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Yamaha YC-45D 8' & 4' stops not working on Ab³ (top manual)

DemeterOphelia

New member
Hello,

I just acquired a Yamaha YC-45D yesterday. I was informed that the 8' and 4' stops did not work on the Ab (3rd octave on the upper manual), but for $500 I had to buy it. I spent around four hours dusting with my air compressor, cleaning contacts, wiping everything down with a rag, re-dying the black leather exterior, polishing all visible metal/wood, gluing any peeling leather, et cetera, so everything looks great... Unfortunately, the stops still do not work (I didn't really expect cleaning the contacts to help, but I knew it also wouldn't hurt).

Along with the 8' and 4' stops, the 'piano', 'harpsichord', and 'marimba' stops also do not work on the Ab³ key. I can hear what sounds like a 1' muted stop on the marimba setting which leads me to believe that those are just presets of the 8' and 4' stops with some other added effects (bright, vibrato, et cetera).

I pulled all of the circuit board cards out and checked for any obvious signs of blown circuitry. In which I found none. Upon reading the original service manual (found here: http://dl.lojinx.com/analoghell/Yama...viceManual.pdf), I saw that Ab³ was generated by board UE5. That's all I know so far.

Any help on this matter would be greatly appreciated. I'm ready to play this thing!

Regards,
Demeter
 

DemeterOphelia

New member
2wcn1g6.jpg



A. Applied new solder to joint in case of loose connection or 'cold joint'.
B. All diodes to the right of "B." and facing the same direction (i.e. upward current) gave a measurement of 67 kΩ
C. Oddly, the diode for Ab3 gave a reading of .97 MΩ
D. Applied new solder to all joints in wiring buss for Ab3


Shorting "A." to an adjacent contact gave me the Ab3 tone in all registers, including the stops that weren't working before. So, I'm assuming that this is an issue in the keying circuit and not the stops.




My questions:


Why would the diode to the key that is not working give a higher reading than the other working keys' diode?
Shouldn't the reading be lower if the diode is bad?
Does this mean that a resistor is bad and letting more power through?


Again, thank you for any help!


Regards,
Demeter
 

Diode

New member
Hi Demeter,

I assume you are using a digital multimeter to check the diodes, in which case you may get misleading results. Some digital meters output a very small voltage when used on the ohms ranges, too small in some cases to turn the diode 'on' when the probes are connected in the forward-bias direction. The readings you have quoted seem rather high in both cases - which suggest to me that you may be seeing the combined resistance of many interconnected diodes and resistors in the keying matrix - rather than the resistance of diode that you are connected to.
Do you have a diode-test function on your multimeter? If so you could try that on the diodes rather than the ohms ranges. With the probes connected to a diode one way round on the diode-test range you should then see a reading of approx, 300-600. This is roughly the voltage drop (in milli-volts) across the diode in the forward direction (not resistance). Reverse the probes and the meter should show the same as when the probes are disconnected. Alternatively, if you have access to an old analog multimeter you may get better results on the ohms ranges when testing diodes. Diodes can fail both ways - they can either go to a high resistance state in both directions or else a low resistance state in both directions when faulty.

In case you are wondering about the coincidental nickname I use on this forum, friends started calling me 'Diode' many years ago when I bought 10000 'reject' diodes at a bargain price and manually tested them for use in the diode-keying matrix for an electronic organ I was building then.

Diode
 
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