Marantz SR7007 repair - Page 1
Forum section: electronics / repair

author: _Emil
author_rank: Contributor
timestamp: December 19, 2021, 06:32:39 pm
content: 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!

author: commongrounder
author_rank: Frequent Contributor
timestamp: December 20, 2021, 07:40:13 pm
content: 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.

author: _Emil
author_rank: Contributor
timestamp: December 21, 2021, 01:52:02 am
content: Thanks for your replyI 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 courseEdit: I had to try... It works! I will order a new transistor

author: _Emil
author_rank: Contributor
timestamp: January 07, 2022, 06:03:25 pm
content: 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

author: Chris56000
author_rank: Frequent Contributor
timestamp: January 07, 2022, 08:46:53 pm
content: Hi!Any help?https://www.google.com/url?sa=t&source=web&rct=j&url=https://pdfcoffee.com/hfe-marantz-sr7007-service-en-pdf-free.html&ved=2ahUKEwjVitzlv6D1AhWT_7sIHWaZDlAQFnoECAUQAQ&usg=AOvVaw2ckC1OjSZbEykQ5YRWoud5Chris Williams

author: RtBNL
author_rank: Newbie
timestamp: January 07, 2022, 10:39:58 pm
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 07, 2022, 11:05:22 pm
content: Then if you have sound but no measurable idle current start by checking diodes between R7129 and R7130 for shorts.

author: _Emil
author_rank: Contributor
timestamp: January 09, 2022, 06:47:14 pm
content: Thanks for all replys!Just tested it, no output on surround channel... Will check the circuit further once in better shape haha. Damn covid

author: _Emil
author_rank: Contributor
timestamp: January 09, 2022, 07:52:51 pm
content: Quote from: RtBNL on January 07, 2022, 10:39:58 pmNo output... R7129, R7131, R7132, R7133, R7134 are all showing high resistance. Will do some more research before ordering parts

author: RtBNL
author_rank: Newbie
timestamp: January 09, 2022, 09:36:23 pm
content: Quote from: _Emil on January 09, 2022, 07:52:51 pmDid 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?

author: _Emil
author_rank: Contributor
timestamp: January 09, 2022, 09:47:13 pm
content: Quote from: RtBNL on January 09, 2022, 09:36:23 pmI 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 badI 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?

author: RtBNL
author_rank: Newbie
timestamp: January 09, 2022, 10:18:56 pm
content: Quote from: _Emil on January 09, 2022, 09:47:13 pmThey won’t measure any lower0,5 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.

author: _Emil
author_rank: Contributor
timestamp: January 09, 2022, 10:31:16 pm
content: And with driver transistors you mean Q7131 and Q7132?I appreciate the help!

author: RtBNL
author_rank: Newbie
timestamp: January 11, 2022, 08:08:29 pm
content: Quote from: _Emil on January 09, 2022, 10:31:16 pmI 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 12, 2022, 01:59:26 am
content: q7418 is the driver transistorq7425 is the biais transistorsq7420 q7421 are what we call a differential pair transistor amplifierif 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 themThe 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 ....