Amplifier output got shorted on right channel, Left channel is now louder than right

[S1L03T]

Hey guys, new to the forum. I tried finding a home electronics forum and it brought me here, seems this is all to do with guitar amps though.. My problem resides with an AKAI AA-R21 20 year old amplifier that sounds amazingly good compared tho some things out these days. I'm an audiophile and i think that this amp sounds absolutely terrific.. Not so much now though. Lately i have been teaching myself to look for blown capacitors which i have successfully fixed an LCD monitor before. So i am confident i can repair this amp, i just need to know what to test, i have a multimeter. It has 4 IC chips, 1 for primary channel and another for secondary, should i try swapping the left channel IC's to the right side?? What do these IC's do? I ran the amp, and the right channel IC's get alot hotter than the left channel.. I have a schematic for this amp. Can someone guide me to repair this amp? Thanks

[S1L03T]

The problem is now it has a blown channel...

[Jazz P Bass]

Can you scan & post the schematic?
Typically, these types of power amps use a hybrid output IC.
STKxxxx something or other.
Try to find a replacement IC.

Edit: Uses a Discrete Output Section. Not an STK IC

[soundguruman]

The problem is now it has a blown channel...

the problem a lot of times is that people want to repair their own equipment but...
you need stuff like a temperature controlled soldering station, a variac / current meter, an o scope, a load bank, sine wave generator, your DMM, and the knowledge to use / safely use all of the above.
just taking the transistors / stuff off the board can easily damage it, if you have no experience repairing circuit boards, and have no proper tools, it takes some experience and knowledge, especially if you do not want to damage what you are trying to fix......
But if you would rather forgo and try to fix it, there are a few who have succeeded the first time, but not a whole bunch.
Likely that damage is done instead of fixing most of the time. I'm not saying that you can't do it, but a lot of times it just ends up that way.

I mean, compared to bringing it upon a knowledgeable tech, who has the tools to repair it correctly.

And if somebody wanted to get the tools and learn how to do it, go for it. The world seriously needs more people who know how, but it is not an overnight learning experience.

[Steve Conner]

On the bright side, it's not the end of the world if someone destroys a 20 year old Japanese transistor amp. One that was already dead at that. If everyone followed soundguruman's advice, nobody would ever learn anything. However, desoldering the blown module from the circuit board will be tricky if you're a newbie to soldering.

In this situation, I'd be looking for another of the same amp on Ebay, Craigslist, charity shops, dumpsters, etc. with a view to scavenging the needed parts, if not getting another working unit for 30 bucks.

I've heard that the STKxxxx modules are quite commonly faked. They've been out of production for so long, it is probably easier to find fakes than originals.

Maybe someone over at diyaudio.com or antiqueradios.com can help too.

[soundguruman]

" If everyone followed soundguruman's advice, nobody would ever learn anything" "desoldering the blown module from the circuit board will be tricky if you're a newbie to soldering."


That's what I'm saying, they have learned something.
They learned that it's not as easy as it looks.

As for the ones who are brave enough, buy your tools.
You have committed to fixing amplifiers, commit to the tools to do it.

If you wanted to fix them without the soldering and test equipment, in many cases that's just unrealistic.
In some cases, true you can...but that's not the majority of cases.

[J M Fahey]

Hi SILOET Post your schematic, at least to know what we are talking about.
Yet, as mentioned above, probably it has an STK module, very hard to impossible to find.
STK closed its operations in USA, its main market, so imagine the rest of the world.
Sorry.
And agree that even if you paid 10X its real value it would probably be fake, not to mention that your amp may still have other problems.

[Jazz P Bass]

AKAI AA-R21 Schematic

Well. color me wrong
The Akai uses a discrete output section.
Definitely repairable.

[Jazz P Bass]

Here is the complete Service Manual.
Nice manual!

[S1L03T]

Tried uploading schematic and it was a few kb over the limit lol.. Yeah it doesn't use a STK IC..

[S1L03T]

This is the parts that were fried and that i have removed. This happened when i accidentally shorted out on of the transistor pins with the multimeter probe on the heat sink :\
If i could get a generic part number for those resistors and the IC's or anything that can be replaced that is equivalent to those parts so i can go fetch them that would be great.. i think i can fix this, am i looking in the right spots from my first initial problem?

[Steve Conner]

I see. Well, for a start then, they're transistors, not ICs.

The blown components you identified there look like a pretty standard failure scenario. The output transistors (that is the large 3-pin devices attached to the heatsink) will probably also be shot. You may not be able to find exact replacements, but the 2SD716 could perhaps be replaced by the MJL3281, and the 2SB686 by the MJL1302.

The burnt resistors should be available from any decent electronic parts store. The small ones are probably "quarter watt" size, the large ones 5 watt cement blocks.

It's only fair to point out that several of the small transistors are probably blown too. Learning how to test transistors would be a wise investment about now. The small Japanese transistors have a different pinout to the American 2Nxxxx types.

Good luck!

