Marantz 18 Bias Test Point Help

mikiekimi

mikiekimi

USAF Ret 1992 - 2012...Korea/OEF/OIF
Looking for clarification on location of R27 and R28. I can't find in the SM where they are located and any thread that discussed setting the bias either has broken links, or no images of where these two resistors are.

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The following link discusses some big green resistors...of which there appear to be three on either 'side' of the receiver coming off of the heat sinks. I've poked and prodded and can't pinpoint the test points...

Marantz Eighteen 18 Bias adjust

Can anyone help by providing a good pic of the location of the two resistors in question and/or exactly where the probes should be positioned?
 
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Is it adjusting at all? See if you can upload some pictures of the amp so we can see how original it looks.

Tony
 
tonebells said:
Is it adjusting at all? See if you can upload some pictures of the amp so we can see how original it looks.

Tony
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Sorry to take so long to get back - had to finish up another project.

There's more to the story...

The receiver had a shorted output on the bad channel. After replacing the mismatched outputs (4 different makes/manufactures), that channel is now distorted and won't bias.

I checked the emitter (tested OK) and temp replaced it with a .47 ohm 5W cement resistor (to verify) with the same result. No bias and distorted sound.

Doing a R/L comparison this morning I see the voltages on this transistor (2nd pic below) are not stable and are + .3 volts higher than the good channel. I think this is Q8...found this thread and it looks like a sub for Q8 is a 2N5192.

I've worked on tons of other SS gear as well as several McIntosh receivers and amps, but this is my first Marantz 18.

Any guidance would certainly be appreciated.

Pics of the amp board and the transistor below.

IMG_7008.jpeg
IMG_7009.jpeg
 
tonebells said:
I use 2N5192 for that one. Replace Q7 at the same time. Use 2N5195.

Tony
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Thanks for the assist on the part #s. Q7 was toast...all is good now, both channels dialed in at 80mv. I don't recall another time where it took nearly as long to replace two transistors! The recap ought to be a hoot :)
 
Great job!

I have gotten to the point where i just remove the heat sink and replace the wires coming from the driver board.

Tony
 
tonebells said:
Great job!

I have gotten to the point where i just remove the heat sink and replace the wires coming from the driver board.

Tony
Click to expand...
Since you're one of the 18 experts on here, I'm going to hijack my own thread and ask about the scope.

I confirmed 500v via 12L on the scope board after my PS recap (well, most of the PS). I found one of the 4 transistors bad (replaced all 4 w/3503s per this thread), and attempted to get a point on the tube by using the scope adjustment procedures in the manual and referencing this thread.

R1223 adjusts the intensity, but R1216 doesn't seem to do anything. With the intensity turned up all the way I see what appears to be a dead spot in the tube. When I power off, a light does move to the center of the screen and fade away.

I'm not much of a tube guy, and less of a CRT guy - is this tube bad, or might this be a bad FET or other component? I haven't started probing for voltage yet...the risk of zapping myself w/500v isn't worth it if I have a bum tube.

IMG_7021.jpeg
 
It sounds like you are still not centering. Could be a bad fet as well. Does the focus effect the dot you see when powering off?

Tony
 
We have our scope back :)

IMG_7037 1.jpeg
Once again...thanks for the assist. Onto finishing the recap...
 
If the scope centers well on tuning and audio then save the cash. YMMV

That tube looks pretty good. A lot of the tubes I have are pretty shot in the center.

Tony
 
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