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domenica 13 ottobre 2019

Fixing a problem with HF reception on a chinese RTL SDR

I recently bought one of these little SDR boxes from a chinese vendor on the web. It was expected to allow reception of both VHF/UHF frequencies and HF frequencies.

Internally, the unit contains a small PCB from a typical RTL SDR dongle (built around the RT820T + RTL2832U chipset), mounted on a larger PCB and connected to it by means of few solder joints (on the back side of the larger PCB).
The larger PCB also hosts two SMA antenna connectors (labeled "UV" and "HF" on the aluminium enclosure), the USB connector for communicating with the PC (which runs the SDR sofware, typically the SDR# application) and a "power on" blue LED.
The "RTL SDR dongle" part of the unit is in charge of managing signals coming from the "UV" antenna connector (a SMA female port), just like on any typical USB-pluggable RTL SDR unit.
On the remaining area of the larger PCB, the "HF" antenna input is connected to a LPF filter followed by a single transistor stage, which purpose is to take the HF single-ended signal from the LPF filter and to translate it into a double-ended signal for the RTL2832U chip ("Q" port of the internal ADC, pins 4 and 5).

The RTL2832U must be configured for "direct sampling" mode of its "Q" input, to properly manage the HF signals received on its pins 4 and 5. This can be made by means of the SDR software application running on the PC (for example, the SDR# application).

That said, after having connected the unit and properly configured it on SDR#, I had to discover it didn't work on HF. After some investigation, I found that the transistor at the end of the HF branch (which is mainly in charge of generating a double-ended signal for the RTL2832 pins 4 and 5) did not show any voltage on the collector terminal.
I connected it to the 3,3V output from the voltage regulator on the RTL dongle PCB and things got better. Following photographs show the wiring from the AMS1117 voltage regulator (which takes the nominal 5V level from the USB port and produce a 3,3V level for the on-board devices) to the collector load resistor of the transistor (look at the blue arrow on photograph below).

You will also notice (in a yellow circle) a non-SMD ceramic capacitor (10 nF) that I have had to solder in place of a SMD capacitor that unfortunately I had broken during initial tests.

giovedì 22 agosto 2019

First tests with my build of an "hybrid feedback" regenerative receiver

First tests with my last build of a tube regenerative receiver.
The original design is by Vladimir Novichkov (please look for thread "6AS6 regen, mk2" on for a detailed description).
I opted for a simplified build (no varactor-based fine tuning, no tube-based AF amp, no AGC). Still, probably one of the ugliest buids ever seen :) Anyway, after some tweaking it's starting to work.
The global tuning range is about 3600 kHz to 10700 kHz.
Yes, I need a reduction drive for the "band spread" capacitor, otherwise it will drive me crazy very soon :)
The small card at the centre of my kitchen table is a small JFET-based audio preamplifier.
The hairy hand appering in the video is not King Kong's, it's mine 😁

lunedì 8 luglio 2019

A strange LSB demodulation problem on my Yupiteru MVT-9000

I bought my Yupiteru MVT-9000 from an on-line auction at what seemed quite a fair price. I was a former nostalgic owner of the MVT-7100 model, that I had sold when I thought I wasn't interested anymore in using a scanner receiver.
The radio was in very good functional and cosmetic conditions, but I soon realized that it was affected from a strange LSB demodulation problem, that I documented in this short video:

At a first impression, it seemed that the unit worked in USB mode even when the LSB mode was selected.
Some researching on the web and questions posted on specialized forums produced no solutions in terms of unit configuration, reset, etc.. Also, it was clearly not a common issue for the radio: no trace of a similar problem experienced by others.
Finally, I decided to inspect the schematic diagram. Quickly enough I found the area of the circuit where the BFO was located. It was apparent that the selection between LSB and USB was performed by slightly modifying the output frequency of the BFO, as described in diagram below:

The BFO section of Yupiteru MVT-9000

There was two control signals from the CPU that seemed to play an important role: first, CARSEL (CARrier SELect) was in charge of adding/removing an additional capacitor in parallel with the crystal X3, thus causing the BFO frequency to switch between the USB value (lower) and the LSB value (higher). In fact, the BFO of the MVT-9000 is actually a VXO, with two selectable output frequencies depending on CARSEL value.

