Re: Tuning Fail and THX effect (Fixed!)
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Just a quick note about a successful Xpander repair: ?My?Xpander was suddenly failing all tuning tests.? Consistently at power up, all VCOs and VCFs ramped up in pitch until they were ~4 octaves too high, then stayed there.? Fixed by replacing IC U814.
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Some details for future generations, and a confession:
I actually caused this problem myself when using canned air to dust off the processor board while doing some scope work.? Held the can in just the right (wrong) way to invoke the ice breath powers of difluoroethane, which is about -50¡ãC.? It foamed all over the upper section of the DAC/tuning ICs, (U801 through U806).? Then I realized I had left the power on¡? (insert eye-roll emoji here.)
After the stuff eventually evaporated, (technically it boils at room temperature), I found the symptoms described above.? Holding down a chord at power up sounded like the THX ¡°deep note¡±; pitch on all voices rose ~4 octaves, then stayed there.? I was able to trace the problem backwards from the DAC output.? The scope showed VCO1&2¡¯s pitch CV and VCF¡¯s were affected, but PW, level and VCA were still stable.? This pointed to U814, which is responsible for assembling the final DAC output by switching between the high and low resolution CV signals on their way to the CEMs.? (Pitch for VCOs and the VCFs frequency get the high res version, PW, level and VCA get the low res.)? Oddly, U814 was not in the line of fire, but even the service manual reminds that that IC is extremely sensitive to shorts (it¡¯s a CMOS), so it must have temporarily suffered a voltage spike, or perhaps a static shock from the blast, as the other components temporarily froze.
I replaced U814 and all was good.? It¡¯s a CMOS analog multiplexer switch, part # CD4053B, $0.51 at Mouser.? I also replaced all of the ICs that were hit, and all the rest in the U800 section, just as a precaution.? Total cost was probably less than 5 bucks.? (Freezing ICs is actually a common technique to isolate problems in circuitry, so those in the line of fire were probably fine, but I figured why not.)? The THX effect was probably caused by a nearby capacitor soaking up voltage unchallenged by the failed multiplexer.
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Anyway, just had to share with someone.? Hopefully someone might find this useful in the future.? And if you¡¯re wondering, the canned air and I are no longer on speaking terms.
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