Author Topic: Marantz SR5200 amp damaged center channel + DAC issues (Read 7274 times)

poot36 · « **on:** August 17, 2018, 03:45:06 am »

I got the amp from a friend and they reported that the digital inputs as well as any analog input that was not set to sound direct would not produce any sound out of any of the speakers. When I was trouble shooting this I found that the surround sound DAC was getting very warm and was not outputting any sound out but there was signal into it. During this process I had no speakers connected to the amp and was measuring the output of the transformer when I had a fire ball and smoke come out of the amp for no reason. I do not believe there was any audio source but I could have been on the internal tuner. I traced this down to the SAP16NY Darlington transistor on the center channel going short between source and collector. I disconnected the emitter source and collector of both the SAP16NY and SAP16PY Darlington transistors and started checking the driver circuitry and found that I was missing the negative section of the sine wave at the base of both Darlington transistors all together. I have tested all of the resistors and they all check out as well as all the pre-driver and driver transistors. The caps are not shorted and the few I was able to test in circuit passed. Is the reason that I am getting half of the sine wave because there is no feedback from the collector pins of the Darlington pairs? I don't want to buy parts and have them blow as well due to another fault in the circuit. I have attached a screen shot of the relevant section of the schematic to this post.

coromonadalix · « **Reply #1 on:** August 17, 2018, 10:51:35 am »

I think you have to remove q 205, q206, q207, q 208 q 209 check them all, check all your supply rails

Q204 Q203 act as an current regulator for the differential input transistors, they may be damaged

This amp is direct coupled, if something at the output transistors goes bad, they can damage almost entirely the amp section up to the input.

Check all capacitors, all the low values ohms resistors (all of them loll) , and your r212 resistor in the squared markings.

And finally your dc output protection circuit q 215.

poot36 · « **Reply #2 on:** August 17, 2018, 01:10:37 pm »

I have checked all the transistors, the only one that was shorted was the SAP16NY and that has been removed from circuit except for its base and drain connections. All of the resistors check out. I will test all of the capacitors that I cannot test in circuit. I used one of the cheap component testers from ebay to test the capacitors and transistors. All the supply voltages are ok and the rest of the output channels work with no issues.

Zucca · « **Reply #3 on:** August 17, 2018, 01:44:16 pm »

No scope, uh?

poot36 · « **Reply #4 on:** August 17, 2018, 05:11:29 pm »

I have a scope, that is how I figured out that I was missing half of the sine wave. I have removed the amp section and have hooked it up to a +- 30V supply (it is normally run around +- 40V in the amp) and with no signal attached to it Q205 and Q209 get warm and on both of the bases of the output transistors I have around -19V. On the good channels I get +- 1V on the bases of the output transistors. Is it because there is now no bias/feedback loop from the emitters of the output transistors? Since only one of the outputs transistors failed can I just replace the one or will I have to replace both of them? I know I cant get the SAP16 transistors anymore and will have to replace them with STD03 along with a emitter resistor.

Zucca · « **Reply #5 on:** August 20, 2018, 08:20:27 am »

Q213 and Q214 must be symmetic, so if you replace SAP16NY Q214 with a STD03N you should replace also the Q213 with a STD03P.

If you are operating the circuit without one of the two above, the circuit is not symmetic anymore and funny things could happen.

Then do me a favor, take a DMM and write down all the VDC value on the schematics node and post the result here...

coromonadalix · « **Reply #6 on:** August 20, 2018, 10:20:43 am »

The term is matched pair or have the same hfe ( amplification gain )

But i don't think it will be an issue in this case, sure it doesn't have an biais adjustment, finding parts or substitute them is the problem ... never saw those transistors number here.

poot36 · « **Reply #7 on:** August 22, 2018, 02:15:43 am »

I have confirmed that the only damage is the blown SAP16NY transistor. I connected the working feedback line from another channel to this one and was able to making the -19V go positive. I have attached a picture of the voltage readings with reference to ground on all transistors as well as the voltage across various resistors to give an indication of the current drawn by each stage of amplification with no signal input. If you want any other measurements just let me know.

