| jkramer |
I have a Sony TA F555 ES amp I'm trying to diagnose to either repair myself or determine whether it would be worth bringing to a repair shop.
There are four 2SC2922 NPN power transistors attached to the large heat sink on the back of the unit. All 4 are smoking, some have burned through the center of the chip.
Any ideas of why the chips are failing? I presume there is another problem that needs repair before these power transistors are replaced. |
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| dswiston |
| The amp was never shorted during operation was it? That would be my first thing to check. Look to see if the speakers they were driving have a shorted voice coil. Also if the output trannies are fried, you may have to check other components in the amp (like resistors) because other components have a tendency to be fried from whatever caused the output transistors to fail. |
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| EchoWars |
| quote: | | There are four 2SC2922 NPN power transistors attached to the large heat sink on the back of the unit. All 4 are smoking, some have burned through the center of the chip. |
There's a chunk of the problem right there...this isn't a quasi-complimentary design. There should be two 2SC2922's and two 2SA1216's in that amp. |
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| lubbie |
| quote: | | There's a chunk of the problem right there...this isn't a quasi-complimentary design. There should be two 2SC2922's and two 2SA1216's in that amp. |
I Disagree.. Quasi-complimentary uses only NPN devices. The quasi-complimentary are notorious for oscillation. True complimentary uses NPN/PNP matched pairs but not the quasi... Being only 4 NPN output devices tells me that there is only 2 output transistors per channel..
Do you have anymore info on how it ended up in this position. Did someone make adjustments inside?? Or is it something someone dropped in your lap with no history.. Sometimes if you can find out what went on before, will leed you to troubleshooting it effectively.. |
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| anatech |
Hi jkramer,
Step one would be to get a schematic. The outputs are blown, so are the drivers and most likely, more stuff. If another technician has had a go, you may be in for a tough repair. As in "struck by technician". I hate those.
-Chris |
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| wrenchone |
| The last Sony product I fixed ( a long time ago) had an open pot in the Vbe multiplier bias network for the output stage. This saturated both sides of the output stage, toasted the ouput transistors, and blew the fuse. It took me a while to figure out what was going on, even with a Sams schematic, but when I did, I replaced the cheap open-face carbon bias pots with good conductive plastic sealed trimmers. I lost a lot of respect for Sony quality after digging into the gizzards of that receiver. Just a small rearrangement of components in the Vbe multiplier would have made the amp fail-safe, but they didn't do that.... |
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| robert_chien |
| You may check the transformer also to see if the output voltage is ok without any loading. Power stage short circuit can cause transformer overheat, and thus windings get short circuit internally. If the transformer goes out of order, you need to get one to fit in the case before reparing the circuit. |
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| Big_Tires |
| its a sony amp they never last for more that a year usually , cause they either start on fire of just quit because a chip has fried |
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| anatech |
Hi Robert,
The transformers have thermal fuses in the primary winding. The fuse would open first.
I have no love for Sony, but their products are better than many others. Keep in mind the level (mass market) they are in. Pioneer actually has the crappiest output stage I've seen in a while. That's the block 'o parts with a cheap fan on the end.
-Chris |
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| jkramer |
Thanks for all the replies.
EchoWars you're right I gave the wrong info, there are a pair of 2SC2922's and a pair of 2SA1216's. All 4 are burned through the center.
I don't know much of the history on the amp. A friend gave it to me and told me it worked until his kids "played with it".
The main power fuse was blown, so I cut it out and installed a replaceable one, but when I powered it up the power transistors started smoking. It was clear they had been burning before the original fuse blew.
I'm going to attempt to find the SAMS schematic for this unit. Then per Robert's suggestion I'll check the transformers - Is there a way to isolate the transformers or at least remove the power transistors for the circuit so that I can begin testing the other components without burning up what's left of the amp?
Any other tips for a novice to check out this unit? |
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| anatech |
Hi jkramer,
The power transformer is likely okay except for the blown thermal fuse.
Both channels of the amplifier are blown. Fix them first. If you pull the outputs and power up you will take out many more parts. The drivers & bias transistors probably need replacing now, maybe more. Do not just power the unit up. A variac will be your best friend here.
If you do replace these parts and plan to just plug it in - throw the unit out now and save yourself the money on parts.
-Chris |
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| wrenchone |
| The fact that both sides of the output are toasted tends to indicate that something happened to the bias pot on the amp similar to the Sony that I repaired. I would strongly advise replacing the bias pots before powering on the unit again, otherwise you will most likely immediately blow any replacement drivers. Just cranking the bias pot around might restore the unit to operation, but I liken that to resetting the fuse on a time bomb. If you can get to a place where you can properly cut the +/- DC supplies to the amp, it would probably be a good idea to wire some automotive bulbs in series to soak up any shorts. This might save you some drivers. |
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| johnsciolino |
| Sony is famous for cold soder joints. I repaired a 555 due to cold solder joints. Variac is a must |
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| anatech |
Hi wrenchone,
Both sides blown would normally indicate they were connected together, or to another amplifier. It is always good practice to check the bias controls as I always do. They may be damaged when the output stage fails, resulting in a burned carbon element. When they are intermittent, either the slip ring is oxidized, or the tension has failed on the wiper. The former case requires replacement.
Given the difficulty in getting exact replacement parts, I only replace them if I have good reason. There is no reason to use a conductive plastic control for this, the carbon types handle surges better.
-Chris |
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| cunningham |
Another scenario is that the outputs were shorted, a voltage line spike caused a voltage spike on the secondary of power transformer exceeding breakdown of one output, or any other way either output could have shorted. If the filter caps are large, and hold a lot of charge, then when one output fails, the feedback of the amp circuit will cause the other to saturate and the current surge through the unshorted output will be huge and will melt the unshorted transistor long before a fuse breaks!:att'n: :hot: :dead: :mad: :smash: :bawling: and then you have no good outputs. :(
Anyway you should take the amp with all of the outputs removed. Place a resistor in series with a diode, 10to 100Ohms or so, from were the base and emitter of each output connection is removed, and power it up with no load. This should somewhat represent the forward PN juction and re (resistance of base to emiter) of the output transistor, with re 100X what it would actually be to limit current in case the outputs are not the only failed componant. Measure the output voltage relative to ground, should be 0V, and then check the voltages between the bases of the outputs. If something still smokes, then the drivers are probably melted too. Look for this base to base voltage to be no more than 1.5V or 2V. Maybe slightly more if the outputs are darlingtons. You should be able to adjust this voltage with the bias pot.
Then again I may be totally wrong, I don't have a scematic for this amp. |
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| anatech |
Hi Cunningham,
I've done that with STK driver IC's to check them before giving an estimate (two diodes + resistor). In this case, it is easy to check the drivers individually. I would recommend that route. Also, aways replace transistors one stage back from the last damaged part. This means, the drivers go for sure.
It's a good short cut for more experienced techs. At least outputs don't go poof!:hot:
-Chris |
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