yamaha CS-10 stuck LFO!

[colmon]

good day sirs *twirls end of moustache*

has anyone ever encountered a stuck lfo on the yamaha CS series synths? i assume it is stuck because the led has gone out and anything effected by the lfo remains at a single point in its cycle, so for example if i flip between the two lfo waveforms while pressing a key the vco pitch will change slightly

will replacing the led remedy this? anyone know what value i should use?

any advice will be greatly appreciated!

[rhino]

Looking at the schematic:
1) check the LFO speed pot for failure: wiper should go from 0 to 10 volts as you move it.
2) check trimmers VR12 and VR13 for dirt or corrosion giving an open circuit
3) the LFO ic has failed - IC4 IG00150

The LED is buffered by a transistor so it should not stop the LFO if it fails.

[colmon]

thank you so much for such a quick and succinct reply!

the pot reads smoothly, but only 0 to 8 for whatever reason

the trimpots are clean and shiny, but vr12 has a *tiny* spot of oxidisation. shall i replace it or is it possible to clean it?

once again, many thanks

[rhino]

Mark where the trimpots are set, then turn them back and forth a few times then reset to the original positions and see if that makes any difference. If not, contact Synthparts (member here) about a replacement IC chip.

[Dr. Phibes]

I have a CS-15 that I think has some very slight LFO bleed-through which I haven't got round to investigating yet (undoing all those pot fastenings to get to the PC boards is bloody chore). I'm hoping it's just a calibration issue.

The LFO circuit is identical to that of the CS-10.

Do check trimpots as rhino said. Also worth checking is the small tantalum capacitor between pins 3 & 4; it seems to set the base frequency of the LFO chip. It worth's going over all the minor things first because those proprietary ICs yamaha used are fairly rare and expensive. You may be lucky. Incidentally has the power supply been seen to recently?

[colmon]

hmmm adjusting the trimpots did nothing. however i can't get a reading off the little blue tantalum cap. i'll replace it and see what happens, other wise i guess i'll see about replacing the ic if it's not too expensive

yes it is a bit of a hassle getting inside this synth, but the pcb is extremely neat and well laid out. i haven't replaced any of the caps in the power supply if that's what you mean, but there is no sign of leakage or anything so might just leave them for the time being

thanks for your replies!

[colmon]

for anyone else who may encounter this problem in the future and come across this thread, synthparts does indeed stock the lfo ic (IG00150), costs $50

http://www.synthparts.com/

[colmon]

Also worth checking is the small tantalum capacitor between pins 3 & 4; it seems to set the base frequency of the LFO chip.

hey doc, just to double check, do you mean the capacitor labelled 0.68/25 on the schematic? i can't seem to measure stable resistance on this, only occasionally a quick jump in values then down to 1 (this could be a failing of my extremely cheap digital multimeter). if this was dead would it mean a short in the circuit, therefore causing the led to fail? it's gotta be either this capacitor or the ic, obviously one is far cheaper than the other so...

[rhino]

Are you testing the cap in or out of the circuit? Capacitors should show a very high resistance. Tantalum caps like that (esp. in Yamaha) can die and short. Since this is the "timing" for the LFO, try replacing it.

[Dr. Phibes]

hey doc, just to double check, do you mean the capacitor labelled 0.68/25 on the schematic? i can't seem to measure stable resistance on this, only occasionally a quick jump in values then down to 1 (this could be a failing of my extremely cheap digital multimeter). if this was dead would it mean a short in the circuit, therefore causing the led to fail? it's gotta be either this capacitor or the ic, obviously one is far cheaper than the other so...

Yep, that's the one. Out of the circuit, a failed cap usually results in a short but it's harder to tell within the circuit. Otherwise, as rhino said, they should show a very high resistance. This is just my supposition though - don't be too surprised if it is the chip itself at fault.

Are you testing the cap in or out of the circuit? Capacitors should show a very high resistance. Tantalum caps like that (esp. in Yamaha) can die and short. Since this is the "timing" for the LFO, try replacing it.

Did they use the wet-slug type then? To be honest I just saw a polar cap at first and thought it was a standard electrolytic until I saw the little triangle symbol.

[colmon]

thanks so much for all of your advice gentleman, however i have somehow managed to inadvertently destroy the lfo circuit. somehow the trace from one of the ic lugs decided to come unstuck from the board. the annoying thing is when measuring both the new and old capacitor out of circuit they exhibited exactly the same behaviour so it seems clear that it was the ic that had failed. but now, even if i were to purchase a new one, it won't work! guess i'll just have to live without an lfo on this one...

[colmon]

and as if things couldn't get any worse... no sound

[rhino]

Step back, take a deep breath, smoke, coffee, etc. and start again.
Check that you may have shorted something with solder.
Check that all connectors are plugged back in.
Remove the chip and replace with a socket.
Restore the broken trace with small gage wire.
Go ahead and use the new cap.
If careful study of #1 and #2 do not restore sound, maybe time to take it to a tech.
If you get sound back, the replace the chip. DO NOT TRASH the old one until you get things sorted out!

Let us know.

[Dr. Phibes]

Sorry I didn't respond before now - have had stuff to deal with.

Don't get too worked up over it. Everyone pulls traces now and again. I know I have. Accidents happen. As rhino said, try to find a local tech to relay it with a new chip, if you haven't already of course.

[radek tymecki]

lfo chip is dead... i'm more that sure