Zucca · « **Reply #8 on:** August 22, 2018, 10:31:47 am »

Thanks!

It looks good, now R205 and R206 have almost the same voltage across, I can't understand why one is warm and the other not.

poot36 · « **Reply #9 on:** August 23, 2018, 04:00:02 am »

I screwed up... I measured the good channel thinking that is what you wanted measurements of. On the bad channel the base ofQ202 was around 0.5V due to no feedback from Q214 and Q213 due to them been disconnected. This caused Q202 to be turned on hard turning off Q206, Q207, and Q208. This is why I got negative values on both bases to the output power transistors. Also probably caused Q205 and Q209 to be turned on almost fully causing them to become warm. I have ordered replacement STD03N and STD03P power transistors as well as 2 0.22 ohm 3 watt emitter resistors. Will the power rating of the emitter resistors be adequate for this amp?

poot36 · « **Reply #10 on:** August 26, 2018, 06:47:13 am »

Well I have received the replacement parts and have installed them. The center channel now works and the voltage measurements are very close to the working channels. I have also found out the hard way that when testing dual rail amps on the bench to make sure both rails are connected or the speaker likes to start smoking and that is the weardest smell I have sniffed to date. The amp still works after this incident so that is good. I think what saved it was my current limited 0.5A max test supplies from old printers. I still need to figure out the digital section of the amp but that is going to be another topic in its own right.

coromonadalix · « **Reply #11 on:** August 26, 2018, 11:59:09 am »

You may had an voice coil problem on the speaker, some deformation, and the wire enamel began to short to the metal ...

poot36 · « **Reply #12 on:** August 26, 2018, 06:50:06 pm »

I know that when I was trying to attach a audio source to the amp that at minimum the ground rail got disconnected and at some point the amp started motorboating. After I unplugged the power supplies I found out that the negative rail had become disconnected. I suspect that the full ~30V from the power supply in dc got applied to the speaker coil and smoked it. I don't really care because I got the speaker for free.

poot36 · « **Reply #13 on:** August 27, 2018, 06:27:58 am »

Well I just did a full power test in the amp and that was a success so I think this issue of the amplifier is fixed. I did notice that there is around 0.063V dc output on the center channel with no speaker attached and no signal input compared to around 0.030V dc on the other channels. Is this because my emitter resistors are 5% tolerance? It does not trip the protection circuit so I am not sure that I should worry about it.

coromonadalix · « **Reply #14 on:** August 27, 2018, 10:29:45 am »

You dont have any bias adjustments for the center channel, it is an very basic amplifier

The important parts are between the bases pins of the output transistors, 5 resistors 3 capacitors.

poot36 · « **Reply #15 on:** August 28, 2018, 03:59:57 am »

So should I bother trying to correct the offset or will it make no difference? All the output channels are clones of each other. I assume I would want to adjust the two 22K resistors to adjust the bias correct?

coromonadalix · « **Reply #16 on:** August 28, 2018, 10:24:08 am »

If it work

Don't play anymore with it, an 0.063V dc is okay at the output, it wont fry the speaker, enjoy your cinema ...

poot36 · « **Reply #17 on:** November 24, 2018, 09:25:41 pm »

Sorry for the late reply. I wont mess with it. I now have the replacement DAC installed and I still have no sound from any of the speakers unless I put it in stereo direct mode which bypasses all of the digital sound processing circuitry. Interestingly I do get some nonsense sound for a second or so when changing the surround sound mode but that is it. I have power to the DAC and SCLK as well as LRCK. I am not sure about SDATA and the AMUTE and BMUTE appear to be operating in reverse to what I would expect (high when there is sound and low when there is no sound). On SDIN1, SDIN2, and SDIN3 I get a constant high or low but no changing data when selecting the different surround modes. Got any ideas?

