Hey guys, I have a Sony STR-DE335 stereo reciever and I let one of my friends borrow it for a few weeks. When I came to pick it back up he said there was something wrong with it. At certain volumes it would go into protection mode, when I looked at it the stereo go into protection mode about 3 seconds after you cut it on.

I took it home and with nothing connected to the unit it goes straight into protection mode when turning it on. Now the unit will not even turn out. When its plugged in the unit flashes and the relays click but nothing come up now. I noticed my friends had a couple speakers connected to each channel so maybe he overloaded the amp.

Whats the most common things that would cause these problems. The unit is a few years old so its not worth paying someone to fix it but I would like to get it working again if it is something simple.

Appreciate the help.

 

one or two of its output transistors got overloaded and are now shorted.

Doubled speakers demand twice the current. Too much current blows up electronic parts.

 

These are the transistors that are located on the heatsink right? How can I test these to find out which on it is?

 

It is difficult to measure power transistors in a circuit. They can be tested with a 1.5V and 0.2V ohm-meter when they are removed from the circuit. My Fluke DMM has both.

It is difficult to get parts from Sony since most electronic items are now disposeable when they fail.

 

So basically I need to cut each transistor off and test them and then put them back on. I would have to order the parts from sony? I can't just order them from somewhere else?

The unit doesn't work anyways so its not a big deal if I end up breaking it or not.

 

You don't "cut" electronic parts. You unsolder them.

I ordered parts for my TV from Sony. They didn't know how to make my order. It arrived with parts I ordered missing and two parts were "substitutes" that were completely different from what I ordered and would not fit. They did have the parts that they "substituted" and the parts that were missing then sent them in a second shipment.

After that I saw the "same" parts on the web but they didn't say who made them. They might not work.

 

Normally when the output transistors go faulty they will measure shorted between the Collector and emitter. Even if they are paired up and you cant distinguish which transistor in that particular channel is shorted, I'd be replacing all the outputs in that channel anyway. Would also pay to check the driver and biasing transistors...as well as resistors around the area.
Certainly sounds like its an output, although I once had an intermitant protection fault in a Technics amp that actually turned out to be a faulty cooling fan motor. Doubt thats the case here however.

 

Thanks for the advise, I will see what I can find out.

 

one or two of its output transistors got overloaded and are now shorted.

Doubled speakers demand twice the current. Too much current blows up electronic parts.

They could be transistors, but I've seem some stereos with power amplifiers. Nevertheless, the cause is the same you pointed, not depending on the case. If your stereo uses power amplifiers (can be a single dual channel amplifier instead of two, or two bridged amplifiers per channel) you probably have to replace them.

 

I have the amp disassembled now. I found 5 MP1620 and 5 MN2488 transistors. The amp has 5 channels so 1 of each transistor per channel. Im still unsure how to test these transistors to see if they are good or not. Also is there a website that would have the schematic diagram for this amp?

 

You lucky lad, you get to build a transistor checker!

See attached:

 

Try here:
http://www.free-service-manuals.com/Detailed/3312.html
You have to register, but looks like registration is free, and you can download up to 3 items per day.
[eta]
Oops, looks like they took the free option away for manuals > 10mB, of which category yours appears to fall into.

 

amps go into protection mode if they have an excessive DC offset coming from the amplifier. this is most common, but it could be other things. adjusting the DC offset is easy if you have the service manual, schematic, or the procedures, impossible without. i recommend you get the service manual.

 

You lucky lad, you get to build a transistor checker!

See attached:

Will this one test the transistors in circuit. I see some of the other diagrams on the internet will test the transistors in circuit.

 

Ok guys I had some free time and messed around with the stereo some. I was testing the output transistors with my multimeter on the transistor/diode setting or something like that. I was comparing the resistence value with the other transistor of that kind. Well I found one that was way off from what the others tested at. It was one of the transistors on the right channel which makes sense. He on used the right and left channel to push all of the speakers.

Well I went ahead and unsoldiered that transistor and tested the stereo out. Now the stereo does not work at all. When the power button is pushed nothing come up on the screen. The setup button glows dimmly but thats it.

Would not replacing the transistor cause problems like this or is something else going on here also?

 

Will this one test the transistors in circuit. I see some of the other diagrams on the internet will test the transistors in circuit.

It's always been advertised as an "in-circuit" tester, however, you can never be as sure as with testing out-of-circuit.

I've always found stereo "protection" circuitry to be somewhat lacking and just more overhead that gets in the way of circuit operation. What tends to happen, even with "protection" circuitry, is the cascade effect. When the output goes it also takes the pre-drivers and possibly some of the protection circuitry along with it. Resistors may have heated up and burned and changed value (sometimes difficult to tell). Transistors can seem okay until they actually have voltage applied and try to operate, then they crap out.

As techroomt mentioned before, one of the main things to measure for is excessive offset at the output. With no input applied and no speaker or load applied, measure the DC level between the output and ground. It should certainly be less than 1V, typically ~200mV or so, but closer to zero the better (+/- doesn't matter). If it contains an output relay, then measure before this because the relay may not be engaging to protect the load.

Excessive offset is because the top (+) half is not matching the bottom (-) half. This can be caused by unequal output transistor biasing due to bad or missing driver transistors, or because of bad pre-driver transistors, or the offset compensation (biasing) circuitry is off adjustment.