JVC R-S77 running hot, DC offset -27.0 mV both sides.......

I bought this receiver a couple of months ago and have just started sorting it out. I purchased it, along with a pair of Norman Laboratories Model 10a and an Infinity subwoofer, intending to put together a nice second stereo system, excluding the sub. I also have a nice Barzilay cabinet and a Denon turntable to complete the system; all the wood matches .

I have cleaned out the inside of the receiver and have done a visual inspection - so far, so good. I tested the DC offset and both sides tested an equal -27.0 mV. I have read throught the DC offset sticky, although it has been a little while. From what I recall, offset should optimally read 0<X<+10mV. My JVC reads a little high, but not terribly so, in magnitude, but the polarity is opposite. Should I be concerned?

I installed new surrounds on the Normans and wanted to hear them, so I hooked them up to the JVC - man, do they sound sweet together! However, I did notice that the JVC is running hot. I can keep my hand on the receiver cover, but it is almost uncomfortable. Is this typical?

Finally, I have noticed a certain smell that I associate with some older receivers that I have come across. It reminds me of the smell of a bad ballast in an overhead fluorescent fixture. I was wondering if that smell is characteristic of aged components in a receiver, such as capacitor electrolytics, indicating that they are past their service life?

I could use some advice on further use of this receiver as is - it sounds great! Or, should I not play it at all? Any experts out there on this particular unit? The front of the receiver says, "Super A". Does that indicate a Class A amplifier circuit?

offset should optimally read 0<X<+10mV. My JVC reads a little high, but not terribly so, in magnitude, but the polarity is opposite. Should I be concerned?

Click to expand...

Polarity not and issue. Not sure about the Super-A....If bias is to high it will run hotter.If Class A it will run hotter..It may be a warm running amplifier by design..
http://www.audiokarma.org/forums/showpost.php?p=596155&postcount=13

Thanks for the reply, avionic. I don't have the schematics or manual for this unit. I see two adjustment pots, but I don't know their purpose; if this is a class A amplifier, then I assume from the information you linked that the bias is variable, is automatically adjusted as needed by the circuit's design and will run hot. If it is not a class A, then I may or may not have located the bias pots, but I don't know where to locate the test points.

I am bumping this to see if anyone else has input that may have missed my original posting.


Not that I crave attention or anything........

I would use some extra caution if the amp is suspect...don't use good speakers with it until your DC issue is resolved...

As I recall, the Super-A design is intended to jack up the bias on small-signal operation, and jack it back down for large-signal operation.

The first thing I would do if I were you is to acquire service literature (service manual) -- and then attempt to do the power amp setup adjustments.

Pay particular attention to whether BOTH channels are acting the same way or whether the over-heating is coming from one channel. You can determine this by measuring the voltage across the emitter resistors on the left and right channels, and comparing them.

In the bias circuit (near the drivers' bases) there's a little super-A controller IC. I don't know if that little single-inline IC is available any more from JVC parts distributors, but it has been known to fail.

Hope this is useful to you.

Fred

So I'm off to source service manuals. Thanks, Fred.

I have a very nice Fluke infrared temperature probe and will get some readings at the heatsink and at the transistors. I will also check the voltage across the emitter resistors, once I get them identified. More to come.....

I just purchased the service manual and schematics. The manual outlines setting the idle current very nicely. Would that also be known as the bias? If I remember correctly, bias is the term for tubes, idle is the term for transistors, more or less.

I just joined this forum and found this thread.

I'm the original owner of a R S-77 and still have it.

I can answer the heat question which is yes it runs hot.

There were times at parties you could not put your hand on the metal case, it was so hot.

It still performs great, though the LED's for the signal strength, locked & stereo are out and I want to fix this and get them working again.

I just ordered the service manual and am waiting to down load it.

How's everything functioning on yours?

Glenn