Conversion of Japanese standard Pioneer F-C3 AM/FM tuner to Philippine standard.

(Long) Many years ago, I went to Paranaque to visit a guy who advertised used Japanese HiFi equipment and a smallish Pioneer AM FM tuner caught my fancy. Since it is a digital tuner and the FM band can only receive a couple of local stations (because the Japanese FM broadcast band is from 78 to 90MHz, while ours is from 88 to 108 MHz) and haggled with the shop keeper to get a good price. Unfortunately back then, no information can be obtained even from the internet on this particular tuner model. Two weeks ago, I got a small 40W p.a. and thought I'd dig up the old Pioneer tuner and listen to AM. By the way, it turns out it is one of the rare models that could receive AM stereo (C-Quam standard -used in the states and Japan. Unfortunately, AM Stereo boradcasting has been on the decline and Japan stopped AM stereo broadcast a few years ago. DWWW at 774kHz still lights up the stereo indicator but the audio received is monoraul. I don't know why but maybe the engineers at the station did not strive to use the C-QUAM they have for whatever reason. Neverhteless, the old Pioneer tuner had very good AM reception. As a matter of fact it has been described as a super AM tuner. A few days ago, while browsing for info on the tuner I stumbled upon a Taiwanese HIFI forum post which showed how to changed the frequency step as well as the FM frequency range. When reading the post (translated by google translate) it sounded straightforward enough so I decided to try it out. After doing the hack on the microprocessor to change the FM frequency. I discovered I could not get the PLL to lock. When I touch the circuitry of the oscillator, I can pull in some strong stations but no matter how I flex the oscillator coils, it still would not lock. Bear in mind that the coil is inside a shield and it was a minor pain to access it. Before assembling the tuner FM front end and VCO I used a chassis nibber to cut a hole in the shield, so I can manipulate the VCO coil. After a whole day of frustration,it occurred to me that what I was probably listening to when I "tuned " the oscillator by flexing the coil was just the "image" which meant my LO is below the RF frequency. I consulted the spec of the PLL IC and it specified that the LO frequency was 10,7MHz above the RF. This meant I have to modify the VCO coil to oscillate at a higher frequency. Thanks to the window I nibbled on the shield cover, I was able to unsolder the coil and coax it out of the hole. I simply removed one turn and resoldered it back to the pcb. I verified if the free running frequency changed and it did - it moved upward. Now is the time to flex the coil and see how the tuning voltage behaved. I found that the PLL locked at the lower portion of the band. A few more flexing of the coil (opening) and it was now locking throughout the Philippine FM band. A new problem became apparent - only the strongest station can be received. I then worked on 2 or the tree front end coils and remove one turn each and the improvement in reception was dramatic so much so I did not bother to touch the coil neares the antenna input anymore. I worked first on the coil closest to the mixer and worked toward the antenna. I reasoned with myself that the coil at the antenna was probably low Q being integrated with a matching transformer for the 75/300 ohm input.

In restrospect, the post written by Taiwanese HIFI enthusiast did give me an initial impression that the modification to the Pioneer tuner was easy. He did not mention that he made coil mods but just bent open the windings of the coils, but maybe I lost something in the translation. Well I don't blame him. The purpose of his post was to inspire others to try it out and any writer wont discourage his reader from trying a mod due to difficulty. And thanks to him, I now have a dandy Japanese vintage tuner with superb AM and full coverage FM as well. Although this post involves modification of a broadcast receiver, it is nevertheless good exercise for radio hams. Half of our transceiver is a receiver and it doesn't harm being intimate with the technologies of receivers, as they say you can't work them if you cannot hear them.