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Hot transistors in Yamaha C-2A

Discussion in 'Yamaha' started by mtjade2010, Feb 1, 2010.

  1. mtjade2010

    mtjade2010 New Member

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    Recently my almost 30 year old Yamaha C-2A preamp started giving sharp punchy pops. When it does that it gives consecutive 3 - 5 pops then stops. It may do it again a few minutes later or it may not do it at all in a week. But it always comes to annoy everyone in the middle of a piece of wonderful music it plays. The pop sounded quite loud and I am afraid it may cause damage to the power amp and my Magneplanar speakers eventually. It may eventually cause death of the C-2A itself.

    I opened up the bottom cover and flipped the preamp over and played it upside down unclothed for a minute. I quickly noticed that there are 10 transistors that are very hot. 4 of them are actually too hot to touch without getting burned if I leave my fingers on them longer than a few seconds. The transistors are TO-5 type transistors. They do not have any heatsinks on them.

    I have a C-2A schematic. I discovered that these transistors are inside the equalizer and the tone control amps. There are 3 of them on each side of the equalizer and 2 on each side of the tone control amp. It has two hot transistors on the collector circuit of the differential input FET of the equalizer. There is one more somewhere in the middle of the equalizer. That's total of 6 for both sides.

    On the tone control amp there is a NPN/PNP complementary pair very hot output transistors. These transistors look like those in the output stage of power amps except that they are lower power and smaller in the tone control amp. So there are 4 total for both sides of the tone control amp.

    The DC power for both the equalizer and tone control amps is +/-40 volts DC. This is much higher than conventional +/- 18V or +/-24V something like that. I think this is one reason why those transistors are so hot. Well, apparently the C-2A has lived so hot for 30 years without problems. But once I discovered that they are so hot I am reluctant a bit to play it long hours any more, especially when it started to do those unexpected pops I almost stopped using it.

    I know that most commercial semiconductors and transistors should be operated under 70 degree C. I believe those transistors are probably near that limit as I really can not keep my fingers on them longer than a second without getting burned. I am considering to add heatsink to them but the space in there is very tight. The addition of heat sinks will increase the chance of shorting the collectors to ground when the case is opened for any reason.

    My question is if I should replace them now with new compatible ones and with heatsink. I am afraid that if one of a pair goes it will shut down the equalizer or the tone control amp beyond repair. Any of those transistors with a sudden shorts or open can cause destruction to the amp as the entire circuit is DC coupled or biased. The amp will be probably beyond repair after that. By replacing those hot transistor it may solve the current random popping issue now.

    Does anyone have similar experience to a Yamaha C-2A? Is it a good idea to replace those hot transistors now? I wonder why Yamaha did not put heatsink on them in the first place. They are so hot and it sure isn't a good idea to design it that way. Oh, I checked the DC supply voltage. It is right on at +/- 40V. The preamp sounds absolutely beautiful except when it is doing the pops.

    MJ2010
     

     

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  2. Martin.Ca

    Martin.Ca Super Member

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    I'd be very concerned about that. How do the supply capacitors for the RIAA circuit looks? Those transistors shouldn't be anywhere near that hot (is my understanding).
     
  3. mtjade2010

    mtjade2010 New Member

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    I don't know about the condition of the two big supply capacitors. But I hooked up a oscilloscope to check the DC voltages there isn't any AC ripples. The DC supply is heavily regulated out of unregulated +/- 60V. The regulator transistors have heat sink (bottom chassis) on them.

    I did notice one thing last night. When it was acting up I gave the C-2A some shocking knocks (using a pen to tap on the chassis repeatedly) the preamp did more pops. It behaved like having some contact problems. But if it is behaving normally it is all good no matter how hard I knock the preamp.
     
  4. junkaudio

    junkaudio Super Member

    the c2a runs hot had 2 so far
     
  5. avionic

    avionic " Black Knights " Subscriber

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    TP8 is + 40vdc....TP9 is -40vdc
    Exactly what its suppose to be..:yes:Yes, the transistors mentioned will run very warm..:yes:
     
    Last edited: Feb 1, 2010
  6. avionic

    avionic " Black Knights " Subscriber

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    The new ones will run just as hot..No room for heatsinks..
     

     

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  7. avionic

    avionic " Black Knights " Subscriber

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    Technically not so.. Its cap coupled to the output with a couple 4.7uf/100v Bi-polar caps.
     
    Last edited: Feb 1, 2010
  8. mtjade2010

    mtjade2010 New Member

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    There are 4 TO-5 transistors that run really hot. After 10 minutes of playing they are so hot that it will burn your skin if you dare to touch them. A few resistors nearby those hot transistors are apparently protected by teflon tubes.

    The amp modules (equalizer and tone control) are capacitor coupled only at the input and output. Inside they are DC amp modules. Any of the transistors developing a short or open circuit can cause other transistors burnt out. The entire circuit looks exactly like a power amp. Differential input, cascode, current mirror, complementary driver and output transistors, etc. With +/- 40v supply (yes TP8 and TP9) I believe a short of a transistor can cause many burnt ones.

    I played the C-2A this afternoon for almost 2 hours and there was not a single pop I could detect. I have this hot baby on hand that I don't know where to start.
     
  9. ryuuoh

    ryuuoh FFXIV Summoner Subscriber

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    How do the electrolytic caps look? If they are original, the unit could probably benefit from new ones.
    Not saying that's your heat problem though.
     
