Finding the reason for PROTECT MODE in my Marantz SR4500

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Hello friends,

I really need your advice for some problem in my Marantz SR4500.
The problem started when I got really crunchy sound on the Front Right channel, something really horrible (attached sample of the crunchy sound -
So, I downloaded the schematic of the amp and started to check the circuit (with a basic experience in circuits or fixing...just knowledge for reading schematic and recognizing the elements..) , I found that one of the transistors (the one in the picture - B1560 is not soldered appropriately, so I re-soldered the transistor, after that i did a horrible mistake. :(
I turned ON the receiver and played a music, the sound was steel crunchy, so I looked at the Base leg of the B1560, and I tried to push it with a screwdriver, (while doing it with full stupidity), I think i touched both of the Base and the Collector legs of the transistor.. and then there was a light and Pfff in the speaker...and since then ->
PROTECT MODE and turn off. every time I am turning on the reciever..
I looked around , all the elements are looking good, but still I dont really have an idea where to start...
Where to start? what can i check?
the B1560 acting good while checking with a FLUK so i think it something else that burned..

I will be grateful for any help from you. even if its a link to some guide or same story..

Thanks in advance and best wishes for you all.
Joined 2010
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The sound I hear is that distinct sound of a blown output stage - like about 90% of all power amplifier faults. With a Meter, test for Collector-Emitter shorts of the output pairs. Compare with the good channel and you'll see significant differences. Before anything though, remove the input signal and speaker connections and turn volume right down. You are testing DC conditions and you already know the audio is dead.

Then, assuming you are competent to work safely with mains powered wiring, fit a bulb limiter to limit power to the amplifier and prevent further damage.
An externally hosted image should be here but it was not working when we last tested it.

You can then measure the voltage with respect to signal ground at the output and probably find, right at L502 on the PCB, that the voltage is not within 50mV of zero and this will probably trigger the protection circuit to disconnect speakers and prevent you from frying them with DC. Fix the output stage and the protection circuit won't trigger any more. Note that the bulb limiter remains in circuit until the repair is complete and satisfactory or you may just repeat the problem.

You'll need to find replacements for your output transistors which are Sanken Darlington types, still in production.
2SB1560 pdf, 2SB1560 description, 2SB1560 datasheets, 2SB1560 view ::: ALLDATASHEET :::
2SD2390 pdf, 2SD2390 description, 2SD2390 datasheets, 2SD2390 view ::: ALLDATASHEET :::
Considering the likely fakes around, you'll need to be careful where you buy. Using Ebay from a Hong Kong store is just asking for fakes and more trouble.
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Check driver transistor Q882, too. Also, when checking Q652 remember that it is a PNP transistor so it will work in the opposite direction.

in all of my measurements the reciever is disconnected from AC input, source input and speakers also diesonnected.

so I checked the Q882 in the FR and it looks same as the FL Q882
+ -
Base - Emitter 0.7-0.8
Emitter - Base 0.7-0.8
Base - Collector 0.7-0.8
Collector - Base 0
Emitter - Collector 0.7-0.8
Collector - Emitter 0

Regarding Q652 it is same as in the FL...

Any idea what else can i check?
Hi Ian, Thanks a lot for your detailed explanation, I just dont understand something, Do you recommend me to measure the reciever while it connected to ac power? anyway i cant do this because every try to power up the unit it'll enter to protect mode and it turn off...

Does your explanation is the fix for the noise or the fix for the protect mode?
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The amplifier does not turn off in protect mode. It remains on with the speaker(s) disconnected by the appropriate speaker relay(s). Therefore you can measure what you need to know before the relay by testing at the amplifier side of the relay contacts or the corresponding node in the amplifier circuit itself. When there is no signal and volume is down, any channel with an output DC voltage above about 50 mV, indicates an amplifier fault.

The point I have tried to make is very simple; the protection is triggered because there is an amplifier fault. Fix the fault and the protection will no longer trigger - right? The complicating factor is that there are multiple channels and you must identify the correct faulty channel(s). Unless you have experience with HT receivers, that's not always easy and I won't try to teach a long and complex subject that I also find difficult, in a forum. Perhaps you should consider a professional repair rather than struggle with amplifier basics.

The point Willi made about checking driver transistors when the output transistors fail is sensible, except that the driver transistors are included in the Darlington output devices here and you have to consider that the NPN and PNP pairs together make up the output stage of the amplifier - drivers and outputs together. That is what Darlington transistors are, shown in simplified NPN format below:

I suggest when testing anything with meter probes, that you fix one probe, probably to ground, so that you are not waving both about causing more damage than you can locate and repair. You can do this with insulated clip leads or clips adapted to probes or any number of ready-made meter leads fitted with hook probes.
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