC bands, 49M is not one of them. There were a few things I found interesting about the design. The use of a phase splitter transistor to convert the single-ended output of the detector to a balanced output in order to drive the LM386 in differential mode was novel.

Paul talks about how much RF is flying around inside regen receivers, and how the common-mode rejection of the 386 when used in differential mode can be advantageous in such an environment. I was also intrigued by the detector consisting of 2 germanium diodes I think I was just looking for an excuse to build something with Germanium diodes again to remind me of my crystal-set building days as a kid :-) If you look at the schematic of the DR2, you`ll see that one of the changes in the design from the original DR is that instead of a variable capacitor, it uses 1N4004 diodes as varicaps.

I have a bit of a thing  for nice air-spaced variable capacitors, and I had in mind a nice Millen 50pF capacitor that I picked up on eBay for a very fair price last year. Combined with a 6:1 reduction drive, it made a good combination with a very useable tuning rate for tuning in AM stations.

Anyway, I`m getting ahead of myself here. I did make a few changes to the original schematic for my version, so allow me to introduce my rather wobbly circuit diagram - - I added an RF attenuation pot at the antenna input. After building the DR2, I found that using arelativelyshort piece of wire indoors as an antenna was causing a lot of common-mode hum.

On top of that, I wanted to be able to increase the signal level into the receiver with the use of my regular outside antenna (A 40M dipole fed with 300 ohm balanced feeder. ) Using the attenuation pot allowed me to use the large outdoor antenna without overloading the receiver.

Use of my outdoor antenna created enough separation between the receiver and antenna that the hum problem almost entirely disappeared. - Earlier versions of the Desert Ratt included instructions for winding the coil on a plastic 35mm film canister and on an IC shipping tube.

The DR2 schematic doesn`t include such instructions, but I wanted to use a toroid, so I experimented a bit and came up with a scheme that seems to work OK. I used a T68-6 former and the turns info is on my schematic above a T50-7 would take up a little less space.

More about this later. - I had a few 2-position center-offswitchesthat I wanted to use, so I used one of these for a bandswitch instead of the SPST switch in NA5N`s DR2 schematic. I had originally thought that using the 50pF tuning capacitor with no padding would make the upper limit of frequency coverage too high, resulting in too large a frequency swing in one band, but there must have been more stray circuit capacitance than I had anticipated, as the coverage with no extra padding was about 7.

3 13MHz. This band became the center position. - I was attempting to power the DR2 from my shack power supply, which is about 45AH of sealed lead acid batteries with a float charger constantly connected. This also powers my K2, and the DR2 was picking upprocessornoise from the K2, as well as a low-frequency burbly  kind of noise of undetermined origin.

The problem went away when I powered the receiver from aseparateSLA. but I decided to add extra filtering to the power line anyway. I found that a 1mH choke as well as a 1, 000uF electrolytic almost (but not quite) got rid of the unwanted interference on the power line. For good measure, I added a 0. 01uF RF decoupling capacitor across the power line at the input connection. - The inputs to the LM386 are the opposite way around from the way indicated in NA5N`s DR2 schematic.

With the inputs connected as shown in Paul`s diagram, the LM386 emitted a loud screeching sound. Swapping the inputs cured this. I was not the only person who had this problem, as I discovered from this post in the GQRP Yahoo Group (you need to be a member of the group to read the post). - I left pin 7 unconnected. I don`t understand the way that NA5N has it connected to the junction of the series resistor and capacitor connected between pin 5 and ground in his diagram.

Most circuits that use pin 7 call for a decoupling capacitor direct from pin 7 to ground (usually about 10uF). This helps reduce large signal distortion, though Paul does say that in this application, it may not do a great deal to help and is therefore optional.

I elected to leave it unconnected. Now for some pictures. I didn`t want to spend a lot of time constructing an enclosure, so decided to make a simple PCB L-shaped chassis and build the circuit directly onto that. With the variable capacitor mounting bracket, it still ended up taking quite a while to construct though.

All my projects begin like this, with the main components and control being laid out on the front panel, while deciding on the basic layout - I`ll spare you the words at this point and apologize for all the pictures that are about to come. If you`re living in a remote area and are still relying on dial-up, then I feel a bit sheepish about the sheer number of images to follow!

