Marantz 500 Power Amp Problem After Rebuild and I Need Some Help.

[Davidmac5]

Hi all,

I know that there are very few on this forum that are familiar with this amp because it's so rare. But if there is anyone reading this post that might be able to help me solve my problem I would greatly appreciate it. Briefly I rebuilt the driver boards and they both work fine, I also rebuilt the right channel amp board that was partly fried when I got the amp and my problem is on that right channel amp board. When I plug in either driver board in the left channel it comes out of protection and both boards work fine, if I plug either driver board in the right channel it will not come out of protection and the rail voltages drop from 81v down to 54v on both channels. Nothing is blown as I have been using a DBT. Please respond to this thread If you think you can help and I will explain in more detail what was blown and what I have replaced and the test's I have ran and maybe we can figure out what my problem is. Thank you.

 

[eizner23]

chime us in on whether the output transistors and emitter resistors were replaced on right ch

 

[Davidmac5]

When I received the amp all the right channel output transistors were blown. I replaced all the output transistors with MJ21195 and MJ21196 transistors and the amp worked but looking at the right channel on a scope the negative side of the sign wave was broken as seen in the photo. I opened it up again and found the 4 emitter resistors were bad either open or incorrect readings and both 2 watt resistors on the negative side were open, a couple of the 10 ohm resistors on the negative side were bad as well. I went ahead and replaced just about everything on the amp board, all emitters all 1 and 2 watt resistors with metal film oxide resistors and a few capacitors and all diodes. I double and even triple checked everything and now the right channel will not let the amp come out of protection with the driver board plugged in, if I remove the driver board then it will come out of protection. The driver board is not the problem. I can plug either of the two driver boards in the left channel and the amp comes out of protection works fine with nothing plugged in the right channel, if I plug either of the perfectly working driver boards on the right channel and nothing in the left channel the rail voltage drops from 81v to 54v and will not come out of protection. So I'm missing something on the right channel amp board but I cannot figure out what it is. I compared the resistance readings on the working left channel amp board to the right channel amp board and they are pretty much identical. I'm missing something but at this point I don't know what it could be.

 

[eizner23]

Measure mV across each of the eight 0.15-ohm emitter resistors. Whichever bank shows current is the side that's conducting. Compare against left ch.

 

[maclvrbr]

I acquired this amplifier from original poster and myself removed the amplifier in question with heatsink and outputs I tested everything on the board and all outputs without finding fault. . I have tested all .15 ohm emitter resistors with zero voltage on them. If anyone has any other suggestions please feel free to reply. thanks