Marantz SR7012 sporadic DC protection fault

[schlot]

Hello,

I have a 3 year old SR7012 AV receiver that played fine until a couple of weeks ago.

Since then sometimes there is a loud pop from all amplified channels (not LFE) and the AVR goes to DC protect mode. I know that as it is possible to read it out via special menu.

It happened the first time when watching TV at low volume (-65 dB) for a while, so the device was at operating temperature.

Since then it has only happened at power up, exactly when the protection relay switches on. Device at room temperature. It seems as if it happens more likely if the device has been used before during the same day, has some time to cool down and is then started again. Overall it happens once in three weeks or so.

Apart from that it plays fine, so it is possible to watch TV for several hours. All channels working (I have a 5.1 front bi-amp setup).

Now I have some questions on how to approach the problem as I work a lot with digital circuits but I don‘t have a lot of experience troubleshooting analog stuff:

- If I can hear all channels popping when the unit shuts down, does it mean that there is a DC problem common to all channels? Maybe a supply rail breaking down? Or does the pop sound come from the protection shutdown itself and is therefore audible in all channels? What I mean is, can the problem still come from an individual amp path, protection circuit, bias network, thermal breakdown of a power transistor or so, or would the loud pop sound before shutdown only be audible from the defective channel?

- As the failure is only sporadic, are there suggestions how I could induce that failure? I do have the service manual, so I know some voltages or waveforms to measure from a couple of test points but as long as the failure does not occur I guess it is meaningless, right?

Any other ideas where to start?

I have a good multimeter and a low-end scope if that helps investigating.

Thanks
Thomas

[macboy]

That's a nice receiver, I have its little brother, the SR6012, and it completely blows away the Sony flagship 5700ES I had previously. It's definitely worth trying to fix the problem.

Sometimes it's the simple things. I'd start by opening up the receiver and disconnecting then reconnecting all connectors possible. This will help scrape away any oxides that might have built up over time causing a possible bad connection somewhere. For the same reason, loosen and re-tighten all screws holding PCBs to the chassis. The chassis is the definitive ground reference of the system, and some boards will use a screw connection to the chassis as an electrical ground connection (especially boards with external A/V connections). I have fixed two unusable receivers in the past just by doing the above.

If that doesn't help, then maybe try to catch the issue when it happens. What do you mean by "low end scope"? If it is a digital scope, then you can probably set it up to trigger (once) on the protect signal, while capturing data on the remaining channels - power rails, maybe one of the speaker outputs (to look for DC). You will want to set up the trigger to for the middle of the display (which might be the default anyway) so that you see some data before and after the fault/protection. The scope will sit there sampling data continuously until the trigger happens, then it will stop, capturing the protection event.

[mzacharias]

As a shotgun approach, replace the larger fixed voltage regulators.

7812, 7912 (or 15's) as well as the electrolytic caps associated with them, 10uF, 47uF 100uf, etc since there is also a chance of oscillation affecting one of them.

TO-220 type packages. Better than even chance it fixes your problem.

[ambrosia heart]

"It happened the first time when watching TV at low volume (-65 dB) for a while, so the device was at operating temperature."

Logical Troubleshooting steps see pm.

What you need to do is to find out what protection is.
How to do see pm.

An expensive hi-end receiver has more components
built on voltage balancing circuit(current detection) comparing  low-end one just resistors and diodes.  LOL. 

This may be a culprit. Analysis of this circuit makes you
understand how protection triggered.  You work on electronics, right? analyse it, (very easy. Any discrepancies
 in components kickoff protection)

Tell me your result if you want.

[schlot]

Hello and thanks for all your replies.

@macboy Thank you for the suggestion to try to fix ground connections etc., but I would prefer to somehow be sure (by measurement or otherwise) what component failed. Because currently it is no fun sitting in front of the TV watching a movie and not being sure whether the speakers are going to jump up 3 ft in the next moment. Sadly I only have the problem once in a couple of weeks which is too seldom to find it and too oftwn to ignore it. My scope is a digital one but when I make recordings of the startup phase for example everything behaves as normal. I just can‘t trigger that failure so I could properly analyse it.

@mzacharias: Do you have an explanation for replacing the 78xx/79xx regulators? As far as I can tell they only supply analog video and preamps etc., but even a failure in preamp could not lead to a DC failure in output stage as the stages are capacitively coupled. In the amplifier supply path there are only transformer, ECO switching relay, rectifier and two dinosaur like electrolytic caps. When I measure voltage in eco or normal mode as well as output ripple from the supply everything is rock solid.

I also have switched from eco to normal mode and back quickly for a hundret times as this quickly charges and discharges the huge caps putting some load on the entire supply system. I had the scope attached in pass/fail mode and everything was fine ramping up and down the voltage.

@ambrosia heart: Yes I do work in electronics. But my problem is that the problem is sporadic only. So, it happens once (boom from the speakers, then DC protect - I have used special mode to see it is DC protect). But after restarting it runs fine for another three weeks or so. This is why I have nothing to measure. I treated the amp and supply boards (not the logic boards) with a heat gun and (gently) with a plastic hammer to find if it responds to vibration or temperature change. Nothing.

Any other ideas?

[ambrosia heart]

Your reciever just 3 years old, the chance
of leakage of  amp. section of transistors
is zero. Basically, the d.c. Off set at speaker
terminal must be this part of amp. Which part?
see pm. The amp stability is highly related to one thing,
see pm. Measure dc offset can only be performed
when metal cover removed. 