mzacharias · « **Reply #18 on:** November 25, 2018, 02:38:52 pm »

I have replaced a number of the DAC/electronic volume control IC, IC720 - a CS4391KZ, mainly on the SR-7200 model. Exactly your problem symptom. They're dirt cheap from UT Source. Also, this series has lots of bad front panel tact switches. Available cheap from eBay (China). Takes a couple weeks to get them.

poot36 · « **Reply #19 on:** November 25, 2018, 05:54:07 pm »

Ok, I have attached a picture of the relevant section of the schematic. This unit has a separate volume control IC on a different board and it does work properly. The DAC I replaced is a CS4228A and as they are discontinued I had to get it from China so I guess that it could be a fake. Interestingly it is connected to another DAC (CS4391) for just the FR and FL channels (it is IC720 in the picture as the bottom of it is cut off). Got any ideas of what to measure now to try and figure out what my issue is?

mzacharias · « **Reply #20 on:** November 26, 2018, 05:15:00 pm »

It's been a while but if I recall correctly, I just looked for output at the related op-amps, and then replaced the CS4391 that was feeding it. Since then - I've seen the problem so many times I just replace IC720. Haven't missed yet. It was a very common problem.

P.S.
They had a lot of problems with the volume control rotary encoders, too. Obviously doesn't directly cause your problem, and you would see the poor response on the VFD display.

poot36 · « **Reply #21 on:** November 27, 2018, 03:13:11 am »

In my case would IC720 being bad result in no sound out even in surround mode due to the way it is attached to IC719? If I remove it would I at least get sound from the surround channels but not the FR and FL channels?

mzacharias · « **Reply #22 on:** November 27, 2018, 06:09:10 pm »

As best as I can recall - yes, no or low sound in any mode, and can be somewhat intermittent. You could try just re-flowing the solder on IC720. I always wondered if it was a solder problem; the chip gets a little warm - but I have always just replaced the chip. I'm retired now but even still, I keep one of those chips on hand. They're cheap, and I really like the SR-7200 model. If one becomes available for very little, I might acquire another.

poot36 · « **Reply #23 on:** November 29, 2018, 06:22:52 am »

Well you were right, IC720 is part of the problem. It was shorting out the SCL line of the I2C bus causing multiple chips to not work. I have removed it. Now I have data on the 3 SDIN pins on IC719 and SDATA on the IC720 pin when switching between the surround modes. I still don't have sound from the surround channels though and when changing the surround mode I still get unwanted sound coming out of all channels like the mute is working backwards. I will order a replacement chip but it will probably take about 2 months to get here from China. In the meantime why are the surround channels not working correctly?

mzacharias · « **Reply #24 on:** November 29, 2018, 02:35:45 pm »

Quote from: poot36 on November 29, 2018, 06:22:52 am

Well you were right, IC720 is part of the problem. (snip) In the meantime why are the surround channels not working correctly?

Don't know really but I'd be surprised if all was not well after you get that IC720 replaced.

When the receivers were still pretty new, Denon tech support could only say the DSP board was bad, but they didn't stock them so if under warranty the receiver just got replaced, otherwise the customer was on his own.

Much later, during some slack time in the shop, I was able to troubleshoot as I said before - looking at the appropriate op-amp outputs, then referring back to the chip that was driving them - IC720. Since then I've replaced at least 1/2 dozen for the same problem.

Other than this and the front panel tact switches, the receivers are quite reliable in my experience.

The tact switches are important, though. They can cause a variety of symptoms, no response to a source selection, wrong source selected, lock-ups, etc.

By the way - here is the reset procedure. It is undocumented both in the user manual and the service manual, if I recall correctly.

1. Unit unplugged
2. press-hold A/D , F/P, and Dimmer.
3. Plug in, hold til unit turns on.

Unit is reset if initial volume is -75db


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Author Topic: Marantz SR5200 amp damaged center channel + DAC issues (Read 7274 times)

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