Hi, this is my first post. I have an amateur's knowledge of electronics and have been modestly successful in repairing old audio gear using a multimeter and a soldering iron.

I'm working on a Yamaha R-90 receiver that stopped outputting sound after the cables attaching an iPod to it were pulled from the AUX jacks while the unit was powered up. The owner heard a loud thump when the cables were pulled. I have determined that the speaker relay does not close when the unit is powered on. If the relay is manually closed, the unit outputs sound through headphones. I did not check it with speakers hooked up, but I assume it will output sound there as well.

A close visual inspection revealed a zener with blackened leads. No other components have blackened leads. I pulled it to test it for resistance in both directions with the aim of replacing it if I got a bad reading. So I tested it using a digital meter that has scales of 200, 2K, 20K, 200K and 2M. The 2K scale has a diode symbol below the 2K designation. Using that scale I get a reading of 715 ohms in one direction and open in the other. This seems like a normal reading, but I am not sure. I then noticed using a magnifier that it had no markings whatsoever. The service manual/schematics seem to be not readily available.

The zener may be good and it may be bad. It could also be that some other component is bad, sending too much current through the zener.

Does anyone have any thoughts on what stress is placed on the circuitry of an audio amp when the cables of an input device like an iPod are yanked while the power is on? And where might I find the schematics for this unit?

Thanks for reading,
Jack

 

There is a protection circuitry that closes the speaker feed relay if after a few seconds from turning on, the output stage is found healthy. Seems yours is not healthy.
Measure the DC voltage at that speaker relay input contacts with zero volume.

It should show some DC instead of nothing due to an output stage failure, usually a semiconductor, caused by the spike you heard, stressful enough to damage something.

Replace the visually determined components that may be faulty.
If only one channel has DC, use the healthy channel to compare voltage readings to track the problem on the bad one.

Schematics may be available on the net by searching for "Yamaha such & such service manual"

 

You should be able to order a service manual here:

http://usa.yamaha.com/support/parts_manuals/

 

Thanks Externet!

One channel reads 15.5 millivolts across the relay input to ground and the other reads zero. I closed the relays individually and the side that reads 15.5 mv outputs sound while the side that reads zero does not. The voltage across the two input terminals reads 52 volts.One channel is blown. time to compare voltages.

 

Thanks, tubeguy!

When I followed their instructions:

"Enter S (space) M (space), then the model number of your unit without a dash."

... I was informed that: "Model doesn't exist"

Oh, well...

 

Try the phone. Yamaha is usually very helpful.
Good Luck !

 

Check for shorted transistors (with no power apllied!) in the output stage of the amp. You have a good channel to compare.

Generally a blown power amp occurs because of abuse and if one or more transistors blow they do so in a big way and turn into silicon blobs.

Some manufacturer's official repair procedures were to "replace all the semiconductors in the power amp" and it sounds like a pain but often is quicker and a more reliable result than the alternatives trying to diagnose and repair piece by piece and hoping you found the original cause of the fault and not just the parts killed by the fault.

 

Some manufacturer's official repair procedures were to "replace all the semiconductors in the power amp" and it sounds like a pain but often is quicker and a more reliable result than the alternatives

and if it's more than ten years old, don't forget to replace the electrolytic caps

 

And, check the output transistor emitter resistors.

 

and if it's more than ten years old, don't forget to replace the electrolytic caps

I still use my very first audio receiver, an HH Scott Stereomaster 341 that I bought in about 1964. It still works perfectly with its 48 years old electrolytic caps. It was one of the first receivers to use ICs for the FM IF amp. It is FM only and does not have an AM tuner.

My main audio receiver is a Yamaha that is about 18 years old and it still sounds great.

 

Thanks for all the excellent advise! Here's where I am at:

I determined that one channel was good and the other was blown. I determined which STK8770 II output transistor pack was associated with the good and the blown channel. With the power on and speaker relays open, I checked voltages at the terminals of the STK8770's and they differed quite a bit, with the one on the bad channel reading rail voltage (~52 volts) to ground on 10 of 14 pins, While the one on the good channel read rail voltage on only two pins. I removed the 2 STK8770's and checked them for resistance across various pins, reversing polarity as needed. The readings varied. I determined from this that one of the STK8770's was damaged. I decided to put the good STK8770 on the bad channel to see of that was the only problem. It wasn't. When I closed to relay for that channel with a chopstick, I still got no sound. With the unit powered up and the relay open, I checked the voltages again. They were different from both sets of readings taken previously. Hopefully, this is telling me that the good STK8770 was not damaged by this operation. So something upstream is not right and that's where it stands. Without the schematics it's a bit tough, but I will see what I can do.

Thanks again,
Jack

 

Seems the common ground inside the bad module is fried open. Confirm it is not a blown ground trace outside on the pc board

You seem to be conducting troubleshooting fine.
Try to obtain the STK8870 datasheet and application notes, which will likely expose a good portion of interest of the Yamaha receiver. I couldn't find it.

Try a similar unit, as in ---> http://diagrams.digitalelectronics.name/yamaha.htm
and ---> http://www.next.gr/audio/amplifiers/

 

I couldn't find the datasheet either. I don't think it is out there to be found. NTE has an equivalent for an STK3082, which is described as an:

Integrated Circuit
Module, Hybrid, Dual Driver for
80W to 90W Audio Power Amp

The unit I have is rated at 70w per channel so it may work, providing the pinout is the same... I dunno. I am assuming that the STK3081Y that is in there now is a mirror image dual channel IC, so reversing it may revive the bad channel and that is what I am going to do next. This unit may not be worth the money in parts, though, even if I do find the problem, but this more of a hobby, and the unit was given to me, so if it proves not to be worth fixing, it's off to the recycle place.

Jack

 

Sanyo in Japan made many audio power amplifier "hybrid" modules with part numbers beginning with STK. They might have made this one only for the Yamaha Japanese receiver.

 

Looking closely at the circuit board where the STK3081Y goes, it becomes apparent that this IC may not be a mirror image inside. In addition, it has 16 pins while the NTE equivalent for an STK3082 has only 15 pins, so it's beginning to look like a downhill slide into the recycle pit with this pup from my point of view.

 

... The readings varied. I determined from this that one of the STK8770's was damaged. I decided to put the good STK8770 on the bad channel to see of that was the only problem. It wasn't. When I closed to relay for that channel with a chopstick, I still got no sound. With the unit powered up and the relay open, I checked the voltages again. They were different from both sets of readings taken previously.
...

It's an inconclusive test, the amp protection systems might be smart enough to detect a bad or missing amp chip and shut down something else, or you may have had a soldering error or something.

I had a lot of years replacing STK and STR chips in commercial HiFi amps, and the best test to to check the PSU main +/-rails, and preamp +/- rails, and if all is good and you are getting signal to the inputs of the amp chips but one chip has a full rail output (like your case) just replace the offending amp chip.

Unfortunately i think STK8770 is getting hard to find, I checked my catalogue and a specialist supplier of amp chips has a couple hundred STK variants but no 8770. Maybe you'll get lucky and pick one up. Trying ringing Yamaha in the big city and ask for their service dept, (tell them you are a repairer not a consumer) and ask their guys if STK8770 is still available.