Muffled/bad sound on 1 channel of Marantz PM4400 amplifier
The right-hand channel of my Marantz PM4400 amp produces very distorted and low volume sound compared with the left hand side which seems to be ok.
Obviously I am no expert but I have tried swapping speakers, speaker cable and using different phono cable from various inputs so I think I am now fairly safe in assuming the problem lies with the amp. I took off the cover but could see only one glass fuse, it looked absolutely fine although I have not tested it (I don't know how!).
Does anyone have any idea what this could be and if so how I could repair it with my limited (i.e. practically non-existent) knowledge?
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Re: Muffled/bad sound on 1 channel of Marantz PM4400...
Go directly to the main amp, follow the leads from the speaker terminals. And you should find something on an heatsink. If there are two transistors see if they are running hotter than the other channel. If you see any damage to them (compare with good channel) replace both!
If all you see is a black box with pins coming out of it you need to replace that. They sometimes have the letters STK on them.
Replace both types with the letters and numbers of the same type. IE 2SC 1220 or STK 589.
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Check the balance or fader control and turn it to the left channel and if that doesn't provide quality volume from the left try swapping the speakers.
If there is no improvement there is an amplifier fault...
If the sound is common to both channels plus on different functions, it is either a part that is common to both channels or is in the power supply. It will be either in the main pre-amp or power amp, or power supply. If you can turn the bad sound down, it's before the volume control and it's unlikely to be in the power amp section. If you can't turn the bad sound down, it will be in the power amp section.
Remember stereo amps are two mono amps joined together. So the two channels have separate parts for each channel. However some use IC's that have both channels running in them, they can be suspects in this case.
Guitar amps have preamp sections (think unpowered audio mixer) and power amp sections (Crown rack-mount amp). An excellent way to isolate audio problems if amp is still producing audio is to check the preamp out (line out). Preamp output distorted = preamp failure. Connect MP3 player to "Power Amp In"...sound distorted = power amp failure. In your case the latter is the case as your "line out" is clean. Also test each speaker for clear sound as speaker cone damage from being overpowered is fairly common. At 300w. I'll assume the failure was not from cranking the volume to loud but a shorted speaker or speaker wiring could zap the power amp in an instant.
Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.
It sounds to me that all you are getting is the pre-amp stage of the amp and nothing from the power amp stage. As it is on both channels then it is something that is common to both. This could be something to do with the power supply of the power stage. Or whatever is on the heatsink. It could be that the amp uses a single audio amp IC for both channels. If it does my bet is that it's gone. If it uses two sets of semiconductors for the right & left channel it would be highly unlikely that both channels have gone, but not impossible.
The distortion might be caused by a resistor. The loss of volume could be the capacitors, but you could try injecting signals into the transistors to see if any has gone. They can cause the distortion and loss of signal. If you apply a soldering iron to any part it will heat up quicker, or you could also use a hairdryer for the whole thing. You can also get a freeze spray that cools things that have heat sensitive faults (available from electronic parts shops).