Marantz 4240 output out of phase

saabracer23

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I thought I’d start a new thread since I have a new issue. One, I have a bad waveform, but my big concern is the output is out of phase. What would cause that? I know, it looks bad.

2E741D01-E2BD-492B-ABB9-6F0CC46FC44D.jpeg

Dan
 
Also this amp has problems and I’m guessing it has a lot to do with grounding issues.

So if I hook up my scope leads to my dummy load which is connected to the outputs I get the wave form above. Left channel only has output if I connect an alligator clip from the left negative out to the chassis. Now, if I connect my scope leads directly to the amp (neg to chassis and pos to directly to the output I get this waveform. Looks good

D38209DE-C212-4E4B-8E25-3C28CE5476CD.jpeg

And this is how it’s hooked up.

61BB607B-3218-4F9E-AF23-6EEFC3FD5599.jpeg

Dan
 
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Are you running in stereo mode - IIRC it is a bridged output, which could explain the inversion. You also run into grounding issues when in bridged stereo model. Best practice is to use a floating load, and connect scope ground to chassis, and look at the manual to find where the signals are. The manual should help....My 2 cents
 
Are you running in stereo mode - IIRC it is a bridged output, which could explain the inversion. You also run into grounding issues when in bridged stereo model. Best practice is to use a floating load, and connect scope ground to chassis, and look at the manual to find where the signals are. The manual should help....My 2 cents

I’m sorry, but what specifically do you mean by using a floating load? Running into grounding issues in bridged, do the grounding issues come from the actual bridging switch on the back? I’ve been tracing the signal on the schematic, but have no idea where they’d go out of phase from each other.

In bridge mode is one entire board used for each channel? So left board for left channel and right for right channel?

Dan
 
Bridged mode - the rear channel is inverted, and fed to the front minus terminal.
Floating load means that neither end of the load resistor is connected to ground. Grounding the load in bridged mode causes the rear channel fuse to go or worse.... FWIW it is probably best to trouble shoot the channels in quad (4 channel) mode to make sure they are all good, then go to trouble shooting in bridged mode.

Bridged amplifiers are always a PITA when it comes to testing on a bench setup. When I setup my bench switch box (long ago....) I put in a switch to isolate the ground on the load, but was still hard to keep it all sorted out.
 
Bridged mode - the rear channel is inverted, and fed to the front minus terminal.
Floating load means that neither end of the load resistor is connected to ground. Grounding the load in bridged mode causes the rear channel fuse to go or worse.... FWIW it is probably best to trouble shoot the channels in quad (4 channel) mode to make sure they are all good, then go to trouble shooting in bridged mode.

Bridged amplifiers are always a PITA when it comes to testing on a bench setup. When I setup my bench switch box (long ago....) I put in a switch to isolate the ground on the load, but was still hard to keep it all sorted out.

Thank you. So I placed it in four channel mode and I regain the ground on the left front channel. All channels have ground now. The two front channels are still out of phase from each other. The rear channels I got no output at all. I found a blown fuse on the left board, replaced it and then had output on the left rear channel. Still non on the right.

Dan
 
Alrighty, I got the channel with lower output taken care of! This board right here was a mess!

FEB5F5E1-B0B8-47DD-AB11-33AB04513263.jpeg

The transistor HE04 (2sc1327) had a gain of 8 while the other 2sc1327s on the board had an hFE of 350-500. A tech had worked in this in the not so distant past and they had recapped the board and I found a few of the 0.47uF caps had been replaced with 4.7uF. When some of those caps had been replaced there were traces that had been broken and I guess they had not seen them. Now I get a beautiful sine wave on all four channels and the two front when I’m two channel mode.

I only get these nice wave forms when the ground of my scope leads are connected to the amp chassis. As soon as I hook up the ground lead to the negative speaker output then I loose the nice sine wave. Checking continuity I have found I am definitely loosing my speaker output ground at the 2 channel/4 channel switch on the back. I’ve cleaned it 4 times and looking closely I’m just not seeing how the ground pin on the switch can touch the center pivoting wheel.

Is there any reason why I wouldn’t be able to solder a wire from the negative speaker output terminal to somewhere on the chassis? That way it’ll have a good ground? Will it mess anything up when it bridges the channels? I wouldn’t think so since whether in 2 or 4 channel mode they’re supposed to have a very low level of resistance to ground.

In 2 channel on the right side from chassis to output terminal I read 147 ohms and on the left I read 143 ohms.

In 4 channel on the front right I get 0.1 ohm and the left 0.2 ohm. On the rear channels I get .1 ohm on each as they are physically tied together with a solder joint.

Thanks,
Dan
 
In 4 channel mode, the - terminal should be ground on front and rear speakers.
In 2 channel mode, the - terminal on the front speakers goes to the rear channel amplifier of that channel and should have an inverted version of the front channel. In 2 channel mode there is no ground for the speaker connections. Grounding the - side will blow the fuse (and maybe cause more problems) in 2 channel mode. The manual does not explain very much - I expected a little explanation and trouble shooting info for modes.
In stereo mode, do not hook the - terminal to ground. I would use a ground to the chassis, and then separately test the + and the - terminals as independent channels. (Same signal but inverted on the negative). Both will have signals on them.
You can also test in 2 channel mode by using both channels of the scope for
+ and - terminals for one channel of amp, then invert the second channel and display in sum mode. (or if your scope has a differential mode, that does the same thing.)
 
In 4 channel mode, the - terminal should be ground on front and rear speakers.
In 2 channel mode, the - terminal on the front speakers goes to the rear channel amplifier of that channel and should have an inverted version of the front channel. In 2 channel mode there is no ground for the speaker connections. Grounding the - side will blow the fuse (and maybe cause more problems) in 2 channel mode. The manual does not explain very much - I expected a little explanation and trouble shooting info for modes.
In stereo mode, do not hook the - terminal to ground. I would use a ground to the chassis, and then separately test the + and the - terminals as independent channels. (Same signal but inverted on the negative). Both will have signals on them.
You can also test in 2 channel mode by using both channels of the scope for
+ and - terminals for one channel of amp, then invert the second channel and display in sum mode. (or if your scope has a differential mode, that does the same thing.)


Okay thank you!!!! So the manual doesn’t explain that very well. So if I just hook up a speaker to it then it should be fine (not a nice pair) as that one side of the coil is technically not ground. In two channel should the negative be measuring open to the chassis the? Is it a bad thing that I’m measuring 145ish ohms?

Dan
 
Okay thank you!!!! So the manual doesn’t explain that very well. So if I just hook up a speaker to it then it should be fine (not a nice pair) as that one side of the coil is technically not ground. In two channel should the negative be measuring open to the chassis the? Is it a bad thing that I’m measuring 145ish ohms?

Dan
In 2 channel mode, the resistance + or the - terminals of the front speakers to chassis should be about the same value to chassis. However, it doesn't mean much as it depends on so many factors.
If you put a speaker on the + with the other side going to the chassis ground, and a second speaker on the - to the chassis ground, they should partially cancel out if in phase, and reinforce if you reverse the phase (polarity for the nit pickers) of one of the speakers. The signals are the same, but the polarity isn't.
 
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