Second, the SSB/CW control signal from the CPU (together with signals AM/W-FM and N-FM, which have a similar function) was used to switch the input of the AF (audio) amplifier onto the proper demodulation path and bandpass filter. When SSB/CW is active, the audio amplifier takes its input from the mixer stage (transistor Q12 in previous image), which combines the BFO signal with the received RF spectrum (that is the output from the 455 kHz IF filter).

Well, it was time to dismount the radio and to inspect things directly during its behaviour.

Disassemblyng the Yupiteru MVT-9000 was fairly easy (below some photograps taken during the process).

Then I gained access to some important measurement points:

The measurement of the DC levels of control signals CARSEL and SSB/CW in modes AM, LSB, USB showed that they was working as expected.
On the other hand, it seemed that the BFO output frequency was changing only by a very small quantity when the operating mode was switched between USB and LSB. Respectively, 453.65 kHz in USB mode (which is a pretty good value, that is about 1.5 kHz below the center frequency of the IF filter) and 453.77 kHz in LSB mode, which is wrong. A value of about 456.5 kHz would be expected (about 1.5 kHz above the IF frequency).

Even if I was measuring the BFO output frequency with a tiny, very old and cheap frequency counter (a little SOAR FC-841), the results matched well with the observed erratic behaviour of the receiver.

Well, so it resulted that the VXO was actually switching between two frequencies, but one of them was out of specs. This seemed to suggest that one at least of the trimmer capacitors (TC2 and TC3), in parallel with the X3 crystal in the BFO circuit, needed to be realigned.
So I needed to furtherly dismantle the radio to reach the involved devices.

In the image below, you can see the other side of the printed board which hosts the BFO circuit, with trimmer capacitors TC2 and TC3 and the test point TP1 where to check the VXO output frequency. Also visible is the Murata CFJ455K device, which is the IF filter.

Exactly when I was ready to try a realignment, my old SOAR FC-841 frequency counter decided to stop working (to be honest, I broke it accidentally, by connecting a power supply with inverted polarity). So I had to buy a new frequency counter.
When it arrived, some days later, I re-checked the BFO frequencies in LSB and USB and (surprisingly) they both looked good! I was measuring about 456.55 kHz in LSB mode and about 453.67 kHz in USB mode, as recorded in the short video below.

I really couldn't explain what happened. After some additional verification (like connecting a temporary antenna to verify that the LSB demodulation was now acceptably clear), I decided to reassemble the radio and to make a live test. This confirmed that LSB was now a properly working mode on my Yupiteru MVT-9000 scanner, as documented in the short video below:

mercoledì 3 luglio 2019

A simple MW box loop antenna revitalizes a poorly sensitive pocket radio

As one of my summer projects, I decided to build a simple MW box loop antenna, just to put into practice the idea of making it using PVC cable ducts like the ones in the image below:

The construction was fairly simple, as documented shortly by the following series of images:

I used some PVC-specific glue to connect the various pieces together. The inductive section is made by 12 turns of  AWG 14 electric wire, covering completely the 40 mm internal width of the PVC duct. The size of the square loop frame is 60 x 60 cm.

For the capacitive (tuning) section, I recycled a polyvaricon capacitor and a small fine tuning capacitor that I had salvaged from an old transistor receiver. They proven to perfectly fit the purpose of tuning the whole MW band. It was only necessary to add a small switch to put the two sections of the polyvaricon in parallel when tuning the lower end of the MW band. With the two sections of the capacitor in parallel, the lower limit of the tuning range reaches about 375 kHz.

When it came to perform some tests, I was not expecting any difficulties. I thought to use my XHDATA D-808 portable radio to verify how the MW reception would have improved by inductively coupling the internal ferrite rod of the radio with the box loop antenna.

Well, after several tests with different approaches, still I wasn't able to assess that the external antenna was behaving as expected. For example, I wasn't able to identify the typical, narrow peak in signal intensity that corresponds to the selective nature of this kind of antenna. On the contrary, there was a distinct, unexpected, narrow notch, along the tuning range, where the signal completely disappeared.

