NIKKO NR-850 tripping Circuit Breaker in rear?

jleon92f

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NIKKO NA-850 tripping Circuit Breaker in rear?

hi, I picked up a Nice looking Nikko NA-850 that trips the circuit breaker on the rear of the chassis?

Would the power supply be bad?

Thanks for any help.

John.:music:
 
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hi, I picked up a Nice looking Nikko NA-850 that trips the circuit breaker on the rear of the chassis?

Would the power supply be bad?

Maybe, maybe not. You've got a short someplace. Your job is to find it. I'd start by pulling the outputs and see if the breaker still blows. I'd also use a dim bulb limiter for the rest of your testing so you're not pulling big bursts of dead-short current through all those old components that are just waiting to fry. If the breaker holds with the outputs removed, you've found your problem. If any of the outputs test bad, make sure and test all of the drivers and emitter resistors too... damn shame to put in brand new output devices and have them blow becuase a 30-cent resistor is shorted.

If the outputs don't test bad, work backwards and test the diodes in the power supply. If these are good, test the power transformer itself. There's other stuff in between, but shorted outputs and diodes is the #1 reason main fuses blow.
 
Maybe, maybe not. You've got a short someplace. Your job is to find it. I'd start by pulling the outputs and see if the breaker still blows. I'd also use a dim bulb limiter for the rest of your testing so you're not pulling big bursts of dead-short current through all those old components that are just waiting to fry. If the breaker holds with the outputs removed, you've found your problem. If any of the outputs test bad, make sure and test all of the drivers and emitter resistors too... damn shame to put in brand new output devices and have them blow becuase a 30-cent resistor is shorted.

If the outputs don't test bad, work backwards and test the diodes in the power supply. If these are good, test the power transformer itself. There's other stuff in between, but shorted outputs and diodes is the #1 reason main fuses blow.

hi, outputs? the transistors? There are (4) Here are some more pics.

Thanks for your reply and help. I am trying to learn about electronics. I am good with mechanical repairs but, the electronics still need some time with..

Thanks,
John.:music:
 
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Curious about this capacitor.from the picture it looks like its sittng a little cockeyed. Like its buldging from the bottom side and the curious dark stain around the base looks oily not like glue. (vented cap ? possibly )Potential source for a short circuit as well.:scratch2:
 

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Curious about this capacitor.from the picture it looks like its sittng a little cockeyed. Like its buldging from the bottom side and the curious dark stain around the base looks oily not like glue. (vented cap ? possibly )Potential source for a short circuit as well.:scratch2:

Thanks, I did not notice that. They look easy enough to replace.
Pic of under side of PCB.

Thanks,
John.:music:
 
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By "outputs" I meant "output transistors" as circled in your first photo, the 4 TO-3 guys on the heatsink. By the looks of it they'll need to be de-soldered from the underside in order to remove, see what you find underneath that heatsink. Mark each one for location with a 1, 2, 3, and 4 so you put them back in the right place. To de-solder, get yourself some desolder braid. Apply the braid to the solder joint you want to remove, and apply the iron to the braid. The braid will wick up the hot solder. Make sure you've got some thermal heatsink compound (available anyplace that builds or services computers if you can't find it elsewhere) for when you re-install... it's the white goo you can see at the edges of the transistors.

Once you get them removed, try powering up the amp with a new fuse (and dim bulb limiter). If the bulb only glows dimly, you've found (part of) your problem, and can proceed as follows:

Download data sheets for the transistors to determine Base, Emitter and Collector pins. To test, use a meter with a diode check function... B-E should conduct in one direction only. E-C should conduct in one direction only. B-C should not conduct at all in either direction. Test all four resistors which are attached to the power transistor emitters. They'll be low value high power, like 0.33 ohm or 0.47 ohm 3W. Often there will be two resistors in one ceramic 3-pin rectangular package. Next you'll want to test the driver transistors which feed the output transistors. A schematic will likely be required to find them. They'll be smaller black TO-92's, maybe TO-220's.