Your amp does not have a usb input.     Forget what  I
wrote in pm.  Never think about the step. 

One advice: let it be. 

If you have time, you can use an oscilloscope to trouble-
shoot the amp to see dc offset.  If you want to do so, let me
know.

If I were you, let it be

[mzacharias]

"@mzacharias: Do you have an explanation for replacing the 78xx/79xx regulators? As far as I can tell they only supply analog video and preamps etc., but even a failure in preamp could not lead to a DC failure in output stage as the stages are capacitively coupled."

The protection circuit monitors those supplies, so a fault there will trigger the protect function; also, a fault here may cause a DC offset at the amplifier channels - again triggering the protect function.

[schlot]

I checked the entire schematic and I could not find any connection between the preamp / video voltage regulators (all those 78xx / 79xx) and any monitoring circuit. The DC / ASO which is triggering in my amp however explicitly has a connection to some discrete monitoring circuits inside the power amp section. But this one is only supplied by the transformer / rectifier directly with no regulator in between.

Today I tried to let the amp run with load resistors (8 Ohms, 25 W) at -8 dB for several hours without any problem occuring. For the amp this must have felt like a good party. I also tried to freeze it with some ice spray (amp sections, power supply, all regulators and rectifier). The device does not respond at all to high or low temperatures or vibration.

What I have not tried due to lack of equipment is how it responds to changes in the AC voltage (let‘s say voltage fluctuates or has surges). As the amp section does not have any regulators the voltage their will depend on the input voltage to a certain degree. So if every other week there are 245 VAC on the line for a while instead of 230 VAC, maybe those extra 5 volts on secondary side make a difference.

DC Offset voltages are between 1.9 mV and 4.8 mV on all channels. Measured with multimeter at speaker outputs (if that hopefully makes sense). I have to check again if the speaker outputs are AC coupled.


EDIT:

Something important came into my mind. The problem does only appear once in a couple of weeks. But when it occurs it sometimes persists for few minutes. Then I have the pop noise and it goes to protection right at startup when the speaker relays switch.

But when it occurs I can quickly lower the master volume from my usual power on volume level of -60 dB to something like -70 dB. Then at startup I can hear a pop noise but the amp does not enter DC protection.

This means it cannot be the power amp stage nor its supply because it must be something in the chain before the master volume IC. So, power amps and HDAMs are out of scope because a failure there would lead to a fixed severity, not being affected by master volume, right?

What else can it be? AVCC+7 / AVCC-7? Some problem with DSPs or DACs or their supplies?

[ambrosia heart]

First see pm and photo DC offset explanation.
As a knowledge not related to this part. See below.

DC Offset voltages are between 1.9 mV and 4.8 mV on all channels. Measured with multimeter at speaker outputs (if that hopefully makes sense) DC offset absolutely normal.

But when it occurs I can quickly lower the master volume from my usual power on volume level of -60 dB to something like -70 dB. Then at startup I can hear a pop noise but the amp does not enter DC protection.       I am sorry to tell you that you meet the most difficult problem: HDMI / decoding board faulty.  ALL signal no matter what analog or digital, will be processed by PLD board, then 4 DSPs in your amp and finally converted to analog via DACs then goes to LPF (low pass filter) and amplified by amp section.

But when it occurs it sometimes persists for few minutes. Then I have the pop noise and it goes to protection right at startup when the speaker relays switch.
What else can it be? AVCC+7 / AVCC-7? Some problem with DSPs or DACs or their supplies?

YES, see above, any chips go wrong or software needed in
both PLD and 4DSPs make pop sound.

Why do pop sound cause dc protect?
Due to high volume at first, the sinewave mixed with pop sound makes sinewave be clipped.  Clipped sinewave is
dc, triggering dc protection.

Let me guess you maybe always watch blu-ray films at high volume level or place your amp in a hifi cabinet.

You can still try your best to troubleshoot it stage to stagesee pm if you have time.  Oscilloscope 40 mHz
is needed. See pm (send you tomorrow) .

You are very unlucky. Never buy amps without usb.
Nor buy marantz or denon due to same digital
board with PLD.

In Hong Kong even authorized service dept. will
Not troubleshoot HDMI  board.
customer, cu,: pop sound in amp.
tech: 450HKD inspection fee.
tech: ok, problem found, HDMI board
Cu: how much ?
tech: can‘t repair. Only replacement . 3500HKD
cu: how much? 3500? He go away.
This is how techs scare off customer.

[schlot]

Let me guess you maybe always watch blu-ray films at high volume level or place your amp in a hifi cabinet.

No. Actually the amp is mostly driven at < -40 dB and it is standing freely on top of a shelf.

Never buy amps without usb.
Nor buy marantz or denon due to same digital board with PLD.

The SR7012 has USB. But how does this help?

[schlot]

OK guys, I‘ve fixed it.

I was about to give up on it and had already ordered another AVR when suddenly my Sony UHD Blu-ray player (UBP-X800) died from a faulty power supply.

I checked the player (which was obviously connected to my Marantz AVR via HDMI) and found a faulty freewheel diode in the switching power supply. This fault seems to have generated intermittend voltage spikes not only inside the player but also on the HDMI lines which were connected to the AVR.

I fixed (upgraded) the UBP-X800 by replacing the diode with a higher rated super-barrier diode that should generate less heat, so it should be good to not fail again.

The AVR did not have any problems since and that is more than 8 months ago.

Just wanted to share, in case someone else has a similar setup or issue.