After some reasoning, I concluded that probably the XHDATA D-808 was too sensitive on its own to take advantage from coupling with such a box loop antenna. Probably the narrow notch that I was finding during tests was caused by overloading of the input stages of the radio when the antenna was tuned exactly to the received frequency.

So I decided to try with a little, very cheap, poorly sensitive pocket radio and finally things started to behave as expected, as documented in this short video clip:

lunedì 1 aprile 2019

Setchell-Carlson model 524 receiver: the BC-1206-C

I found one of these tiny WWII A-N radio range navigation receivers on a famous web site for on-line auctions. It looked in nice overall conditions, complete and unmodified, both externally and internally. It was untested, but for me as a collector of vintage radios this was not a big resistor. The price looked fair for this kind of unit so I decided to buy it, despite of the relevant cost of shipment from the U.S.A.

The Setchell-Carlson model 524 was an A-N radio range navigation receiver
Once it arrived in my hands, of course I started to inspect it and I found it was really in good conditions as it promised to be. It had the serial no. 5888 on the front plate.

The Setchell-Carlson model 524 a.k.a BC-1206-C.
The short wire in the foreground is the antenna connection, the longer wire
from the back of the unit is the power supply (+ 28 VDC) connection

Internal view with tubes. From left to right: 28D7, 14R7, 14H7, 14J7, 14H7

Internal view with the tuning capacitor

The back of the unit with the +28 VDC and ground connections. The two capacitors and
the two coils form an input filter on the power supply voltage.

View from the bottom. All components and wires look original.
Only the white wire on the right is more recent (may be a repair).

Generally speaking, the construction tecnique is rather simple and low cost, compared with bigger WWII receivers like the venerable BC-348 or the BC-312, just to mention two. Only tuning and volume controls, a few cheap connectors, a very simple (and rather rough) tuning mechanism. Probably there was no provision for real maintenance and no needs for an higher accuracy.

I found a very nice description of the role of BC-1206-C during WWII in this post of "The US Militaria Forum": US Navy marked Setchell Carlson BC-1206 CM2 radio range receiver.

Forum user MIFlyer wrote: "In the 1940’s the standard aircraft radio navaid system in the U.S. was the AN Range.   The AN Range predated the widespread use of Automatic Direction Finding receivers and used the 200-400 KHZ aircraft navigation band.  Use of the system only required an ordinary AM receiver tuned to the band.  Pilots would tune in the station that best corresponded with their destination, listen to the signal, and hear either the Morse code signal “A” which indicated they were to the left of the desired course, the Morse code signal “N” that indicated they were to the right of the desired course, or a continuous tone, which indicated they were right on course.  Everyone with an aircraft sophisticated enough to have a radio used the AN Range, and so many light aircraft were equipped with only receivers for the 200-400 frequency that most control towers were set up to transmit on 278 KHZ for the purpose of giving landing and takeoff clearances to aircraft.  For this reason, when the USAAF adopted the SCR-274N  “Command Set” receivers and transmitters early in WWII, a standard equipment set consisted of a BC-453 receiver for 190-550 KHZ, a BC-454 receiver covering 3-6 MHZ and a BC-455 for 6-9.1 MHZ, together with a couple of transmitters, enabling coverage of both the AN Range as well as the standard control tower frequencies and other required military communications.

When the USAAF got to Europe, it found that the RAF had adopted VHF for fighter aircraft communications, using a crystal controlled set known as the TR1143.  The Americans had to be compatible with the British when it came to fighters, and VHF gave far superior short range communications anyway, so the U.S. built the British set as the SCR-522, and adopted it as the standard radio for fighters, at least in Europe.

VHF was a big advancement but it caused a bit of a dilemma, especially for fighters operating in the U.S., which had to still use the AN Range for navigation.  Overseas, low frequency beacons were less available and a lost fighter pilot would call for DF steer from a ground station.  But fighters flown in the U.S., at least, had to have the low frequency receiving capability.  Problem was, the SCR-522, while no more bulky than the SCR-274N receivers, transmitters, and modulator, still took up virtually all of the available room in the aircraft.  Also, the fighters deployed to Europe would not necessarily require the low frequency capability, so an easy add-on capability was desirable to keep things as standard as possible.