Good luck. If you have questions about what you're doing, ASK QUESTIONS. The likelihood of your guessing the right answer is almost always close to zero, and if you take a guess on something and it's wrong, it generally causes more damage.
 
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By "outputs" I meant "output transistors" as circled in your first photo, the 4 TO-3 guys on the heatsink. By the looks of it they'll need to be de-soldered from the underside in order to remove, see what you find underneath that heatsink. Mark each one for location with a 1, 2, 3, and 4 so you put them back in the right place. To de-solder, get yourself some desolder braid. Apply the braid to the solder joint you want to remove, and apply the iron to the braid. The braid will wick up the hot solder. Make sure you've got some thermal heatsink compound (available anyplace that builds or services computers if you can't find it elsewhere) for when you re-install... it's the white goo you can see at the edges of the transistors.

Once you get them removed, try powering up the amp with a new fuse (and dim bulb limiter). If the bulb only glows dimly, you've found (part of) your problem, and can proceed as follows:

Download data sheets for the transistors to determine Base, Emitter and Collector pins. To test, use a meter with a diode check function... B-E should conduct in one direction only. E-C should conduct in one direction only. B-C should not conduct at all in either direction. Test all four resistors which are attached to the power transistor emitters. They'll be low value high power, like 0.33 ohm or 0.47 ohm 3W. Often there will be two resistors in one ceramic 3-pin rectangular package. Next you'll want to test the driver transistors which feed the output transistors. A schematic will likely be required to find them. They'll be smaller black TO-92's, maybe TO-220's.

Good luck. If you have questions about what you're doing, ASK QUESTIONS. The likelihood of your guessing the right answer is almost always close to zero, and if you take a guess on something and it's wrong, it generally causes more damage.

I removed the one output that the previous owner said was sparking. The breaker does not trip anymore. The picture shows a resistor that smoked as I increased the voltage, but breaker does not trip out any more.

Am I close to the problem?

Thanks,
John.:music:
 
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Test the transistor you removed. Is it bad? If any components "smoked" AFTER you removed that transistor, you've still got problems. I'd be pulling and checking the driver transistors, at the very least the one that drives the bad output transistor. What is the resistor that burnt up connected to?
 
Test the transistor you removed. Is it bad? If any components "smoked" AFTER you removed that transistor, you've still got problems. I'd be pulling and checking the driver transistors, at the very least the one that drives the bad output transistor. What is the resistor that burnt up connected to?

I will have to get a schematic for this amp to see where I am going. What and where are the "Drivers transisitors" are they big or small?

Thanks,
John.:music:
 
I will have to get a schematic for this amp to see where I am going. What and where are the "Drivers transisitors" are they big or small?

Schematic = very good.

Driver transistors are small-signal transistors that feed the signal to the output (or power) transistors. They're generally either TO-220 in larger amps or TO-92 in smaller ones. Google those terms, you'll find some pictures. They're very often heatsinked (or should be), relatively close to the output transistors. There's probably about 5 small transistors per channel, plus or minus. Two are used for the input differential pair, one is a bias transistor, and two more are the drivers. Depending on the topology of the amp there could be a couple more.
 
Test the transistor you removed. Is it bad? If any components "smoked" AFTER you removed that transistor, you've still got problems. I'd be pulling and checking the driver transistors, at the very least the one that drives the bad output transistor. What is the resistor that burnt up connected to?

Hi, Yes the transistor I pulled tested bad! I will pull the rest and test them.

Thanks again, feels good learning something new! at my age.

John.:music:
 
By "outputs" I meant "output transistors" as circled in your first photo, the 4 TO-3 guys on the heatsink. By the looks of it they'll need to be de-soldered from the underside in order to remove, see what you find underneath that heatsink. Mark each one for location with a 1, 2, 3, and 4 so you put them back in the right place. To de-solder, get yourself some desolder braid. Apply the braid to the solder joint you want to remove, and apply the iron to the braid. The braid will wick up the hot solder. Make sure you've got some thermal heatsink compound (available anyplace that builds or services computers if you can't find it elsewhere) for when you re-install... it's the white goo you can see at the edges of the transistors.

