Marantz 4230 right power amp board is missing H705 (2SC1318Q)
- Thread starter vferdman
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vferdman
Active Member
I did just that and everything is great in 4 x 12W mode. All the adjustments are good and power is closer to 18W before clipping and sounds super clean and lovely. I use a 1kHz tone into 8 ohm resistor load to test the output with a scope. Then after I am satisfied with that I give it a listen to music on my 8 ohm test speakers. In 4 x 12W mode things are great. In 2 x 30W mode right channel measures way too high during power test on the pins I use to measure bias (compared to the left channel) and sometimes gets stuck in 1.3V even after I turn the volume all the way down. In other words, does not come back to idling at 15mV I set it to. Power cycle brings it back to proper idling, but again, if turning up the volume it plays well for a while (albeit at higher levels of DC on those test pins), then jumps to that dreaded 1.3V and stays there. It still sounds good and output waveform is fine, but the output transistors begin to overheat. I have replaced the 2SA641 PNP transistors in what I think is the biasing part of the circuit (please set me straight if I am wrong here), H709 and H708 with old 733 transistors I had on hand because I do not have too many PNPs kicking around to try. I think those 733 are compatible, but perhaps not... I will search some more replacements for 2SA641, I guess. It may be that circuit is misbehaving, but I am guessing here.
Did you resolve this? I'm working on a 4230 and it sounds like a similar problem. 3 channels bias fine. one has 300mV bias. All in 4 channel mode. I've replaced all the caps except the 3300uFs. All the transistors have been replaced. This section of the board was all but on fire. 4 or 5 black resistors. One cap had exploded, its guts everywhere. Must be a bad resistor? I can adjust the R737 to match the other side at the diode pin J726 and J724. I'm pulling my hair out over this one.
Edit: found it! .5 ohm resistor was measuring 1.xxK ohms. Replaced it and it is holding bias at 14-15mV. I overlooked it because I already replaced it. But fried some things when I hooked up both scope probes. That was before I read about not hooking up common ground scope.
Edit: found it! .5 ohm resistor was measuring 1.xxK ohms. Replaced it and it is holding bias at 14-15mV. I overlooked it because I already replaced it. But fried some things when I hooked up both scope probes. That was before I read about not hooking up common ground scope.
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