JVC RX-8010 Receiver: Are 75°C Power Transistor Temps Normal at Half Volume?

I am trying to figure out if this is normal or one of the components failed.
Some people say it's normal some say it's not, I would would really appreciate a second opinion.
Those two parts of the schematics are from a JVC RX-8010 receiver.
I was wondering if these 6 power transistors suppose to be so damn hot that I can't even touch them or its just normal.I have used a infrared thermometer and on the heat sink it reads somewhere around 75 C degrees when the volume is just 1/2 way up.
Comparing these power transistor with the output transistors, the output ones are just barely reading 25 C degrees.

thanks!!!

Need the entire circuit could be any number of reasons

Mark www.harrington.force9.co.uk

Also need things like the voltage applied to the collectors, the load on the emitters.

BUT, your title for this post is deceptive. It asks "why" they are getting so hot, yet the text of your post is more about whether it's "normal" for them to be that hot. I will take that to mean whether it is injurious to the transistors for them to be that hot.

According to the datasheet for a 2SD2395 the absolute max junction temp is 150ºC so if your reading is correct, then you're probably OK, but I couldn't find a thermal resistance from junction to case on the datasheet, so can't be sure.

Wouldn't hurt to throw a fan on the thing, or install a bigger [better] heatsink (or both). Also, is this something that you constructed yourself? If so, did you use a heatsink and if so, did you use some sort of heatsink compound?

I think the others have given you enough to work on, but it is maybe normal to run at the 70 degrees. I noticed some little triangular warnings next to each of the power transistors. Maybe there my be a hot warning in the text somewhere. Just wondering.

That why I say we need an entire schemtic Steve You cant really tell anything from this at all except there a power transistors used in the circuit thats all you could verify and that they seem to be part of a power supply

Two of which is suppplying 5v6 the other 6v2

Other than that what they are supplying or where those supplies lead too or what is feeding them and at what voltage and controlling what part of the circuit we dont know

The other circuit seems to be plus and minus 12 volt rails I assume for opamps and also dovetail pair on amplifier if it is an amplifier and plus 9volt supply I would have thought for tuner being as he is suggesting an RX-8010 tuner amp

Hence can we have the whole schematic

Too, true.

I noticed that the power transistors have a small wattage resistors in the collector circuits. If the resistors haven't burned up then I would say everything appears normal

Took another, closer, look -- these look like crude series pass voltage regulators.

If the voltage at the collector is fairly high (like 3 volts or more), then these transistors will have a lot of voltage across their Emitter-Collector junctions (_a lot_ is more than 3 volts or so). Judging by the fact that the second diagram has a couple of regulators that appear to supply 12V, and assuming that all of these little regulators are being fed by the same super supply (except the -12V source), it won't take much current for them to get hot (at least for the 5V regulators) .

On ones with the MTZ5.6JC zeners, if I assume that "5.6" means 5.6 Zener Voltage (couldn't find a datasheet), and because these transistors have an hFE minimum of 100 [at Ic = 500ma and Vce 3V], and if I assume these transistors are being driven by no more than 800ma (and judging by hte "USB5V" lable, this will, at least, go to 500ma) (max collector Pd = 2W which, at Ic =800ma is 2.5V and I'm pretty sure the Vce voltage will go higher than that), then the max base current (Ib) will be 8ma. So, lets say the zener has around 10ma flowing through it at quiescence: that's 10ma * 2.7K = 27Volts.

That seems a bit high, so, lets look at the voltage across the parallel resistors: R941 & R942. Each is labeled as 33Ω and they are in parallel for a total resistance of 16.5Ω. At 800ma, that would be: 16.5Ω * 800ma = 13.2V. Assuming all of these little regulators (at least the ones supplying positive voltage) are supplied by the same super supply. Then the super supply would have to be higher than the highest zener voltage, which is 13V (if my assumption about the zener part number is correct), and the fact that the 13V zeners both have 2K resistors in series with them, I would say that voltage _is_ a bit higher, then, I think it is safe to say, that these transistors have a fairly high series pass voltage. That is tempered by the series resistors on the collector side, but unless the currents in the circuits they supply are quite stable, then this is going to very a lot (esp on the one with the "USB5V" designation.)

Thanks for your replies and critics guys!!

I'm still a newbie in electronics , 3/4 way into electronics technician course( so please bear with me + english is my 2nd language)

So, lets start with this, had it cranked up around 70(max volume is 90) at my b-day party and it went 2 times in overload and then stop working.

Opened it and found the fuse blew;changed it, turned it on and blew again.

Looked a bit further and found a Darlington pair transistors E to C shorted on Front right board channel, pulled the board out. turned it on( of course after changing the fuse),then after a few mins of playing the Front left board channel Darlington pair transistors E to C shorted .

Got 2 new pairs of transistors(2SD2560 & 2SB1647) , set the idling current as the service manual said and now it works fine , had it playing for the past few days with no problem.

What I am trying to find out is if its normal for the other 6 transistors 2SD2395 (Q921-Q971) suppose to get so hot.

When I have the volume @ 55, the temp on their heat sinks is about 75C and higher)

P.s.: I have attached service manual for this receiver and a couple of pictures with transistors that I am talking about.

Thanks!!

Are your speakers 8Ω?

Also, it is definitely reasonable for those transistors to get so hot when you turn the volume up so high. Especially if the sound is "fat" and has a lot of base (the more sound, the more current demand -- assuming a Class B or AB output driver.

yes,they are 8 ohm.