I'm having a problem with my CR-820. The outputs were blown, they were changed, the amplifier L + R channels were re-balanced as per service manual. It works , but right channel is over heating. Any ideas?
Damage to the outputs can cause damage in the driver and even pre-amp stage. I would test all resistors to make sure they are not open or increased in value. I would do a resistance test of the bias diodes (D707, 709); they should measure the same as the good channel. Check them both directions; open one way, conducting the other polarity. The driver transistors need to be tested; personally, if I even suspect them I change them out (TR 715, 717, 719, 721). I would also check any electrolytic capacitors in the audio amp.
Hope this helps; that's a nice receiver.
Thanks for the info. Will let you know.
Mike, purely by chance I have logged on to this site today and saw your request. I managed to repair my CR820 in 2011. I described what I did in the AudioKarma forum- see http://audiokarma.org/forums/index.php?threads/yamaha-cr-820-success.348...
Basically I had something similar to you and I went back and changed the driver transistors as well, and some of the resistors. I recorded my method at the time. I posted it in the AudioKarma forum and I hope it is OK if I repeat it here....
"I've finally managed to get my CR-820 back to working order. I've had it from new (about 1979) but one day one of the power resistors started to emit smoke, and the power output transistors in one channel blew. I'd replaced the transistors piecemeal but they always seemed to blow immediately. I gave up for several years but decided to have another go, a bit more systematically this time. I am hoping that if I record what I did it may possible help someone else in a similar position.
1. Replace the fusable resistors FR701 and FR702 with 300 ohm 1/4 watt resistors. Raising the resistance from 68 ohm was suggested by 'ecluser' as a way of protecting the driver transistors from failure - and this seems to have worked. I also tested the emitter resistors (in white woven sleeving) and found that two of them needed replacing.
2. Remove the existing power output transistors and drivers. Then replace the drivers with TIP31C and TIP32C. These are pretty cheap.
3. Do in-circuit multimeter tests of the other transistors in the drive chain, comparing the readings from each channel.
4. Power-up the unit without the output transistors. I checked that the transistor voltages were the same on each channel, and that all the driver transistors became equally hot.
5. Replace power transistors with MJ15016G and MJ15015G. These are TO-3 cans with identical leadouts to the originals. I bought heatsink paste and new mounting insulators to be on the safe side. These transistors are not expensive.
6. Power up and adjust bias current to the required value in the service manual. One has to wait a few minutes for the current to stabilise because it starts off by drifting alarmingly upwards.
7. Enjoy the result. The sound remains superb, especially using a decent record deck.
Thanks to all the posts that have helped, and of course a special mention to 'Merrylander' on AudioKarma who seems to understand this receiver better than the guys who designed it."
If it is any comfort to you, I was unable to repair it for years. Every time I changed the power transistors they got hot and blew. But I managed it in the end and the receiver has worked well ever since.
Thanks for the comments. I swapped all the caps. I did not changed out the drivers, on my test set they looked good and the bias was OK. I never considered the resistors, I'll try that.
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