Marantz SM500dc repair, help?

Mikefc

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Screen Shot 2018-12-12 at 5.14.41 PM.png I'm stuck and a little help will be greatly appreciated. There is a negative 12vdc bias on the right channel. the diagram above is where the issue lies. With Q715 out of the circuit all voltages and transistor biases are correct. With it in the circuit the 12vdc results on the collector and therefore all the way through to the output transistors. It is not the transistor though as I have swapped left and right channel. All other transistors test good out of circuit and all base/emitter potentials are correct even when the error is showing. Likewise the difference across the two parts of the output resistor (0.47ohms) is correct also. R729 even has the same drop as it's mirror on left channel. (about 75vdc), the diode(s) between the collectors of Q711 and Q715 are good as well. C715 electrolytic is new and tests good. It appears to me that Q715 is passing too much of the collector voltage but the base emitter potential is 0.6vdc as spec'd. BTW the speaker relay circuit works as it should if the logic chip sees the proper 0vdc for both channels. with the -12vdc lurking it of course stays deactivated.
 
Check the board for continuity between the collector of Q715 and the cathode of Q713, also to the base of Q719 and related components.

Craig
 
Q715 is passing too much

Just as likely that Q711 is passing too little current. (My intuition says more likely.)

Q715's job is to pass a fixed amount of current. Exactly what amount is not ultra-super-critical. What is critical is for Q711 to pass a matching amount of current when there is no input to the amplifier, which will result in the amplifier's output being 0V if all else is well.

So, I would include Q711 itself to be on the list of possible culprits, as well as the differential pair that drives it (Q701), and the DC servo (Q709). I think it would be worth testing R731's resistance, too.

Just suggestions, no certainties here. Good luck,

chazix
 
I thought about the DC servo also but the other channel seems to be working, the other half of the opamp. I guess one half could go bad. My thought is it's missing the positive voltage from Q711 to balance out the circuit. My first suggestion would have fried the outputs if no continuity in the bias circuit.
 
I thought about the DC servo also but the other channel seems to be working, the other half of the opamp. I guess one half could go bad. My thought is it's missing the positive voltage from Q711 to balance out the circuit. My first suggestion would have fried the outputs if no continuity in the bias circuit.
thanks guys, it was the opamp, I had a generic one from years ago in a parts bin and it did the trick. Uint running noice and cool, bias current balanced, both channels have great freq response on the o'scope
 
Glad to hear this. I and everyone on AK, love hearing a piece of equipment brought back to life, and making music. Enjoy thy music!!!! One minivan load of Karma to you!!!
 
agree...... the owner of this Marantz was afraid he'd never see it working again but as I fix things for cheap, ( basically for records ) he's going to be a happy man.
 
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