Yamaha R-50 IC Amplifier

motfinoski · 12th December 2010, 05:25 PM

I am new to this forum and need some advice with ny restoration project, early 1980's Yamaha R-50 receiver. I replaced the STK Module and caps, checked transistors and resistors, cleaned switches. Unfortunately, I powered up the unit and noticed the dc-offset having a -12mV at left channel and 38mV at right channel, unlike my other amplifiers with discreet amplifier transistors. Sound will be produced but only through the left channel with the right channel speaker wire disconnected. With 2 channels connected the protection relay trips. Even with minimal load and with 2 speakers connected the protection relay trips, as if there is a short? Is the dc-offset indicative of the tripping problem?

motfinoski · 14th December 2010, 10:23 AM

Quote:

Originally Posted by motfinoski

I am new to this forum and need some advice with ny restoration project, early 1980's Yamaha R-50 receiver. I replaced the STK Module and caps, checked transistors and resistors, cleaned switches. Unfortunately, I powered up the unit and noticed the dc-offset having a -12mV at left channel and 38mV at right channel, unlike my other amplifiers with discreet amplifier transistors. Sound will be produced but only through the left channel with the right channel speaker wire disconnected. With 2 channels connected the protection relay trips. Even with minimal load and with 2 speakers connected the protection relay trips, as if there is a short? Is the dc-offset indicative of the tripping problem?

CORRECTION - sound produced through RIGHT cannel without creating tripping/pulsing relay.

motfinoski · 22nd January 2011, 12:45 PM

I have replaced the STK module with another after market (non-Sanyo) brand rated 50 W instead of 35 W due to availability, and the DC Offset settled to - 4mV and -8 mV. 0.47 ohm Emitter resistors were replaced (one off by a factor of three!) and the amplifier is steady under all loads.

12th December 2010, 05:25 PM	#1
motfinoski diyAudio Member Join Date: Nov 2010 Location: North Branford, CT	Yamaha R-50 IC Amplifier I am new to this forum and need some advice with ny restoration project, early 1980's Yamaha R-50 receiver. I replaced the STK Module and caps, checked transistors and resistors, cleaned switches. Unfortunately, I powered up the unit and noticed the dc-offset having a -12mV at left channel and 38mV at right channel, unlike my other amplifiers with discreet amplifier transistors. Sound will be produced but only through the left channel with the right channel speaker wire disconnected. With 2 channels connected the protection relay trips. Even with minimal load and with 2 speakers connected the protection relay trips, as if there is a short? Is the dc-offset indicative of the tripping problem?

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22nd January 2011, 12:45 PM	#3
motfinoski diyAudio Member Join Date: Nov 2010 Location: North Branford, CT	I have replaced the STK module with another after market (non-Sanyo) brand rated 50 W instead of 35 W due to availability, and the DC Offset settled to - 4mV and -8 mV. 0.47 ohm Emitter resistors were replaced (one off by a factor of three!) and the amplifier is steady under all loads.

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