Author Topic: Marantz SR7007 repair  (Read 3843 times)

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Offline _EmilTopic starter

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Marantz SR7007 repair
« on: December 19, 2021, 06:32:39 pm »
Hi,

I have a Marantz receiver which a friend of mine accidentally shorted a surround channel on. When i connect power cable it starts, but when relay clicks it turns off with standby led flashing red. If i unplug power amp board it stays alive. I inspected power amp board and found a shorted out transistor (2SD2560).

Removed it but same problem. When i check protection history it says "DC". Some reading on Google tells me DC error = DC output of the power amplifier is abnormal.

I'm kind of stuck here, what else should i check for damage? Maybe it's just a matter of replacing the transistor? I have no previous experience repairing amplifiers... :)

If someone have a service manual i would really appreciate it! :)
« Last Edit: December 19, 2021, 06:34:17 pm by _Emil »
 

Offline commongrounder

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Re: Marantz SR7007 repair
« Reply #1 on: December 20, 2021, 07:40:13 pm »
That output transistor is one of a complimentary pair. Removing it will cause the output of the channel to swing  negative, throwing the same DC offset protection error. Assuming the other transistor is okay, as well as the emitter resistors and drive circuit, replacing the blown output should restore DC balance and allow the amp to function.
 
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Offline _EmilTopic starter

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Re: Marantz SR7007 repair
« Reply #2 on: December 21, 2021, 01:52:02 am »
Thanks for your reply :) I will order a new transistor and check if that fixes it. You say it will swing negative, would removing the other transistor make it temporary work? With one channel missing of course :)

Edit: I had to try... It works! I will order a new transistor :)
« Last Edit: December 21, 2021, 02:24:41 am by _Emil »
 

Offline _EmilTopic starter

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Re: Marantz SR7007 repair
« Reply #3 on: January 07, 2022, 06:03:25 pm »
Both output transistors were bad. 2560 and 1647. Replace both, no more protection message. But I'm not able to read any idle current... 0mV, trim pot does nothing.

Any input on what i should check next? Kind of stuck at the moment :)
 

Offline Chris56000

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It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 

Offline RtBNL

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Re: Marantz SR7007 repair
« Reply #5 on: January 07, 2022, 10:39:58 pm »
You don’t mention if you have any output on the channel? Check the emitter resistors in the faulty channel, usually they have a low value, around .22 or .47 ohms. It’s likely they have been taken out by the faulty transistors.

Edit: They use two .47 ohm 2 watt resistors in parallel, see the attached screenshot of the front left channel. CN715 is the connector you use to measure the idle current.

« Last Edit: January 07, 2022, 11:20:03 pm by RtBNL »
 

Offline shakalnokturn

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Re: Marantz SR7007 repair
« Reply #6 on: January 07, 2022, 11:05:22 pm »
Then if you have sound but no measurable idle current start by checking diodes between R7129 and R7130 for shorts.
 

Offline _EmilTopic starter

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Re: Marantz SR7007 repair
« Reply #7 on: January 09, 2022, 06:47:14 pm »
Thanks for all replys! :)

Just tested it, no output on surround channel... Will check the circuit further once in better shape haha. Damn covid :)
 

Offline _EmilTopic starter

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Re: Marantz SR7007 repair
« Reply #8 on: January 09, 2022, 07:52:51 pm »
You don’t mention if you have any output on the channel? Check the emitter resistors in the faulty channel, usually they have a low value, around .22 or .47 ohms. It’s likely they have been taken out by the faulty transistors.

Edit: They use two .47 ohm 2 watt resistors in parallel, see the attached screenshot of the front left channel. CN715 is the connector you use to measure the idle current.

