Of more practical use was a batch of ten LM567 PLL tone decoder ICs that came yesterday. These can be put into use to detect tones, latch onto a particular tone for decoding, act as an FM detector, or work as an oscillator. Ive not used them before as they have always been just a bit pricey. Buying from the Far East, the costs a peanuts! However, there are fake semiconductors out there! So, each and every one was put into a simple dual frequency oscillator circuit to check it - and they all passed. One of these will now go onto become the detector part of a Morse Reader.
Ive also finally tested the Frog Sounds 40m CW QRP transceiver. All seems well. The receiver gives a Minimum Discernible Signal of -110dBm into a set of cheapo earphones, and the transmit side will run a tad under 2W when given 13.8v, and 1W on just 9v! Harmonic suppression isnt great, around 30dB down at 2nd Harmonic, but its adequate for a radio of this type.
So, onto the title subject! What the heck is a 'Paeansonic'??? Well, I spotted this little AM/FM portable radio kit on ebay, item number 231603813152
Clearly the name is supposed to sound like Panasonic. It intruiged me, so I thought i'd give one a go.
For a nats over £4, I wasnt expecting much. But when it came, I was quite surprised to find it looked like it might not be too bad. The circuit is based around two common radio ICs - the CD9088 FM radio subsystem, and the TA7642 AM TRF IC (the modern version of the old ZN414), along with a TDA2822 dual AF amplifier chip. Its a very simple circuit with minimal alignment, however more about that later!
The kit contained everything, including case, antenna, and even the wires to the loudspeaker! The one thing it does lack however, are English instructions! What you get is a single double sided sheet of Chinese idiograms, interspersed with the odd Latin character here and there. Luckily, the circuit diagram is clear, and there is a PCB layout diagram, which is actually backwards! There is a parts list as well, but quite a few parts are only listed in Chinese!
All that said, its really not that hard to build! The PCB is rather thin, so take care handling it. It does develop a bit of a curve during soldering, but its nothing to worry about. The first thing to do, is to solder on the CD9088 chip. Now, this is a surface mount device! But its quite a wide pin spaced package, so I managed it even with the big tip on my 25w Antex!
With that done, I followed normal build convention, starting with the resistors. These are all a bit longer than the space between the holes, so its best to fit them at a bit of a slant. My PCBs silk screen had the part value/marking on it rather than the component number. With all the resistors installed, I moved onto the ceramic capacitors. Theres a lot of these! Note that when fitting the resistors, theres a link, marked 'J' on the board, thats worth fitting at the same time! Just use a cut off leg from one of the other components.
With these fitted, your not far off done! I did the few electrolytics next, then the switch, volume pot, tuning capacitor and headphone jack. The two coils went on next, and finally the AM radio IC and the audio amp IC.
In the picture above, you can also see the LED fitted. This has to fit into a hole in the case, so I found it best to fit the PCB on the case mounting studs with the LED in place, let it slip to its place in the hole, and then solder it, ensuring its proper alignment.
At this time, I soldered the wires to the loudspeaker and installed that in the case front half. A few small chunks of hot melt glue were melted onto the rim to secure it. The red and black wires were soldered to the tabs on the battery contacts, and the contacts installed in the slots in the case. These were the only parts that needed any force - a firm push with the tip of a pair of long nosed pliers to push them into place.
Next, I assembled the AM ferrite loop antenna. This came as a rectangular ferrite rod, a round prewound coil, and a fixing clip. The rod pushed firmly into the clip, and with a gentle squeeze the coil slid securely onto the rod. The silk screen on the PCB shows the correct positioning of the coil on the rod (about 1/3rd of the way along) The clip then fits in a slot on the PCB. This was somewhat loose, so more hotmelt to secure it. There are PCB holes for the coil wires, but I routed them around the back of the PCB and soldered them directly to the pads.
The telescopic antenna fits into a slot in the rear case half, and secures with a small screw. There is no solder tag, so prior to fitting the antenna to the case, its best to solder the remaining yellow with onto the antenna base next to the screw hole. Give it some heat and it tins nicely, being plated brass. The wire and then the antenna threads through the hole and is screwed into place.
Pretty much all thats left now is mechanical, and alignment. There are essentially three alignment points, and this is where it gets tricky! AM is aligned by adjusting the tuning capacitor units trimmer capacitor nearest the edge of the PCB. FM however is more complicated. The other trimmer has some effect on the tuning, but the principle adjustment is L2, the 8.5T coil. This has its turns spread to shift the frequency. The trouble here is knowing how much! I found it best to temporarily fit the PCB and the tuning indicator, adjust the tuning to put the pointer where you know there will be a known station (I set mine to 90, knowing that BBC Radio 2 is on 89.3MHz) and then remove the board again without moving the tuning!!! Then, temporarily connect up a 3v supply, and splay L2 and adjust the trimmer to get your known station. Of course, to do that, you need to fit the tuning indicator! This is fun!
It took me quite a while to get the coil and the trimmers all set up just right. I'd advise as well not having your mobile phone in your pocket whilst doing this, as the beeps and buzzes of GSM interference are pretty irritating. Once aligned, the PCB fits on three fixings, with a fourth screw hole between the tuning capacitor and the volume control, a 3mm self tapper fits there to secure the board. With all the wires connected and pushed out of the way under the board or wherever, the two halves of the case snap together, and secure with three long self tapping screws and another 3mm one in the battery compartment. I found that the hole for the headphone jack needed a bit of a ream out with a drill bit, but other than that the case went together nicely.
With the case screwed together, the final task is to fit the mode switch! This snap fits through a slot on the top onto the AM/FM switch, but be aware its a slightly odd shape and fits best just one way around!
The battery compartment lid is captive, so you cant lose it when fitting the batteries! Which is a nice touch. All in all, once complete, it works surprisingly well. FM is a bit fiddly to tune a station in due to the small dial but once done its nice and clear, and surprisingly loud! AM works pretty well too. The antenna folds nicely away when not needed.
For what it cost, its a fun little kit. Build time for me was about one and a half hours. And the result is a basic but functional AM/FM pocket portable, which is also rather cute!
Its an ideal kit for a 'guided' build by a beginner with assistance from a more experienced builder, although due to the SMD part and the tricky alignment maybe not for a beginner to tackle on their own. Im going to get another for Sam to build!
So to end this post, i'll also mention that ive finally had success in receiving the downlink on 70cm from Saudisat SO-50. I have the FT-857D set up in the car with split frequency memories for this. I copied a number of stations during a lunchtime pass on saturday, during my lunchbreak at work. Im now awaiting the arrival of a couple of tape measures, and will then start building a dual band crossed yagi tape measure antenna for working the FM satellites.