Once you get them removed, try powering up the amp with a new fuse (and dim bulb limiter). If the bulb only glows dimly, you've found (part of) your problem, and can proceed as follows:

Download data sheets for the transistors to determine Base, Emitter and Collector pins. To test, use a meter with a diode check function... B-E should conduct in one direction only. E-C should conduct in one direction only. B-C should not conduct at all in either direction. Test all four resistors which are attached to the power transistor emitters. They'll be low value high power, like 0.33 ohm or 0.47 ohm 3W. Often there will be two resistors in one ceramic 3-pin rectangular package. Next you'll want to test the driver transistors which feed the output transistors. A schematic will likely be required to find them. They'll be smaller black TO-92's, maybe TO-220's.

Good luck. If you have questions about what you're doing, ASK QUESTIONS. The likelihood of your guessing the right answer is almost always close to zero, and if you take a guess on something and it's wrong, it generally causes more damage.

I used the Vacuum pump to remove the solder, Thanks for explaining how to use the Braid method.

Thanks,
John.:music:
 
I can see this amp NEEDS a lot of repairs. Driver transistors and main transistors, some cap work, Etc. I paid $25.00. Is it worth the repair in your opinion? I liked the look of the meters and silver...

Thank you for ALL your help, I will secure a SM and take it from there.:thmbsp:

Some times you win some times you loose....:scratch2:

Oh well..

Thanks,
John.:music:
 
I can see this amp NEEDS a lot of repairs. Driver transistors and main transistors, some cap work, Etc. I paid $25.00. Is it worth the repair in your opinion? I liked the look of the meters and silver...

Thank you for ALL your help, I will secure a SM and take it from there.:thmbsp:

Some times you win some times you loose....:scratch2:

Hi John:

Of course value, like beauty, is in the eye of the beholder. It's a good solid amp, and properly repaired there's no reason why it shouldn't play for another 25 years. The parts aren't particularly expensive, and if you're enjoying yourself and have the time, go for it. The driver transistors may or may not be bad, it takes quite a whollop through the mains to fry them. Just make sure when you power up you do so with a lgiht bulb or variac so you can check whether what you've done has fixed the problem without introducing new ones.

I'd expect the output transistors to die in pairs, but that's not the case 100% of the time. Most times though the speaker leads get shorted and it affects both the PNP and NPN equally (badly).

Good luck

Dave
 
Hi John:

Of course value, like beauty, is in the eye of the beholder. It's a good solid amp, and properly repaired there's no reason why it shouldn't play for another 25 years. The parts aren't particularly expensive, and if you're enjoying yourself and have the time, go for it. The driver transistors may or may not be bad, it takes quite a whollop through the mains to fry them. Just make sure when you power up you do so with a lgiht bulb or variac so you can check whether what you've done has fixed the problem without introducing new ones.

I'd expect the output transistors to die in pairs, but that's not the case 100% of the time. Most times though the speaker leads get shorted and it affects both the PNP and NPN equally (badly).

Good luck

Dave

Thanks Dave,

I have a Variac which I use when testing unknown stuff.

I am having a problem finding replacement Transistors, di you have a source?

They are marked 2SB541 (2) and 2SD388 (2)

Thanks,
JOhn.:music:
 
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Hi, what is the difference between these resistors? and what do I order?

It says 220ohmsJ on it. No stripes?

I am ordering the parts now and don't see this one listed.

Thanks,
John.:music:
 
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I am having a problem finding replacement Transistors, di you have a source?

They are marked 2SB541 (2) and 2SD388 (2)

2SB541 use Onsemi MJ15004G
2SD388 use Onsemi MJ15003G
 
Hi, what is the difference between these resistors? and what do I order?

It says 220ohmsJ on it. No stripes?

I am ordering the parts now and don't see this one listed.

Thanks,
John.:music:

220 ohm is correct I'd get at least 2 watt silicone wirewounds
 
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