Pioneer SA-1050 Channel Noise

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I have this unit on the bench. I has in the past been serviced and by looking at it ,the right channel suffered a gunshot wound at some point.
A lot of work was done, 2sc2705 were for the most part all replaced, right power transistors replaced, a few resistors replaced.

Now Its on my bench initially the right channel was crackling ,and sounded like the speaker was blown.

After some probing around I found that there were some dry joints and one transistor was loose(2sc2705) and pads broken, Q520 I believe it was.,

Re-attached everthing and the crackling is gone ,but still sounds like a blown speaker.
Using the left channel as reference, I checked voltages on major components. Everything appears fine..Moving any of the audio control's dosent aggravate the noise.

I'm left now cleaning away old flux ,and tomorrow I'll grab some Mr. Freeze AKA: Key Board duster.

Looking for a new direction here , I'm sure others have encountered this .
Where else should/could a guy look.:wchair:

For reference I have been using the SA-1040 service manual ,,which is pin for pin identical
I'm assuming the 1050 designation is to note it was intended for Canadian distribution.

http://madamebutterface.com/assets/documents/Pioneer SA-1040 Service Manual.pdf
 
So Far I have found
1- 4.7 volt diode
1- C2705 transistor

Both faulty

A left over I believe from the last repair were two resistor's 8.2k these were out of tolerance they were both in the high 9k ohm range so I replaced these along with the two other parts.

The 2SC2291 ...when hit with the key board duster the static did get worse..
left channel was unaffected by this.

Isnt C2291 a simple current divider?
Couldn't a person super glue two equivalent transistors together to achieve the same result?

I'm noticing other anomalies the .33ohm dual 5 watt resistor was registering .66ohm while the left channel was bang on.
Other points of interest are lower voltages when compared one channel to the other with the right channel lower..

Would dirty relay contacts on the speaker protection circuit cause this ,along with the static..
I'm tempted to jump the terminals temporarily ...but feeling very shy about doing this.

No scope ...but I have compass ..this problem I think will require a lot of leg work.
 
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Hi Chris,
Voltage measurements sometimes will miss open emitter resistors, or open emitter-base connections. Use your ohmmeter and your DVM on diode check to make sure the outputs, drivers and associated resistors are okay.

You said some transistors were changed in the right channel. Were all the output and driver transistors changed do you think? Normally you would replace all these if even one failed. Also, if the "new" transistors are out of production part numbers, you may even have some fake transistors in there. Never assume they are okay. These days I'd recommend the On-Semi parts for replacement use. They will work fine as long as you choose the proper substitutes.

-Chris
 
I'll give that a go..
Fakes yes ..rebuilt a Marantz 1070 that was blown ,had previous repairs ..output transistors ..one pair anyway the Marantz logo came of with parts cleaner. Ironicly the same ones that were blown

The output and driver transistors all appeared to be pulled at some point ,as they all exhibit the same buildup of coarse flux on the solder joints.

I don't know how long ago this repair was done.. My friend here got it from his father-inlaw it was sitting on the back shelf for who knows how long..Im thinking this is why if found its way into storage.

The 2sc2291 from what I gather was discontinued back in 1995 , which explains the price tag..and would also lead a person to believe that fakes are abound.
Found a few threads across the internet indicating that there are a lot of duds.

Found two more solder joints that appeared cracked .

It appears that most of the C2705 transistors were replaced but the complimentary transistors A1145 were not.

Thanks for the advice..I'll root around some more and see if my friend wants to put a few bucks into it for a few parts.
Its a nice amp.
 
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Hi MoreCoffee,
Well, I guess the A1145' s are on the list then.

See what you can use from current parts. This repair sounds like it will be super cheap for your friend. It's really worth doing since you are his friend. These problems are very likely why the amp was stored.

-Chris
 
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