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.
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:
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.
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?
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?
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 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
I am having a problem finding replacement Transistors, di you have a source?
They are marked 2SB541 (2) and 2SD388 (2)
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.
2SB541 use Onsemi MJ15004G
2SD388 use Onsemi MJ15003G