  10. junkaudio

    junkaudio Super Member

  11. mtjade2010

    mtjade2010 New Member

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    The Japanese site shows some replaced transistors (2SC1913 and 2SA913). Based on the pictures the 4 hot transistors on the tone control amp are replaced. This confirms that these transistors are way too hot. They are likely to die and need to be replaced. I am considering to install a very small fan inside to cool them as an alternative.
     

     

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  12. mtjade2010

    mtjade2010 New Member

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    The electrolytic caps inside the preamp all look like new. There is really no way to tell if any of them have aged to a point needs to be replaced. I checked TP8 and TP9 with a scope and there is no AC ripples. So I can assume that the power supply regulator is working just fine.
     
  13. mtjade2010

    mtjade2010 New Member

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    I need to have 2 fans to cool the equalizer and the tone control amp. I think this will make a big difference if I want my C-2a to live longer. I have got some idea how to mount then and power them. I will need to find appropriate DC fans. It will take some time. But I would like to stop the popping issue still. Just listened to it for an hour and there was no popping sound still.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    Trivial tidbit..2SC1913 and 2SA913 both have a junction temperature( Tj spec) of 150°C.. thats 302°F..for what its worth..And internal fan would be useless IMO. The transistors are fastened to the extruded aluminum control amp enclosure.And external fan blowing across the exterior of the enclosure would accomplish much more cooling.
     
    Last edited: Feb 2, 2010
  15. mtjade2010

    mtjade2010 New Member

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    I know those transistor's Tj is 150°C. But I am talking about the ones not fastened to the chassis. I am talking about the ones that have no heat sinks on them whatsoever. They are visible from the component side. They are much hotter than the TO-3 power transistors on my power amp. That doesn't make a lot of sense for a preamp to running hotter than power amp.

    The fans will be blowing at the hot transistors directly on top of them. I am talking about super thin cooling fans for laptop computers. They are of the right size, thin and quiet, requires little power to run. There is room inside C-2A. Believe me they will cool the hot transistors. Cooling fans blowing from outside of the chassis won't do much help to the super hot transistors inside.
     
  16. mtjade2010

    mtjade2010 New Member

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    C-2A's equalizer has 4 such output transistors. But they are mounted on the chassis for cooling. They don't get hot at all. The tone control amp also has these 4 transistors at the output stage. But they are not mounted on the chassis. They are standing right on the PCB without cooling. This doesn't make sense.

    If you look at the component side of the tone control amp you will see why. There is no room. The Yamaha designer had no more room to mount those 4 output transistors on the chassis.

    Same thing for the 6 hot transistors at the differential input stage of the equalizer. I think they are slightly cooler but still hotter than anything I have ever seen. In my opinion they should all be cooled by some sort of heat sink.
     

     

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  17. mtjade2010

    mtjade2010 New Member

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    Yamaha C-2A looks like 4 little power amp inside. Two of them with feedback loop to make them RIAA equalizer. The other two with bass and treble control to make them do tone control. Some how the output transistors for the RIAA equalizer are mounted on the chassis for cooling. But the output transistors for the tone control amp are not. They are super hot in normal operating condition. The input stage of the equalizer also have super hot transistors that have no heat sink. Believe me they are not just warm or hot. They are extremely hot. They get hot in 1o seconds, super hot in 1 minute. Had I known they were designed to run so hot I would not have bought one 30 years ago. Those transistors are almost hotter than tubes on a tube amp.
     
    Last edited: Feb 2, 2010
  18. avionic

    avionic " Black Knights " Subscriber

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    So you've been using it for 30 years..How many times have you replaced these particular transistors?
     
  19. mtjade2010

    mtjade2010 New Member

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    Actually I have not used it for 30 years. My buddy bought it (5 years used) about 20 years ago from a private owner. I went with my friend to the previous owner's place and bought it there. My friend had problems with it from time to time and eventually lost his confidence with this C-2A. He unloaded it to me a few years back. I have been reluctant to use it and it has been put aside for many years. During those years I put it in use from time to time but every time I pulled it aside again because of the same issue. It's a 30 year old preamp, that's what I am trying to say. I don't actually own it for 30 years. Now that I am at it I see these supper hot transistors. I wonder what the wisdom was to make the output transistors work so hot, while the same output transistors for the equalizer are mounted on the chassis to cool. What is the benefit to run them supper hot? Why is the difference? The circuit of the equalizer and tone control amps are almost identical (identical on C-2 by the way), except in the feedback circuit.

    I suspect that the problem of the intermittent popping noise issue is caused by transistors that are dying. I suspect that the hot ones are the ones questionable. Why do people recap their amps? Same thinking. They may be dying. I am surprised to see the Japanese site showing the picture of the same transistors replaced.
     
    Last edited: Feb 2, 2010
  20. mtjade2010

    mtjade2010 New Member

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    I just did an experiment. I put a cooling pad for laptop computers over the hottest 4 transistors with one of the 2 fans directly over those transistors. I turned on the fans after 20 minutes of listening to the C-2A. I wait for about 2 minutes then check the temperature of those transistors by hand again. They are warm, not hot!!!

    The cooling pad costs less than $5. They are used under laptop computers and blow air upward to cool laptop computers. I only need the fans. They have two fans in it and uses power form a USB port of a computer. The fans are quiet and use very little power. I think I am going to make this "cool" mod to my C-2A.
     

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