Need help with Yamaha Ca 820.

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.

Check your emitter resistors for the left channel.

Avionic, thanks for the reply. I'm not quite sure what you mean by emitter transistors. I will have some time tomorrow night to look at the receiver. I really appreciate your help.

Thanks,

Scarab said:

Avionic, thanks for the reply. I'm not quite sure what you mean by emitter transistors. I will have some time tomorrow night to look at the receiver. I really appreciate your help.

Thanks,

Click to expand...

Emitter resistors on the left channel.
R772,R774,R776,R778 all four are suppose to be .22 ohms.

Avionic, I was able to adjust the DC idle current last night. I found that the resistor R789 had been damaged and was not measuring the correct resistance of 4.7. I also replaced the lamps last night and they are now working. I am now concentrating on the hum. I did look at the .22 ohm resistors that you recommended and they all look good.
Thanks,

I looked at this unit again last night trying to figure out where the hum was coming from. I discovered that the AM receiver side is not working. I get nothing out of it. It is perfectly quiet. Started to study the schematics today. Hope to have some time to look at it again tonight. Any ideas or places to start looking at for troubleshooting this unit?
Thanks,

Could be indicative of a cap that's nearly shot, and is forming up under voltage. Check the power supply caps. Old trick- wire a new one in parallel, even if lower in value, but full voltage rating of course. If the hum goes away, replace the old cap.

Do you have a scope? First thing to do with radio is check to be sure the local oscillator is running and near the right frequency. Make sure the circuit is getting power- some switch off the power when other functions are selected. Then signal trace through the circuit. Sometimes the 1st transistor or FET gets zapped by static or nearby lightening. Fortunately AM is pretty simple and within the capabilities of just about any scope. Might be a bad part, or maybe somebody just messed with the adjustments. Once it works, you can align AM with broadcast signals. Not so for FM.

Conrad, thanks for the response. Yes, I do have a scope. I will look at and try your suggestions. I have been studying the schematics today to try and understand how the unit works. This is one of those units that breaks off the +12V power supply from the non-used AM/FM receivers. I will also check the local oscillator for correct frequency. I have a RF/Audio signal generator and a frequency counter. Thanks for the suggestions. One question, with a scope should I be able to see the input signal from the antenna going through the first transistor?

Thanks,

At the input I doubt it, and you'll load things down, but you should see at least a trace (no pun intended) of noise or signal after the first device. Usually people start further down the chain unless there's some reason to suspect the input. I'm a bit spoiled because I use an ancient scope with an extremely high gain plug-in, so almost everything anywhere is visible. Since you have an RF sig gen, I'd find an alignment procedure and see if the signals are present through the IF. If the IF works, work your way forward or backwards from there. If you haven't worked with tuners before, learn how a generic AM tuner works and the schematic will make more sense. They all seem to be pretty similar. FWIW, even if you don't have the unit, download the instruction manual for a Sencore SG-165. It gives a huge amount of general info.

Conrad, after looking this unit over this last weekend, I am wondering if the bar antenna could be damaged. I have studied and followed the schematic through quite thoroughly and I am not seeing anything coming from the AM side. I have checked to make sure that the proper voltages are there, and have checked all of the transistors and found that they are good. I have also downloaded a copy of the Sencore SG165 manual and will be studying it this week.

Thanks,