A forty seven year old trick. Some of the readers here may remember the post I made on the Pioneer tuner I converted from Japanese to local standard. Before I made the mod. I had to repair the tuner. Right out of storage, when turned on, the tuner audio out would produce what I would described as a loud crackling sound intermittently. To many radio techs, the word "intermittent" is a dreaded word. An accompanying symptom included diminished sensitivity.
I had just repaired the oscilloscope the day before. I had to replace a resistor in the focus section of the CRT circuit as the normal dot trace was as big as a grain of corn. The 'scope is the best instrument to track down the source of the noise. Poking the 'scope probe here and there lead me to a AM section IF amp stage which had an odd reading of voltage. So the immediate thought was that the transformer was shorted. I took it off the PCB and examined using a good magnifier. There was no apparent short between the windings. In the event there was a short between the turns of the transformer windings, that would be real disaster. It took some time before the actual circuit of the transformer formed in my head and I concluded that all windings were wired together in this configuration and something was preventing the B+ from getting to the transistor collector.
Around 1970, I used to hang around a lot at a tv repair shop and I remember one case when an seven transistor AM radio was brought in for repair. What my friend did was to take out all IF transformers and with a small screwdriver, crushed all the tubular capacitors (as can be seen in the bottom of the IF can in the picture), effectively taking them out of circuit. I was sort of shocked when I saw him do it, but I said to myself this must be something he learned form years of experience with this kind of trouble. Once reinstalled the seven transistor worked normally!
Looking at the Pioneer tuner IF transformer I thought "what if the capacitor was the problem?". So I got a small driver and crushed the capacitor and the short disappeared. After-re installing the transformer, the tuner worked normally and no more crackling sound. One may argue: how about the missing capacitor; would reception be degraded without it? Perhaps, there would be some degradation but it may be slightly compensated by re-tuning the slug and perhaps AM broadcast signal strength would be sufficient in the suburbs, any difference would be unnoticeable. Anyway, small replacement glass tube capacitors are unobtainium. A more elegant solution would be to replace the entire transformer itself if available. Some are available in sets but they are becoming rare and on top of that the transformer you get might be wired exactly as the original you want.
Two days after I got the tuner working a stumbled upon a snippet of the schematic for the tuner. The schematic was not complete (the site sold service manuals) but the snippet included the portion of the IF stage I worked on, as it turns out the capacitor was wired so that if shorted, no B+ will get to the transistor. Although I discovered the diagram a bit late it does explains it all.
Anyway, it is comforting to know I can still recall an "old trick" I saw when I was still in fourth year high school. HIHI
Mga Komento
Mag-post ng isang Komento