[S1L03T]

Okay, so i've taken the blown parts to an electronics store, they said they've never seen them before but they'll try hunt them down for me. How do you test a transistor anyway? What small transistors am i looking for that might be blown?

[Brad1]

How do you test a transistor anyway?

Search the 'net for "testing transistors", "testing transistors with multimeter", or similar. And, when you find some things, search further for new things you've learned. (And, don't "just" watch only the YouTube videos. They may "show" you how to do it, but may not explain much).

I could tell you how to do a basic test that usually points to a suspicious device, but you won't understand why you are doing it.

If you dig down through, you'll learn more about transistors, and what to expect when you test them. To understand what your meter is telling you, it's good to know why one transistor acts differently when you measure from a certain pin to another, than it does on a transistor that may even be directly connected to it. You'll learn about NPN and PNP, base, emitter, collector, source, gate, drain, junction voltages, why a diode-check function on a meter works, what a diode is, and other good stuff to know if you want to work on things. You may learn why some will give different readings than others. You may learn why some may test fine in circuit, and some may have to have at least their base leg pulled out of circuit to test properly.

It'll be good for you to do a little homework. The wise guys here could likely answer any question you have, and could probably even take a person step-by-step through a repair (assuming the rookie didn't blow stuff up during the process), but the rookie won't learn much.

It's like if you were totally green to auto repair, and your uncle hired you for his shop. Your uncle says "see that part there...change it with this new part here".

You ask "what is it, and what does it do?"

Your uncle says, "You don't need to know...just change it."

All you got out of it is practice turning a wrench...without knowing what the part is, what made it go bad, why he knew it was bad, or anything else helpful.

A determined and resourceful fella might say 'Well, at LEAST tell me what the %$^*$! part IS, so I can look it UP!", if he was serious about really knowing.

One other thing. We'll assume you have looked up "Safety Procedures" for both yourself, and to protect the equipment? Things like 'don't hold onto the chassis with one hand while your poking around in a live circuit with the other', and stuff about how to properly handle static sensitive components...like transistors and IC's, etc? Stuff like potential heat-related damage from soldering, and how to minimize the risks?

If you REALLY want to learn how to do this stuff, it would be most helpful to yourself to take a new term...whether it's a part, or a test device or procedure, a unit of something (volt, ohm, amperage, wattage, etc.)...and start wading through the Internet to get different perspectives and explanations. The more you do that, the more you'll understand how each always affects all the others.

If you have done some homework, but something is still confusing...nobody here minds answering questions. Most of these guys are fairly good at prying out enough info from someone that they can usually determine what the REAL question is, and give a somewhat "Electronics for Dummies" answer (not calling YOU a dummy...I just mean that you'll get an understandable answer with enough technical terms to connect it all).

The reason I wrote all this is because there is no better way for someone who REALLY wants to learn something than to start digging around for themselves when a question pops up. When you thought about "how do I test a transistor", the very next thing you should have thought about was "Google", and "testing transistors".

Not trying to be mean and beat you up. Just trying to pass along some helpful advice. I'm back in some classes to fill in some holes to be more attractive to employers in Industrial Systems Controls (gotta get the hydraulics and pneumatics...stuff like that filled in to get that piece of paper). Anyway, some of those folks in there (mostly the younger ones) want everything spoon-fed to them, and seemingly can't be bothered to "think outside the box" to do a little research on their own. Granted, the books suck, are full of errors and don't explain some things well at all...and that's when I head to the computer to get a different perspective. The younger ones just bitch and moan because the books suck, but can't seem to extract the smallest amount of relevant info to take 10-15 minutes to search the relevant terms. That's probably why we've lost 5 young guys already...and the class started in January. Not comparing you to them, either. Just using this as a point (and there's already a thread lurking about here that kind of covers this topic).

Anyway, I'll throw in one more bit of advice before i go. Have your schematic printed out, and readable. Have a backup somewhere (a file, or a master copy). Why? So when you do what I'm about to suggest, you'll be able to start with a new copy when you inevitably screw up the one you are using.

You know which channel is not working, right? Grab a (preferably thin) highlighter pen. From the positive terminal of that channel's output, start tracing back through the circuit and highlight everything in path, at least some of the way back. Start looking at all the resistors to see if they look like they are getting hot, and even turning the board dark. You may want to measure some. if they are in the path in series (just one other device connected to each end) you can just measure across it. If there are components attached to either end, you may have to pull up one end off the board to measure, because something in a parallel path can skew the reading.

When you get to any diodes or transistors, try checking those. Again, depending on what's connected to them, you may, or may not, have to pull an end to a diode, or the base leg of a transistor. When you get to one, you'll be able to tell by looking at the schematic, to see what's connected.

Anyway, just start checking stuff, and familiarizing yourself with doing that. Of course, this is all done at this point WITH THE POWER OFF and the AMP UNPLUGGED.

Good luck. Have fun, and learn some stuff!

Brad1

[soundguruman]

Oh, man.