My father gave me his yamaha A-550. Looks like new. The sound is terrible at about 1.5 on the dial. on both side. looks like grinding noise when there is some bass or you know the noise when speakers goes bad (but they are not, i test them with other equipments) The volume is not so high too. I'm ok with a soldering iron as i repair a lot of TV in past years but i don't know a lot about audio electronics. There is 4 transistors underneath, 2 biggers than the 2 others. Looks like the 2 biggers has been change as the soldering is not the same as everywhere. The 2 smaller one look like TO220 but with some kind of wings. Can these transistors be in half fault ans not completely short as in TV? On both side? Thanks!
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Pictures of said transistors would help us assist.
Pictures of said transistors would help us assist.
northpaw
Super Member
It is a ~1980 unit. Gritty noises can often be dirty contacts, in switches, relays, and/or potentiometers. I'd check and clean/exercise those first, particularly if the unit has not been used much recently. For the switches and pots, gently moving them, ever so slightly, while listening to music can help identify the culprits. If the noise is sensitive to those movements, a dirty contact is likely.
northpaw
Super Member
There is an owner's manual and a service manual available on line, for free, if you are so inclined. You need to register on that site, but it is free and and safe; they don't do anything bad with your info.
https://www.hifiengine.com/manual_library/yamaha/a-550.shtml
https://www.hifiengine.com/manual_library/yamaha/a-550.shtml
About the output transistors..
One pair 2SD716/B686 are TO-3P
the other pair are "batwinged" TO-220's - 2SD586A/B616A
Both are OEM pairs strange as it seems. It would come new with one set or the other. Heatsinks are probably factory drilled and tapped for both mounting configurations.
One pair 2SD716/B686 are TO-3P
the other pair are "batwinged" TO-220's - 2SD586A/B616A
Both are OEM pairs strange as it seems. It would come new with one set or the other. Heatsinks are probably factory drilled and tapped for both mounting configurations.
Do you think they were initially all 4 big or small one and a repair shop change only 2 with different one? You ar ok, there is holes that can be use for mounting bigger one.
Yes. Likely it has been worked on at some time.As to which pair were the originals...Which pair has shinier solder connections. They would be the newest pair.
I look closely and there are marks of ''wings'' left by OEM transistor where the 2 big one ar now, so they replaced with different one on left and right channel!!!
Usually no...They are either good or bad. Page 7 of the service manual has the idle current adjustment (bias) and the Center Voltage adjustment (DC offset).. Might want to at least check them to see how close they are to spec.
Those NEC (batwing) transistors are the wrong ones for this unit. In the photo at the top (black) one you can see the original screw hole for the TO-3P transistor. Someone drilled new holes and probably just had those "on hand" instead of getting the right ones. The "bigger" TO-3P ones are the originals. Although the other "batwing" transistors should still function without distortion up to a point, almost full volume since they are only (about) 70w compared to the (about 80w) originals. There is something else wrong. Look for cracked solder joints as these are known to develop them over the years. Those transistors were replaced for a reason. Probably blown. That leads to the question "were the other associated components changed too?" The driver transistors, resistors, diodes, etc in that circuit may, or may not have been tested or replaced as needed. Or one was simply "missed" like a bad resistor that may not "look" bad. If you still hear the right channel with the balance full-left, there could be a problem in the preamp/tone board section, or even a solder bridge, possibly on the volume pot.. Look things over closely and look for a bridged joint. When someone else was into something before, it is hard to tell what they may, or may not have done so you have to look at *everything*. Look for what appears to be "fresh" solder joints (more shiny than others) for a clue to what someone else may have touched.
Look at the A-550 schematic legend and the parts list in the service manual . Both the" batwing" NEC and the TO-3P transistors are referenced.
clinic-audio
all on YAMAHA untill 1990
Those NEC (batwing) transistors are the wrong ones for this unit. sorry but NO ! I refixed more than 100 pieces and all were with these output transistors !
the heatsinks are pre-drilled to help you to use both type of transistors , there is nothing wrong here and as Avionic said the service manual give also this information
If you need original output transistors send me a PM
Regarding your trouble , I got it many times and it is coming from one output transistor (blue one if I good remember) only
the heatsinks are pre-drilled to help you to use both type of transistors , there is nothing wrong here and as Avionic said the service manual give also this information
If you need original output transistors send me a PM
Regarding your trouble , I got it many times and it is coming from one output transistor (blue one if I good remember) only
Well I'll be jiggered!
Yep, staring me right in the lookers. I never came across them in a Yamaha before. Just in Pioneer and Kenwood. Carry on... 
I just begin the repair and i read all your comments. You are right, the blue one is bad but not short. It test like a diode and not a transistor. I also have 2 open resistors and 2 small transistors short.