Quote:
Originally Posted by jayceebee
Don’t have any experience with this model but have repaired sets using the TDA450x series ICs. The unusual audio symptoms you describe are most often caused by the 6MHz detector coil drifting off tune. A very small adjustment should return the audio to normal.
John.
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You were absolutely right!
I would never have thought that would have caused a fault where there was some low audio that disappeared as the volume control was increased past a certain point. It must be to do with the circuitry that we can't see within the chip, but identified as the sound synchronous demodulator, shown as a block on the chip data sheet and connected to L5 via pin 13. I would have expected just a general lack of, or low, thin audio, increasing throughout the volume control range. It's probably down to slight component value variation around the coil, there's an internal 'un-valued' capacitor shown, which may not even be a physical capacitor, but perhaps just the stray capacitance of the coil itself. Anyway, as you described, adjustment of the coil slug cured the fault, so many thanks for the pointer, I think I would have gone round in a few circles fault finding that one.
As shown in the first pictures in this thread there's no mains on/off switch, just a hole where it once was and the set was always switched on and off at the mains wall socket. I assumed that the switch had been previously broken and bypassed, but on looking at it the switch was all intact and the wiring looked original. Shining a light in there I could see what looked like the broken off push button wedged to one side and there also seemed to be a metal washer on the spindle of the switch itself. I removed the switch and retrieved the plastic button which had one side of its fixing broken off and missing altogether. Someone had obviously had it apart before and added that washer for some reason. The remaining side of the switch fixing lug looked brittle and there was a fine crack in it, naturally, a bodge was called for. So out came the reel of RS tinned copper wire of an appropriate gauge and a needle file to produce a couple of notches round the plastic lug. The two bits of wire were wound tight and then the soldering iron used to melt around the area of the crack and the wire - as shown in the pictures below. It's a bodge and it may not last, but so far it's been switched on and off a few times and seems to be holding up well.
While working on the switch I found that the degaussing coils had been disconnected. I thought that perhaps I'd accidently pulled the plug off the board, but the plug had been pulled right up out of the way through its tie. I don't know why this had been done and reconnecting the plug doesn't seem to be causing any problems, so that's a bit of a mystery.
So that's it, the set will live to fight another day, pics of the switch etc. below:-