I`ve talked before about constructing enclosures from PCB material, so won`t repeat that information here. As well as constructing the chassis from PCB material, I also made a mounting bracket for the variable capacitor and a tuning pointer to attach to the reduction drive with 2 small screws all from double-sided copper-clad laminate.

I applied several thin coats of lacquer from an aerosol spray. It was sprayed from a distance, resulting in a light, and stippled coating, which you can see in these pictures. I`d rather apply too light a coat than risk overdoing it. The downside of this is that oxidation will being to affect the appearance of the copper fairly soon.

Oh well. The capacitor mounting bracket received a thicker coat. You can see the smoother, shinier finish. I got the 6:1 reduction drive from Midnight Science. A number of others sell them, and one place that springs to mind is Mainline Electronics in the UK. They are the suppliers for Jackson Bros components (Ithinkthey have the rights to manufacture and sell the parts). They sell on eBay using the name anonalouise. The enclosure looked a little bit different by the time the DR2 was finished, as the hole for the nylon toroid mounting hardware hadn`t been drilled in the base at this point.

A close-up view of the Millen 21050 50pF air-spaced variable capacitor and mounting bracket. This component is silver-plated (the vanes are probably brass), and has double bearings and a ceramic base. It is a very nice variable capacitor, and had never been soldered to before being used in this project.

It is at least 35 years old most likely older! Boy, was I glad to finish the chassis so that I could start work on wiring it all up. I decided to build the AF amp first and work backwards, my thinking being that the AF amp would be relatively straightforward. The act of touching the input with a metal screwdriver and hearing a hearty buzz in the loudspeaker would give a welcome psychological boost!

If I started by building from the antenna end, I`d have to wait until the entire receiver was built before getting any clue as to whether it was working. Here`s the chassis with the LM386 amp, the 2N3904 phase splitter, and the 2N3904 preamp built. As has been the case with all my projects since I started using then, I used W1REX`s wonderful MePADs and MeSQUAREs to build thecircuit- Here`s a close-up.

The 2N3904 preamp is just below the 6:1 reduction drive, and the 2N3904 phase splitter is to the left of the LM386. The 100uF capacitor that decouples the supply line to the LM386 straddles it. I read that it is best to ground it to pin 4 instead of to some other point on the chassis to avoid instability, hence the reason for this placement.

The other electrolytic that is straddling the chip is the 10uF capacitor between pins 1 and 8 that sets it to the maximum gain of 46dB. The black shielded cable connecting the AF gain pot to the circuit on the PCB is lavalier mic cable. It has 2 conductors, each of them in it`s own shield, which is ideal for wiring up potentiometers. It is fairly thin and very flexible. I use it in all my home-brew projects. I bought it from a local pro-audio store which recently closed down, so will now need to find another supplier.

In this view, you can clearly see the extra DC supply line filtering that I added, consisting of a 1mH choke in series with, and a 1, 000uF electrolytic across, the DC supply. After seeing these pictures, I noticed that there wasn`t very much solder on the joint connecting the choke to the power jack, so Ire-flowedthe joint and melted a bit more solder onto it.

The power indicator LED`s main function is as a voltage regulator. NA5N marked the various voltages on his schematic for the DR2, and I chose an LED with a forward voltage drop to match those voltages as close as I could. A green LED in a variety pack I got from Radio Shack had a forward voltage drop of 2. 1V, which seemed about right. The 1N4148 had a forward drop of about 0. 65V. The next stages to be built were the detector and impedance converter/buffer stages. The description of the DR2 on NA5N`s site gives more info on these stages (as it does for the whole receiver).

I couldn`t be sure these stages were working, but bringing my finger close to the diodes resulted in a pleasing cacophony of stations in the headphones and at a louder level than in doing the same to subsequent stages, so I figured there was some detection/amplificationgoing on :-) I didn`t know how many turns I was going to use on the toroid, but using the calculator on W8DIZ` site and an online resonantfrequencycalculator, I figured that 36 turns on a T68-6 should be a good starting point for the whole winding from pin 3 to pin 6. In Paul`s version, with the coweek or so).