Scarab
New Member
Hi, I recently purchased a Yamaha CR 820 from Ebay knowing that there was no output from the unit. After I opened it up I discovered that someone had changed the output drivers TR723 (2SD427) and TR725 (2SB557) with some Motorola equivalents. I tested them with my sencore transistor checker and they tested good. I then tested the TR719 (2SC1624) and TR721 (2SA814) transistors and found that TR721 (2SA814) was bad. I replaced it and then tried powering up the unit. I heard the relay click and now have sound coming from both left and right outputs. I checked the idle current and adjusted the left side for 8mv. The left side would only adjust from 0 to 3mv. I couldn't get it to go any higher than 3mv. After testing the outputs terminals to the speakers to make sure that I wouldn't damage anything, I then hooked up a cheap set of speakers to the outputs to take a listen. The unit sounds great and there is plenty of volume. Other than the idle current issue that I stated above, the only other problem is a hum that happens when I first power up the unit. If I switch off the audio muting switch the hum goes away. This happens for a short while then the hum goes away and the unit works great. I have listened to it for several hours and there is no hum. If I turn the unit off and then back on, no hum. If I turn the unit off for several hours and then power up the unit, the hum is back again for a while then it goes away and the unit works and sounds great. I can also get the hum to go away or at least quiet it down by touching the wires on the signal and tuning meters. I have looked the PC boards over several times and have soldered anything that looked like it might be a cold solder joint. I would appreciate any help that you could give. I have a DVM and oscilloscope to help with troubleshooting.