Working on a marantz 2270, replaced the power switch, found shorted S40442 output transitor on right channel. I replaced both npn and pnp on right side with onsemi mj21193G/94G. powered up on dimbulb everyting is working, came out of protect. set dc offset just fine but the bias is all over the place. the pot which I know is probably shot and needs replaced is soooo touchy. Measuring accross each .2 ohm resister (r788 r789) I get two differnt values. they are about 3-6mV off from each other. Left channel is dead on. What am I missing here? Ive read that bias should be 5mv accross each resistor for a combined 10mV accross both.
Marantz 2270 bias uneven
- Thread starter JagsHiFi
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First try measuring the voltage drop on the resistors with the load disconnected (speakers turned off), as any DC offset will create an imbalance in the voltage drop on those resistors. There are actually 2 different versions of the 2270 main amp. Both are wired differently than most class AB push pull amps. They have the collectors of the output transistors as the common element connected to the speaker output (where most others have the emitters connected to the output). One version has the 0.2 ohm resistors in the emitter circuits between the transistors and the power supply rails, and the other actually has the resistors in the collector circuits between the transistors and the output. In the case where the resistors are between the emitters and supply rails You cannot measure the voltage across the pair, as they are not wired in series. In that case measuring from both resistors will just give you the sum of the two supply voltages. In the case where the 0.2 ohm resistors are between the collectors and the output you can measure across the pair, or just between the two output collectors to set idle current.
yes, my serial number is 462213. It has the .2 ohm resistors on the collectors and outputs. I was measuring with no load on the speaker terminals and no signal going in aux. I replaced the 1k ohm with a bourns multi turn and was able to get 5mV across one .2ohm and the other has 8mV across it. Going to search for a resister out of spec or a transistor.
Last edited:
foldthe
Active Member
I reference this thread whenever I have bias problems with a 2270. Special attention to the post by user "ecluser"
Marantz 2270 bias won't go below 15mV
Hi All I repaired one amp channel which only had one bad output transistor. I used the OnSemi MJ21193/4 pair and up she came, sounding great. The DC offset adjusted just fine but when I went to adjust the bias it wouldn't go below about 15mV when fully CCW. I checked the other channel which...
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