Yamaha psr 70
YAMAHA PSR-70 Keyboard - Lights On - No Sound
This may be the cause if the unit has not been powered up for a while; switched LEDs come on but fails to produce any sound from either or both channels:
Normally one would suspect faulty headphone switch jack or faulty LA4500 power output chips.
Most electrolytic capacitors can be slightly leaky (tens or hundreds ofkilo-ohms) after years of non-use.. They are re-polarized / reformed when equipment is switched on after aperiod of non-use and this effect would probably not be noticed since it make only takeseconds for all electrolytic capacitors to come up to specification...
In the circuit used for the Yamaha PSR-70(see the Sanyo Applications Notes for the LA4500)there is an 'anti-thump' / slow start capacitor (220uf 16V, aluminium electrolytic 'ripple filter' capacitor) connected between pin 9 and ground (of both LA4500 audio output chips in the PSR-70).Since these hold the chip in a power down condition, they cannot 'reform'from a near rail voltage normally present when the chip is turned on, due to lack of power and thus continually hold the amplifiers in a quiescent power down mode..
To remedy, they need to be disconnected and reformed with a 9Volt battery.After the Chips have powered up, there should then be about 90% of the rail voltage on their PIN 9...
They are not necessarily faulty, they just needs for the equipment to be powered up working for a minute (every year or so).Alternatively a4700 ohm(2m/A) resistor temporarily wired from the (9V) + rail(PIN 1) to PIN 9 of the LA 4500 may do the trick..
This reported fault may be very unusual or fairly common (these keyboards now being almost 30 years old...)
Hope this helps someone out - feedback will be appreciated
Apr 23, 2014 |
Keyboards & Mice