Tag Archives: Mehdi

Photos from an EP2C field event

Icom IC-R72: a repair story

Icom-IC-R72-Front-Mehdi-Asgari In this post I’m going to tell you how I repaired my Icom IC-R72 receiver. Although it’s about a specific device, the logic and methodology applies to all radios.

I bought an Icom R72 from a friend for about $200. At first, I just checked 7.0MHZ (40 meter band) and 21.0MHZ (15 meter band) and it was OK. After some time, I tried to listen to some ham radio on 20 meters (14.0MHZ) and it was deaf! I checked everything: antenna connectors, balun…everything.

With some trial and error, I found out that it was deaf from 11-15 MHz. With the help of the members of “Icom R72” Yahoo Group, I found one of the usual suspects: bandpass filters’ switching diodes.

I took a look at the service manual and apparently this radio uses multiple bandpass filters for different frequency ranges.

Schematic-Icom-R72-1

As you see in the above picture (grabbed from service manual), one of the bandpass filters is for 11-15 MHz range–that’s the range where my radio was deaf.

Note that there are multiple ways to test that a radio is deaf at a frequency. One of the simplest ways: connect an antenna or even a long wire to the antenna socket of radio. The noise level should increase; if not, there’s a problem.

Schematic-Icom-R72-2

After testing diodes with a multimeter, I found out that D31 is faulty. Almost all multimeters have a diode-test functionality.

I replaced it. The original diode was 1SS53, but I used a 1N4148 which is very common and found everywhere. Now I have a working radio! 🙂

If you have a radio that’s deaf at a frequency range, there is probably a problem in bandpass filters.

Here are some internal pictures of my Icom IC-R72:

IC-R72-Internal-IF-Stage-Mehdi IC-R72-Board-1 IC-R72-Board-2 Icom-IC-R72-Open

I should thank my friend and electronics mentor, Saeed (EP2LSH) who always helps me in my electronics adventures.

Mehdi Asgari, the author of this post, is a regular contributor to the SWLing Post. Mehdi lives in Tehran and is an active member of the EP2C amateur radio club.

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How to use the SDRPlay RSP as a panadapter

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SDPlay-RSP

In this post, I will show you how to use an SDR as a panadapter for a commercial communications receiver. I’m using an Icom R72 and a SDRPlay RSP, but you can do it with nearly all receivers and SDRs.

The Icom R72 is a double conversion HF (0-30 MHz) communications receiver.

A panadapter lets you see the spectrum of your receiver (it gives you a broader, higher-level picture of what’s around your tuned frequency). After using it, you’ll wonder how you’ve been using your receiver without a spectrum display! 🙂

Almost all the radios we use are of superhetrodyne type. To connect a SDR to the radio, you should first find its 1st IF mixer output (if it has more than one Intermediate Frequency stage).

To find it, the best way is to consult the service manual. The first IF of Icom R72 is 70.4500 MHz, so we can use RTL-SDR dongles too.

Here’s a part of R72’s schematics, extracted from its service manual:

IC-R72-Schematic-Mehdi-Asgari

Look at the R82 resistor; it’s located at the output of first IF stage, so I soldered a connector to its pins (its other pin is connected to the ground)

Between the R82 resistor and the output, we’ve added a series 470-ohm resistor (to prevent loading the IF stage)

Click to enlarge.

The other side of this connector is connected to the antenna input of my SDRPlay RSP.

Now I can view my spectrum on a PC.

Here’s a screenshot of CubicSDR on Mac OS X (you can use other apps like HDSDR, SDR Console, etc.):

CubicSDR-Screenshot-Mehdi

Although this post was about the SDRPlay RSP and Icom R72, this procedure can be done for almost any combination of receivers and SDRs provided that your SDR covers the frequency of first IF stage.

Also, don’t forget that you should tune the SDR to the frequency of IF–in this case: 70.4500 MHz.

Mehdi Asgari, the author of this post, is a regular contributor to the SWLing Post. Mehdi lives in Tehran and is an active member of the EP2C amateur radio club.

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Mehdi discovers ESD protection in the Tecsun PL-680

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The Tecsun PL-680

Many thanks to SWLing Post contributor, Mehdi, who writes:

Once you told me to be careful when attaching an external antenna to my portable radio.

Today I opened it to see whether it has some sort of protection on its external antenna path.

As you can see in the picture below, there are two diodes side by side (D4 and D2) which protect the radio’s front-end against high-voltage inputs.

Description of what these two diodes do from http://www.giangrandi.ch/electronics/diode-clipper/diode-clipper.shtml:

“The basic idea of the diode clipper is that for signals with an amplitude within ±0.7 Vpeak (less than about +7 dBm over 50 ?) the diodes are just open circuits and do not interfere; for higher amplitudes, the diodes clip the signal to about ±0.7 Vpeak limiting the maximum power that reaches the receiver at about +10 dBm over 50 ?, which the vast majority of receiver circuits can easily tolerate. If you wonder why I specified +7 dBm for unclipped signals and +10 dBm for clipped ones of the same peak to peak amplitude, it’s because unclipped signals are supposed to have a nice sinusoidal shape; once clipped, they become more square and their RMS voltage is higher, explaining the 3 dB difference.”

Thanks for sharing this, Mehdi! I’m happy to see that the PL-680 has some built-in ESD protection.

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Now Online: A New WebSDR in Iran

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SWLing Post contributor, Mehdi, has informed me that his ham radio club (EP2C) has just put a WebSDR online in Iran–perhaps the first in this part of the world. Mehdi notes some of the details about the receiver:

[The WebSDR is] installed and operated by our amateur club: EP2C (www.ep2c.ir)

[Perhaps] the first WebSDR in the Middle East (the nearest one I know of is located at Armenia). We have a dipole antenna and an AVALA SDR receiver (going to be replaced by a SoftRock).

For the moment, it just covers 20 meter band, but we may increase the coverage to 30 and 40 meters too.

The server and antenna are all in EP2C club’s office (Karaj/Iran). Iran’s timezone is GMT+03:30.

Because of our bandwidth limitations, the total number of simultaneous users is limited to 10 (will try to increase it in the future).

Our WebSDR address: http://websdr.ir:34567

Thank you for the announcement, Mehdi! I’ve been listening to the EP2C WebSDR this morning–it’s been working flawlessly.

Please keep up informed as you improve and upgrade the EP2C WebSDR!

Click here to use to the EP2C WebSDR online.

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Hearing Ghadir: Iran’s Sepehr Phased Radar System

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In response to Andrea’s UVB-76 recording, SWLing Post contributor, Mehdi, comments:

Thomas, another signal which may sound mysterious:

Iran’s OTH Radar, named “Ghadir”; part of Iran’s Sepehr Phased Radar System.

They’d polluted the 10-meter band [in the past]; now they transmit on 26MHZ. (See spectrum in screen capture above.)

Sound: https://clyp.it/ldpxab2a