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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Wed 30, 2018 3:52 pm 
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Joined: Apr Wed 03, 2013 2:38 pm
Posts: 54
I wish I had more knowledge in how they designed the protection circuit to work but with my limited knowhow and the inputs I've had I'm surmising that it monitors the currant coming in and also monitors the speaker side too.

it also looks like if there is a disruption in one channel both channels will shut down.

The channel I will be replacing transistors in does work but only until the volume level gets to a certain point then both channels are stopped

When I took the bias measurements they were erratic and I could not fine tune either one. I took each control off and bench tested with no problems, very smooth movement with no flat spots

I won't be able to test now until I receive trans on order but it seems that being those trans are regulators it would have some impact on the situation

I'm hoping this will solve the problem but I feel I'm in for more education.


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Wed 30, 2018 6:05 pm 
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Joined: Aug Fri 12, 2016 1:49 am
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Location: Houston, TX
For those who don't want to create a hifiengine account, the service manual is also available for free here.

A complete description of the protection circuit is in section 6.8 (pages 20-21).


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Wed 30, 2018 8:00 pm 
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So far, I've learned that it senses 2 things:
transients on the output---mainly, I assume, the turn-on transient.
imbalance in either channel---measuring the current in the output transistors.

Before trying to replace parts, I would want to see the actual circuit parameters that will make it shut off---and then monitor that parameter as the output level is increased.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Wed 30, 2018 9:15 pm 
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Location: Houston, TX
stratozen wrote:
I won't be able to test now until I receive trans on order but it seems that being those trans are regulators it would have some impact on the situation

Keep in mind that something caused those xistors to go belly up. Semiconductors don't generally fail in a properly designed circuit, as long as they're kept within design parameters (voltage & current). They don't wear or age like other components. You'll usually find passive components that failed and took the semi(s) with them in an earlier stage, often times resulting in a cascade failure of semi's in subsequent stage(s). Some people may call this a catastrophic failure, but I reserve that term for when the final outputs are destroyed in this fashion.

I recommend using this time while waiting on the ordered parts to check the associated passive components in the xistor circuits, since you don't want the brand new parts damaged. It isn't always just a matter of bad lytics being the sole cause.

You may find yourself tracing the cause all the way back to the power supply, as has been mentioned, so I would also verify the supply is providing the spec'd voltages and doing so cleanly.

Tunnel vision (e.g., fixating on the protection circuit alone) can waste as much time as easter egg hunting.


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Wed 30, 2018 10:41 pm 
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Location: Albuquerque, NM 87123
I seriously doubt that all those parts have gone bad. Something like a lightning strike nearby might cause multiple failures in multiple circuits, but that is quite rare. The poster has stated that the receiver is working, but cuts out at higher volume--this would not be possible if all those transistors were really bad.

As I stated before, it sounds like the bias diodes are bad; also, replacing all those parts in a shotgun attempt at repairs is NOT recommended.


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Wed 30, 2018 10:48 pm 
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Location: Fayette County, Pa
Different model, but I recently had a stereo amp through here that acted very similar. It sounded fine till the volume was about half way up then slight distortion could be heard just before the shutdown circuit was activated.

What I found was a poor ground connection on a PC Board. What was happening was at half volume the amp started generating a high frequency oscillation that was exceeding the output capacity yet it could not be heard. The scope showed almost 25 volts going out to the speakers and the amp was shutting down to protect itself.

Put a scope on the speaker output under a proper load and verify nothing unexpected is present. Component wise, nothing was bad in mine, only the poor ground which when repaired fixed the problem.


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 1:02 am 
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We can now give the bias diodes a passing grade as they check good !!!!


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 1:21 am 
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i recommend that you stop testing parts and focus on the functionality. Let's assume that something happens that triggers the protection circuit---there are 2 basic things that it responds to. As you increase the volume, one of those hits a critical value. This kind of thing is tracked down by observing the behavior and making measurements.

Start by watching the parameters that go into the protection circuit (for each channel), and narrow down which one triggers it.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 1:25 am 
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Location: Houston, TX
The OP has already stated he tested the xistors individually and found them to be bad.

I'm not sure whether people in this thread are doubting the OP's ability to determine when a xistor is "bad", or they're just nitpicking the definition of "bad".

I took it at face value that he knew how to test them and that they are indeed "bad" (leaky, according to the OP), since I have no reason to doubt him.

Assuming they are "bad", we then have to consider failure modes. He gets output from both channels, so we know the semi's are neither open nor dead shorted. Could there perhaps be an internal mechanical (weld) fault which manifests after a given amount of current/time is applied? Maybe a thermal intermittent? He can check for mechanical by tapping on the suspect components and thermal via freeze spray.

He has just ruled out the stabistors, so we're again left with 1)assuming he doesn't know how to test xistors or 2)he does and something caused them to fail.


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 1:53 am 
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Vin;
I am not sure what or who you are responding to. I for one am not questioning anyone's ability to test parts.

My point has to do with the order of events in troubleshooting. In my book, it's best to start with symptoms and functional testing. Once an issue is isolated to a specific circuit, then make measurements to ID suspect parts, and then test the parts.
My 2 cents only---everyone has their own methods

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 2:14 am 
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What we DON'T know is what positions those transistors came from. That would seem to be important in this discussion.

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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 2:50 am 
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Location: The Old Dominion VA 23518
pixellany wrote:
Vin;
I am not sure what or who you are responding to. I for one am not questioning anyone's ability to test parts.

My point has to do with the order of events in troubleshooting. In my book, it's best to start with symptoms and functional testing. Once an issue is isolated to a specific circuit, then make measurements to ID suspect parts, and then test the parts.
My 2 cents only---everyone has their own methods


Agree. The manual has idle values for base voltages - monitor these DC voltages (one at a time) while increasing the volume with the amp dumping into a dummy load. Once you see the DC value change greatly and protection kick in, you will likely have found your stage, +/- one stage.

In my 30+ years of working amps (both audio and servo - hint: they act alike!), a DC offset will trigger the protection circuit quicker than any other failure mode. Very often, the culprit in a working-at-low-volume amp is a bias diode or bias resistor - and every time(for me), it's value is less than 2.2K ohms.

I just serviced a Technics SA-800 with a very similar issue - the protection circuit would kick in at very low volumes, intermittently. A pair of bias diodes from another unit, and it improved some, but voltage testing told me the driver stage right after the bias diodes was still off - emitter voltages were too high. A 47 ohm resistor, measuring at 113 ohms out of circuit was the culprit. After replacing the noisy dual transistor preamps and a dreary recap that used 87 capacitors, it was returned to my buddy.

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Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: May Thu 31, 2018 3:11 am 
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Posts: 17931
Location: Albuquerque, NM 87123
stratozen wrote:
We can now give the bias diodes a passing grade as they check good !!!!


Exactly how are you testing these diodes?


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 Post subject: Re: Pioneer sx-1050 HeadAche
PostPosted: Jun Sun 24, 2018 4:30 pm 
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Joined: Apr Thu 11, 2013 11:03 pm
Posts: 464
Location: 649 Lacy Wilkerson Rd Roxboro, NC 27574
So what was the outcome? Is it fixed?

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