Help with sx 1050 after new outputs installed

Hello, I have a pioneer sx 1050 and i need assistance.

About a month ago, while in aux mode for tv, my wife said the receiver just turned off. I got home and turning the main power switch on and off did nothing, was like it was unplugged.

I started by removing the case. I then installed a new main fuse (same rating). Tried to power up, still nothing. Tapped on and around around the protection board looking for something obviously loose. Suddenly the lights came on real dim and was greeted by a nice cloud of smoke off the surge current board. Then main fuse immediately melted.

I ordered a surge board off ebay, installed. Read up on this forum about possible causes of blown fuses. Saw that Bridge Rectifier could be to blame, I removed old rectifier from power board, it had burnt smell so replaced it with new one. It did however check out fine with my meter. I think it smelled burnt from possibly running hot over time.

Then got to testing and removing outputs. Discovered in right channel one bad npn @ 1.6 ohms and one bad pnp at .4 ohms. Orded new set from Mouser according to suggestions (http://audiokarma.org/forums/showthread.php?t=125460). Ordered mouser 863-mj21193g (npn) and 863-mj21194g (pnp) replacements. Got 4 for each for each channel (all 8) and installed.

Read that you should always dim bulb test, so i made a tester out of some things I had hanging around my work area with a 60 watt bulb.

Fired up and immediately the light came on full brightness (short). Pulled all output connectors off amp board. Fired unit up again and all lights came on. Test light came on bright then dimmed.

I attach the outer connectors (npn) and turn unit back on. Test light came on bright and dimmed. However, when I attach inner connectors (pnp), the protection tripped and light came on full brightness and stay bright. I removed and tried only left pnp, same result. Tried just right pnp, same result, both sides short.

I am hung up at this point. Still unsure why connecting the pnp transistor outputs shorts unit out. I have removed the semi and use my meter to look for a short but cannot find one with semi removed. Even printed out full service manual with 3 right binder and been studying the schematics for the last 2 days. What have I missed?

Thanks I appreciate the help.

Greetings;

That's it? a rectifier and transistors? (are they blowing do you mean the fuses are blowing? because I can't see an endless supply of testing and blowing output transistors.)

No test on any other components? rail voltage etc.. drivers caps testing etc. setting bias etc.. Power supply caps good? voltage regulators? Transformer voltages? pnp and npn are installed in proper positions?

Just hitting on extent of checks etc..

I guess I know where to pick up. Will get back to you.

Npn and pnp can only be orientated one way. Fuse initially blew after it shut down. All other power up have been on dim bulb tester.

I also see two rectifiers in the ps. not sure if accurate? but my understanding is power to these matched pairs is important. i.e. maybe get into the shop on a scope.

Binkman said:

I also see two rectifiers in the ps. not sure if accurate? but my understanding is power to these matched pairs is important. i.e. maybe get into the shop on a scope.

Click to expand...

I only found one. It has its own heatsink.

Sounds like you are trying to shotgun the repair. You will only end up with more heartache and damage until you actually take a test meter to things and find out where the actual short is.

In my experience and 99 times out of 100, you have a bad or shorted output transistor but you need to test the drivers and the emitter resistors also that are associated with the bad transistor and/or channel.

Well I've ohmed out everything I could think of. This includes carrying resistors. This is my first experience fixing circuit boards. I was only trained on hvac systems.

If you post a link to the schematic, and detail what board is shorted, we can guide you.
(you already narrowed the problem to the PNP boards. Without that boards connected, the dim bulb is OK, so I guess the PS is OK. It could be that part of the PS is on that board you disconnect tough. I don't know your unit)

It's just a question of keep narrowing the problem in that boards.

You need to start at the Supply Voltages, output transistors disconnected, be sure you have the right +V and -V rails with the right voltages.

With a schematic we can tell you exactly what to measure.

Be patient. If not, you can damage more things. Wait, wait, wait. The unit won't fix by itself, so you don't need to power ON again and again to check it's shorted. You can measure a short with the unit OFF.

FYI, it doesn't matter whether your transistors are PNP or NPN when testing. Check them like you would a diode between the BASE and the EMITTER.

A reading on the ohms scale in both direction would indicate a short. Once you've found the short or culprit, don't just replace it, test back a little further. Usually a shorted transistor will take out the associated driver transistor and/or emitter resistor.

Nick D said:

I only found one. It has its own heatsink.

Click to expand...

there are 2 bridge rectifiers .. one for regulated voltages and the other for unregulated

thnx.. read also associated layout of output matching.. so service caps as looking at pretty low comparative service voltages on both sides. caps likely if old and esr types. You'll need balance to even measure so rails have to be pretty equal at transistor power in and balance.. heavier caps with good esr again.

Sometimes these old units are just tired in the caps and general stuff just works fine. audio path caps a little better.

Good to know! Luckily I got the whole 100+ page service manual. I will upload schematics when the kids go to bed.

BTW it died when kids where watching curious George. How discracful!

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Am I looking to probably just recap power and protection boards?

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I can see the damage and smell the smoke from here already! :sigh:

Where are you so any AK'ers nearby can help?

Westfield, Massachusetts

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century tek said:

I can see the damage and smell the smoke from here already! :sigh:

Where are you so any AK'ers nearby can help?

Click to expand...

Lol and I thought I was off to a good start!

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Ok as promised. Thanks all for the help so far I do really appreciate it. A bit of a learning curve for me.

This are the PCB layouts. The schematic diagram will be helpful too, actually more than the PCB.

But, look at this: at Power Amp assembly, pins 22 and 23 are the emitters of those 2 output transistors (the "arrow" on the transistor symbol). And pin 21 is connected to both collectors. If you have continuity, or zero ohm reading there, there is ashort. Check all 8 transistors. Continuity from emitter to collector, something is shorted.

Pin 24 is connected to both transistor's bases.

Pins 25 to 28 repeat the same layout. And the same with pins 29-32, 33-36.

check continuity and post the results.

Don't power on the amp yet.

OK thanks will get put my ampro and start ohming out parts after work. You guys are great thanks!

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