The answer to this problem were the two cutest aircraft radios ever built; the Detrola Model 438 and the BC-1206.

Both the Detrola and the BC-1206 were designed to operate directly from 24VDC, without a dynamotor, and to be set up so that they could fit into a standard aircraft 3 inch instrument panel hole.  They were small enough to be mounted directly in the cockpit with no more than a power lead and an antenna connection.  Using their headphone jack, the sets could be plugged directly into the same audio circuit used by the SCR-522, so switching between radios was not required.  They could be installed or removed within minutes without affecting the VHF installation.

The Detrola went into the later model P-38, P-51, P-47, and some P-63’s, fitting right into the cockpit, possible because the remarkably small receivers were only about twice the size of the control boxes used for the larger radios.  The P-51 and P-47 had the little set right next to the pilot’s seat, facing upward at an angle, while on the P-38 it was to the right of the pilot’s seat, almost resting on the floor, facing up.  On the P-61 it was in a rather strange installation behind pilot’s right shoulder, the dial facing forward.  The P-63 manual I have describes the Detrola as a “portable” installation, even though it is bolted down under the main radio panel at the bottom of the instrument panel and the P-38 manual says the Detrola “may still be installed.”  Presumably this indicates the possibility that the set was yanked out when the aircraft was deployed overseas.   Interestingly enough, none of these installations used the set’s ability to be installed in an instrument panel hole.  All installations used long wire antennas; on the P-51D, the Detrola antenna is the long wire coming through the hole in the top of the bubble canopy and stringing back to the tail.

In contrast to the Detrola use and installations, the only manual I have found that mentions the BC-1206 shows it installed in a side instrument panel hole in a P-80A, the early model Shooting Star jet fighter.    

The two sets fulfilled an identical function and were clearly made to identical specs, but are actually quite different electronically.  I have always assumed the Detrola and BC-1206 were just related models of the same radio but recently found out they are very different.  The Detrola Model 438 was made by the Detrola Company, and employs five standard WWII type octal tubes, including a VT150 oscillator/mixer and a couple of 6L6’s for audio output.  The BC-1206 was built by Setchell Carlson and uses five 14 volt loctal tubes like the 14A7 and 14J7 and a 28D7 output tube.  Both have an unusual antenna spring loaded lead socket on the left lower side near the front, use a combined volume/on/off switch, and have a ¼ inch headphone jack on the front. While the tuning knob on the Detrola simply operates a geared pointer against a frequency dial painted on the face of the set, the BC-1206 has a window over a dial to show the selected frequency.

While the Detrola was used extensively in the U.S., I don’t know if it actually went to combat.  However, the period photos of P-51’s in Europe show the long wire antenna, even though an SCR-522 usually can be seen behind the pilot, so one would presume those wire antennas were hooked to something 

Postwar, the Detrola and BC-1206 were replaced in refitted WWII aircraft and the newer jets by the BC-453 and/or the new ADFs such as the ARN-6 and ARN-7.  No doubt the larger sets were much better receivers, - but they were not half as cute!"

A good and simple description of the AN radio range navigation system can be read here:
Detrola 43B - Low-Frequency Radio Range Receiver

Well, now of course I was curious about discovering wheter my BC-1206-C was still able to work or not. So I quickly inspected the circuit looking for anything suspect. There was a white wire in the filament power supply circuit which was apparently newer the all other wires around it (i.e. PVC or teflon insulation instead of cloth insulation), anyway the wiring resulted to be functionally the same (not exactly the same) as in the service manual I had found on the web. Also, I checked that there were no short circuits between the +28 VDC wire and the chassis, so I decided to give it a try at 12 VDC, just to see what would have happened.
The power supply current at 12 VDC resulted to be a reassuring 440 mA. But the biggest surprise came after connecting a few meters of random wire to the antenna input and quickly surfing the limited tuning range between 200 kc and 400 kc: suddenly the id of the local NDB (PIS at 379 kc) clearly came out of the LS-166/U loudspeaker that I had connected to the audio output! Not that bad as a first power-on test.

