Faulty Marantz Power Amplifier Supply! - diyAudio
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Old 1st May 2012, 06:29 AM   #1
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Location: Melbourne
Default Faulty Marantz Power Amplifier Supply!

Hey all,
I really need some guidance with this, so I would greatly appreciate any input you have.

I recently bought an Marantz SM80U power amplifier off ebay, cost me $300AUD. After plugging in my DAC/Preamp and connecting the speakers, i turned it on. The lights come on, but no sound is being outputted to the speakers.
After inspecting the internals, vacuuming all the dust and carbon, i replug it in and turn it on. I notice that a very strange almost ceramic smell start ommiting from the amplifier (not sure specifically from where).

Inspecting all the modules, i noticed that a few of the BJT transistors on the board had burnt the PCB surrounding them. Same goes for some FETs adjacent.
Click the image to open in full size.


After taking the module out, i immediately notice that the underside of the board is quite burnt!
Click the image to open in full size.
Click the image to open in full size.

Quite annoyed that the ebayer has sold me a faulty item, I've demanded a partial refund because I think I may be able to fix this.

The module after consulting the service manual for the SM-80U:
Service Manual
Is the Power Supply Regulator for the power amplifier.


The interesting thing is that there are two fuses protecting the module from exactly this damage.
USA/Japan it specifies: 630mA 250V
Europe it specifies: 400mA 250V.

Im in Australia and our mains are 230V (which i think is the same as Europe), which means that these fuses should be rated at 400mA, because this burning has taken place, something tells me they are clearly not.

Looking at the circuit diagram, the LED's power bypass a majority of this module (only the rectifier is really used), which gives explanation that the LED's turn on when I power up the amp.
If this module isnt outputting the correct voltage, then I would expect that the power amplifier itself wouldnt be able to work, hence the output voltage isnt been conducted.
Is this an appropriate assumption?

This is my first repair Job and I can find the transistor parts in the service manual, but should I repaire ALL the transistors, including the mosfets etc?

Should I test each transistor seperately to see if its damaged, (all i have is a multi with a diode test on it). If the BJT's have broken down, the diode test on the BJT's base and collector/emitter shouldnt work, yes?

How can I test the FETs?

If you where me, what would you test first, output voltages etc? What would you do in this situation?

Thanks!!

Last edited by olmeister; 1st May 2012 at 06:37 AM.
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Old 1st May 2012, 07:18 AM   #2
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Location: Dona paula, Goa
The darkening of the PCB is normal, but will the circuit work with this deteriorated pcb material is to be be checked. Measure the output voltages. If they show correct, then forget the blackening.

Gajanan Phadte
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Old 1st May 2012, 09:56 AM   #3
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I'll confirm.

Theres absolutely no sound, not a hiss coming from my speakers, which brings me to the idea that the amplifiers aren't being switched on...
Any ideas?
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Old 1st May 2012, 10:23 AM   #4
pdul is offline pdul  Denmark
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Did you check the switch for mains power, could it be set for 115v?
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Old 1st May 2012, 12:29 PM   #5
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Hey,yes the transformer is 240v...
110v the windings would blow and the lights wouldnt turn on..
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Old 2nd May 2012, 07:16 AM   #6
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Location: Dona paula, Goa
Check whether 62 volts and 53.5 volts are present at the connectors. These r dual polarity.
That means connecting black lead to ground and other to each of these will show +62V or -62V and so on.

Gajanan Phadte
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Old 2nd May 2012, 07:37 AM   #7
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Those are 400mA medium blow fuses, see http://www.ue-soft.de/oilfield/techt...c/fuses001.pdf for a colour code table
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Old 2nd May 2012, 07:46 AM   #8
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Join Date: Jul 2003
Location: Dona paula, Goa
Check whether 62 volts and 53.5 volts are present at the connectors. These r dual polarity.
That means connecting black lead to ground and other to each of these will show +62V or -62V and so on.

Gajanan Phadte
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Old 2nd May 2012, 08:01 AM   #9
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Location: Melbourne
Hey thanks for the replies,

I spent the last few hours examining the circuit "P851" which is the circuit power regulator (images above) from the schematic and testing the PCB.

There is a -35v output from transistor Q859, which should actually be -23.7V according to the schematic. This excess voltage is caused by the -62V rail actually being -95V.

Below i have highlighted the break down of analysis, the -80 -> -62 Module is damaged.

Click the image to open in full size.

The scary thing now is that both this -95V and -35V has been conducted through the main circuit board. Unsure about the transistor ratings, this is a potential catastrophe..
Considering that no sound comes from this (at all), what would your initial thoughts be?
Should I attempt a full-refund?

Last edited by olmeister; 2nd May 2012 at 08:21 AM.
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Old 2nd May 2012, 09:58 AM   #10
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Transistor 2SA970 on the -80V to -62V regulator has blown.
Confirmed with diode test across each terminal of the the transistor.. no 0.7v drop.
I find it quite amazing that this one little transistor can render the entire system useless.
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