jblmar
09-19-2006, 09:02 AM
Developed a left channel problem with my Marantz 300 DC amplifier. The channel would distort and then drop out. No sound at all. 330 mV at the output!!
The connector to the amp board had a broken pin. Fellow AK member marantz3650 was good enough to send me an extra post he had. Thanks John!
After replacing the post, same problem. No sound or distorted channel.
The 300 DC is a fairly easy amp to work on since it's a dual momo design. You can transpose amp boards and power supplys to find the problem. The easier way is to flip the amp over, not too easy. It weighs 44 LBS. On the bottom are the connector pins to the amp board. A quick DC voltage test found that the + 70 volt rail was missing from the channel. A quick look at the schematic revealed Q 804 on the power supply produced that voltage. A quick diode check revealed an open condition between Base and Emitter on the Q 804 transistor. The Emitter produces the + 70 volt rail. The - 70 volt rail was ok.
Replaced the 2SA913 Q,R with a 2SA968 O. End of problem.
It's a good idea to check each component after the E, C, B of a defective transistor in case damage occured.
While I had the amp open, I removed the Peak Indicator board to get to the relay. Desoldered the relay from the board, removed and cleaned the contacts with Deoxit as well as all connector pins on each board.
I used a piece of paper sprayed with Deoxit and passed the paper over the relay contacts while applying lite pressure to the contact arms. Let dry, then repeated. Being 1/4 mile from the Atlantic Ocean causes oxide problems at a fast rate.
DC offset is now < 10 mV on each channel after 2 hours on.
Back in business.
Ron
The connector to the amp board had a broken pin. Fellow AK member marantz3650 was good enough to send me an extra post he had. Thanks John!
After replacing the post, same problem. No sound or distorted channel.
The 300 DC is a fairly easy amp to work on since it's a dual momo design. You can transpose amp boards and power supplys to find the problem. The easier way is to flip the amp over, not too easy. It weighs 44 LBS. On the bottom are the connector pins to the amp board. A quick DC voltage test found that the + 70 volt rail was missing from the channel. A quick look at the schematic revealed Q 804 on the power supply produced that voltage. A quick diode check revealed an open condition between Base and Emitter on the Q 804 transistor. The Emitter produces the + 70 volt rail. The - 70 volt rail was ok.
Replaced the 2SA913 Q,R with a 2SA968 O. End of problem.
It's a good idea to check each component after the E, C, B of a defective transistor in case damage occured.
While I had the amp open, I removed the Peak Indicator board to get to the relay. Desoldered the relay from the board, removed and cleaned the contacts with Deoxit as well as all connector pins on each board.
I used a piece of paper sprayed with Deoxit and passed the paper over the relay contacts while applying lite pressure to the contact arms. Let dry, then repeated. Being 1/4 mile from the Atlantic Ocean causes oxide problems at a fast rate.
DC offset is now < 10 mV on each channel after 2 hours on.
Back in business.
Ron