Pioneer SX-303R Receiver: 1 Channel Dead

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  #1  
Old 11-01-02, 01:18 PM
TexasJim
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Pioneer SX-303R Receiver: 1 Channel Dead

My five or so year old Pioneer receiver has one channel dead. It is a Model SX-303R. Radio Shack also sold this unit with Realistic on it. It is a 100-watt per channel unit. I have looked for burned components, checked voltages as well as I can with a VOM. I find no suspicious components. I got on Pioneer's Website and found no schematic for this. Surely, someone has had this problem. Any ideas? Thanks in advance. Texas Jim
 
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  #2  
Old 11-02-02, 10:21 AM
bigmike
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Cool DVM...

You probably have a blown output transistor. You have a VOM is it digital (DVM)? Does it have the diode check setting on it? Do you have a soldering iron? Do you know how to check a transistor for failure? Answer these first and go from here…
 
  #3  
Old 11-02-02, 06:46 PM
TexasJim
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Yes, I have a Digital Voltmeter with a diode check function, and I know how to check a DIODE, but not a transistor. Also, I haven't been able to tell where the output transistors ARE without a schematic. I tried the section where the speaker/headphone switch is, to see if the switch is the problem. It isn't, the headphones only deliver the one working channel, also. So, the problem is PRIOR to the speaker/headphone switch. There are four big power transistors on a big heatsink, and all the voltages are identical in and out of all four. I would have thought that one of the four would be disfunctional.
If you can help further, let me know what to check next.
Thanks, Texas Jim
 
  #4  
Old 11-02-02, 07:52 PM
bigmike
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Cool Test...

Ok so you know how to “Front to Back” test a diode right? The outputs are either 4 transistors or a large black 60 to 80 pin IC in both cases they will be screwed to the large aluminum heat sink. The leads will protrude thru the PCB or be at the very edge of the board bent over making contact with the PCB. If it’s the individual transistor look at the board, it will be marked with E=emitter, B=base and C=collector. Unsolder the legs of the xsistors. You want to front to back the emitter to the collector legs, if you get any kind of reading (tone) in the diode mode in either direction the xsistor is shorted. You will have readings on all other leg configurations but only in one direction IE: Collector to Base, base to Emitter and base to collector. The one your concerned with is E to C. MOSFET’s will check between E&C but I doubt you have mosfet’s in that. The xsistors can be purchased for about $6/20 each and replace all four of them. The smaller regulators by the xsistors will most likely be ok. Go to Tri-Tronics.com or MCM Electronics.com for the parts. You don’t mention if you are blowing fuses or not but look at the fuses, are they blown black or can you still see thru them? If they’re black, you should really have a variac or some such setup to bring the unit up under low power conditions so as not to blow it again. If they’re clear, you should be ok when you power back up. I can provide you with a schematic to build a load indicator out of a 100-watt lite bulb and some misc hardware, very handy for this type of thing. Plug the unit into the lite, turn the unit on, lite glows brite you have a shorted condition, glows dim you are ok.
To make this even longer, if it is the large blk IC and since you do not have the schematic you will just have to pull it and replace it with the new pack and use the lite setup or just flip the switch and cross your toes. Before I do anything, I would check the solder joints of the xsistors or IC. Solder gets hot, old, the chemical reaction of the tin and lead they lose cohesion and break contact. Lastly or finally, you can look on the IC or transistors and get the numbers off them and look them up in an ECG book or at http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search?OpenForm (NTE site) for the pinouts and description of them. So with this you can check the IC. However, that will take another page minimum to describe…
 
  #5  
Old 11-03-02, 08:53 AM
TexasJim
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Thumbs up

NONE of the fuses are blown. I will do the transistor check per your instructions and get back to you(it does have four large power transistors on a heat sink). This unit is on my boat, so I will have to drag it home to my "laboratory" for the lobotomy. Thanks a bunch. I will be back in touch. Texas Jim
 
  #6  
Old 11-12-02, 02:18 PM
TexasJim
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Question

O.K.: bigmike: The power transistors are Toshiba 2SC3281's and 2SA1302's, two each. I desoldered the legs and front and backed them. NO Continuity of any kind (or diode check tone)either direction between E and B. Continuity, showing about 510(ohms?)(but no diode check tone) one direction between C and B and E and B, but not the other direction. Sounds to me like the transistors passed the front and back test?
What now? None of the four transistors appear to have been hot. No fuses are blown. No obvious "smoke leaks" on any of the other components.

Thanks, Texas Jim
 
  #7  
Old 11-12-02, 02:28 PM
bigmike
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Cool Well...

Well from here we are pretty much going to need the schematic. It's possible that the transformer has went. Or the standby transformer has went. With a no power at all scenario I would say the main transformer. At this point I would say to just replace the unit. You are going to spend more than it’s worth to repair it. Shame too, our landfills are getting clogged with crap like this. If pioneer will sell you the schematic, you might be able to trace this down but I would not hold my breath…
 
  #8  
Old 11-12-02, 03:06 PM
TexasJim
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Red face

One channel works fine. I have been using both sets of speakers off the working channel. I don't think the power transformer would be dead if one channel works? Oh, well.
Thanks for your help. Texas Jim
 
  #9  
Old 11-12-02, 03:26 PM
bigmike
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Cool Sorry didn't pay attention...

One channel dead duh… Still you will need the schematic to chase this down, if I had it in front of me I could probably figure it out. I bet the electronic switch for the speakers has failed, I have seen that several times in Pio stuff but I can’t remember which xsistor it is or its values. On the speaker outputs with the device selector set to AUX or one that has no input check the voltage out on the speaker outputs. Should be around 450 milivolts or less. If the voltage is much higher than 475 Mv then somewhere down the line a preamp or such has failed. Don’t mean to be vague but would have to have the unit in front of me to really get into it ant deeper. My suggestion still stands, replace it. It will cost more to fix it than it’s worth…
 
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