Hi Steve
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Thanks for doing that...I just couldn't make the numbers out. I spent ages trawling data books I have and looking for TI mixers on-line, but nothing came up.
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According to Google, it became the TL442.
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I'll check it out though...hopefully the one I have is okay.
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It appears the previous owner did what Pentagrid suggested and referred to the MSM5525 data sheet when connecting it up; at least in part! The DFC-7 uses a 9-0-9 volt transformer, and gets the 3v for the display filament, by using a resistor from the supply. Said previous owner used an 18-0-18 volt transformer for some reason, and used its 3v AC winding for the display filament.
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Somewhere I have a spare counter chip...probably not a mixer, but you never know. The counter chip will go in a socket and I'll see what happens.
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Thanks very much for all your help Steve, and I appreciate everyone else's input too.
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Regards
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Simon
Simon Robinson