Now it was time to give it the full +28 VDC power supply voltage. I did not have a suitable power supply, so I connected the 12 VDC power supply in series with a 12 VDC battery to get a good +24 VDC for my test.
Measured power consumption resulted to be (approx) 700 mA at 25 VDC.

Local noise (from man made electric sources) was terrible during this test. May be this problem could be mitigated with a better antenna (during test I had attached to the antenna input simply a few meters of thin insulated wire lying on the floor).
At the lower end of the 200-400 kc tuning range, the receiver detects few european longwave broadcasting stations. The level of signals is good but reception is unclear, may be due to passband limitations of the radio, which was intended for the AM reception of Morse encoded signals, not for voice or music. Anyway, I noticed that the readability became good when the unit was powered by a 12 VDC battery only, despite of the fact that the audio output level was much lower in that case, of course.
At the lower end, in addition to the local NDB (PIS 379 kHz), I could hear the signal from GBG 426 kHz (Gleichenberg for Graz, Austria), as documented in the short video below:

giovedì 28 febbraio 2019

My R-326 russian military receiver

This post is simply a short photo album of my R-326 russian receiver.

You can find detailed information about this little military radio at following links:

Russian Receiver R-326
Soviet Era Radio: Dennis reviews the Shoroh R-326 receiver
The R-326 Receiver

My R-326 with all its accessories

The user manual (in Russian)

This nice wooden box contains several spare tubes and rubber parts

The front panel and the power supply

Russian instructions on the front cover

Pages of the user manual

Ready to operate

My modified LS-166/U used as the external loudspeaker

Modifications on a LS-166/U military loudspeaker

I bought a LS-166/U military loudspeaker in good general conditions recently. According to labels on it, my LS-166/U was made in Italy by LA.RI.MA.R.T., in Rome.
I bought the LS-166/U with the aim of adapting it as the external loudspeaker for my russian R-326 military receiver.

As you may well know, the R-326 (or P-326 according to the russian alphabet) is a nice small receiver, which can operate both from batteries and from AC mains (by means of its power supply). It started to be produced in early '60s. It employs 19 miniature tubes and requires only about 1.5A at 2.5V to operate, which is surprisingly a low power requirement for a tube receiver of its complexity.

More on this receiver at following links:
Russian Receiver R-326
Soviet Era Radio: Dennis reviews the Shoroh R-326 receiver
The R326 Receiver

The R-326 receiver (from

The R-326 does not feature an internal speaker. It has a 600 ohms audio output, in the form of a bipolar socket in the lower part of the front panel, that can be used for connecting both heaphones and a 600 ohms external speaker.
Let me say that the original, heavy duty, military headphones that came with my R-326 are not the best you could desire in terms of comfort. So I started to think about how to connect more modern, light and comfortable low-impedance headphones to the radio. First, I looked around for a 600 ohms to 8 ohms transformer to build a simple impendance and connector adapter, then I thought that it could be nice sometimes to have a chance to listen to the radio by an external loudspeaker, possibily a military one. The LS-166/U appeared to be well suitable for my purposes: 600 ohms input impedance, an internal impedance transformer with 8 ohms output, a rotary switch that could well be used to redirect the output to a stereo jack (to be added) for headphones. Finally, there were enough room to add a volume control.
In fact, the R-326 does not offer a real AF gain control on its front panel. Rather, the output audio level can be controlled by acting on the RF gain control. In practice, it is expected that the audio level be controlled externally, on the loudspeaker.

The LS-166/U detailed information can be found here:

This is the original schematics of the unit:

This is how I have modified it (I hope it is readable):

I removed the original cable (terminated by the U-77/U connector) and replaced it with a robust bipolar cable terminated by two very simple banana plugs, suitable for the R-326 audio output socket.
Then I re-wired the internal connection to adapt for the addition of a 100 ohms wire potentiometer and a stereo 6.3 mm jack.

NOTE: after first period of usage, I preferred to re-wire the volume control as a voltage divider, in parallel with the secondary of the impedance transformer. The original series connection did not allow to completely silent the audio output.

This is a short video showing the unit while working connected to the R-326 receiver.

Here below a brief photo story of my modifications.

The re-wired internal connections

The added audio level control and headphones jack

The original switch now selects speaker or headphhones

The new cable with banana plugs