help! logan string melody synth gone sharp

joninc · Post by **joninc** » Tue Jun 09, 2015 2:52 pm

my old logan synth has creeped up in tune over the past year to where it's noticeably sour now against a track recorded to 440.

it's in tune perfectly to itself but is sharp to 440.

it has a pot on the front to adjust tuning but even all the way left (flat) - it's still
sharp.

any tips for addressing this?

The Scum · Post by **The Scum** » Tue Jun 09, 2015 3:40 pm

If it's any relation to this guy:

http://www.synfo.nl/servicemanuals/Loga ... MATICS.pdf

It looks like there's a tunable inductor in the master clock section - about halfway down the left edge of the page, marked AF1.

If it's like a Farfisa, the inductor was plugged with melted wax that you'll have to remove before you can turn it.

kslight · Post by **kslight** » Tue Jun 09, 2015 6:18 pm

I have a synth that wouldn't tune all the way and a new pot ended up correcting the issue. Don't know if related on your instrument but worth a shot.

joninc · Post by **joninc** » Tue Jun 09, 2015 7:27 pm

thanks @the Scum

I have it open and it looks like there's a few little tunable pieces... no wax.

hmmm. not sure how to decipher schematics - maybe i'll try to post a pic... once my phone is recharged

joninc · Post by **joninc** » Wed Jun 10, 2015 11:05 am

here's a pic of the board that is directly attached to the tuner knob (top left of pic)

https://dl.dropboxusercontent.com/u/141 ... .46.31.jpg

this is another board further over inside that has a few tuneable knobs -

https://dl.dropboxusercontent.com/u/141 ... .46.46.jpg

The Scum · Post by **The Scum** » Wed Jun 10, 2015 3:02 pm

The wires (2 purples and a yellow) from the pot disappear into that wire bundle, and it's unclear from the photo where they re-emerge.

Nothing there looks obviously like a trimmer coil. Trimmer coils are often a little metal box about the size of a sugar cube, with a hole in the top for a screwdriver...sometimes you can see the coil of wire inside, too.

Do the PCBs have any identifying marks on them? Like part number etched in the copper? If the device and schems correspond (and it's still not clear that they do), you're looking for board "122-B".

Do you see a chip marked MK 50240 anywhere?

joninc · Post by **joninc** » Tue Jun 30, 2015 12:54 pm

does this look like it might be it?

https://dl.dropboxusercontent.com/u/141 ... %20PM.jpeg

The Scum · Post by **The Scum** » Tue Jun 30, 2015 3:21 pm

Yes, indeed.

That round thing marked "A1 7605K" is the trimmer inductor.

Gilroy · Post by **Gilroy** » Thu Jul 02, 2015 9:57 am

it also is possible that the whole thing could use a recapping. that can also lead to a major shift in tuning.

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help! logan string melody synth gone sharp

help! logan string melody synth gone sharp

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