No output... R7129, R7131, R7132, R7133, R7134 are all showing high resistance. Will do some more research before ordering parts :)
 

Offline RtBNL

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Re: Marantz SR7007 repair
« Reply #9 on: January 09, 2022, 09:36:23 pm »

No output... R7129, R7131, R7132, R7133, R7134 are all showing high resistance. Will do some more research before ordering parts :)

Did you measure them in circuit? Then those readings are explainable. You have to desolder them at least on one side to make a reliable measurement. 

No sound can have multiple reasons. It’s likely those emitter resistors have been fried when the transistors shorted. There is also an option that the shorted transistors took out the driver transistors.

Do you own a scope?
 
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Offline _EmilTopic starter

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Re: Marantz SR7007 repair
« Reply #10 on: January 09, 2022, 09:47:13 pm »

No output... R7129, R7131, R7132, R7133, R7134 are all showing high resistance. Will do some more research before ordering parts :)

Did you measure them in circuit? Then those readings are explainable. You have to desolder them at least on one side to make a reliable measurement. 

No sound can have multiple reasons. It’s likely those emitter resistors have been fried when the transistors shorted. There is also an option that the shorted transistors took out the driver transistors.

Do you own a scope?

I did measure them in circuit. Other channels showed same value as schematic. I don't see how they would read a lower value desoldered in other end. I think these resistors are bad :)

I don't own a scope unfortunately...

Been searching for new resistors, seems hard to find 0,47 ohm resistors. You think 0,5 ohm would work?
 

Offline RtBNL

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Re: Marantz SR7007 repair
« Reply #11 on: January 09, 2022, 10:18:56 pm »

I did measure them in circuit. Other channels showed same value as schematic. I don't see how they would read a lower value desoldered in other end. I think these resistors are bad :)

I don't own a scope unfortunately...

Been searching for new resistors, seems hard to find 0,47 ohm resistors. You think 0,5 ohm would work?


They won’t measure any lower 8)

0,5 ohm would do fine, or a single 0,22 ohm in a higher power rating if there is enough room. The difference is really small so I don’t expect you hear much of a difference in output. You could recalculate the idle current but it isn’t a big difference.

Before replacing the resistors I would also check the driver transistors for shorts.

Offline _EmilTopic starter

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Re: Marantz SR7007 repair
« Reply #12 on: January 09, 2022, 10:31:16 pm »
And with driver transistors you mean Q7131 and Q7132? :)

I appreciate the help!
 

Offline RtBNL

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Re: Marantz SR7007 repair
« Reply #13 on: January 11, 2022, 08:08:29 pm »
And with driver transistors you mean Q7131 and Q7132? :)

I appreciate the help!

I believe those are called the finals transistors in English. Basically in an amplifier line level signals are “amplified in voltage” in the first stage, the transistors amplifying the voltage can’t handle much current so the amplified signals go into the driver transistors and those drive the final transistors. The final transistors are able to handle the high currents the speakers need. IRL there is much more going on as you have to be able to control the DC offset, idle current etc.

When the finals fail in a short I always look at the schematic to see where the current could have been flowing. Paths / parts with a low resistance are more likely to fail then those with a high resistance.

You mentioned that one of the 22 ohm resistors also failed so it’s likely that surrounding parts have failed to. I circled them in red at the schematic. This is the surround left channel. Hope that is the same one that failed as you have corresponding component numbers.



Hope all is clear as I’m no expert either.

Offline coromonadalix

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Re: Marantz SR7007 repair
« Reply #14 on: January 12, 2022, 01:59:26 am »
q7418 is the driver transistor
q7425 is the biais transistors

q7420 q7421 are what we call a differential pair transistor amplifier

if the 22 ohms have busted  i would check all the transistors outside the circuit  up to the driver,  the output transistors are darlington, they may be tricky to test thru a diode test, since the junction voltage is twice a normal transistor and you have an emitter emitter resistor in them

The biais voltage may got to high, cold solder, mecanical failure  etc ...   

i had a similar problem in the past with the biais circuit, it got defective and busted out the amplifier section, since it was direct coupled  lots of parts failed ....
 
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