Yamaha CR-820 left channel out

stevepe

Active Member
My thrift store CR-820 responded very well to a deoxit treatment and has been my primary receiver for the last year. New lamps, too. Love it.

When I turned it on last week I heard a loud pop, and now I have no left channel. :tears: I had been playing a fair amount of moderately loud music for extended periods in the days before the incident. I can hear a very very faint static but, "musical/rhythmic at least", signal on the left side when balance is maximum to the left and volume is at 10.

I noticed some residue underneath the board in the area of the chip that is under the input selector extender shaft about midway. The solder seems a little dirty and dark on the chip, but looks intact. There are three little caps nearby, but they seem to be not quite in the right place to have leaked, and don't look damaged, IMO.

Fuses are all intact.
A or B speakers makes no difference.
Working all the switches has no effect
Haven't worked the headphone input's yet.

I've no training in electronics. I have a decent multi-meter, and I've re-caped a couple of sets of speakers.

I'd appreciate some advice on how to get this great receiver going again.

Thanks,
Steve
 
Just a guess... but if you heard a pop (from the receiver I assume?), then I'd expect that a capacitor has gone kaput. They might look ok... but could still have failed with a small explosion... not sure if that could also be sign of an output giving up? Or another transistor?

The other thing to check would be fusistors - ML curses the use of the things, and I've found several to be way out of spec - or even open - which I guess is the point since they are fuseable... should be white, and they look like a ceramic type material. If you can, desolder one leg of them one at a time, and test the resistance.

Based on the pop though, I'd be looking for suspect capacitors. The question I don't know the answer to though is can a cap blow up, and take out only one channel? Hmm.

I have two 820s open at the moment... I'll take another look at mine if you need more help... I'm sure some of the other much more knowledgeable Yamaha techs will chime in too. I'm nowhere near considering myself a tech, but I'm learning and enjoying it.
 
I'm pretty sure the pop was through the speaker(s). These things are never expected, so I wasn't listening for where it was from. The speaker and and the amp are near each other to my left. I may have just been turning up the volume when it happened. (I sometimes leave the power on)

I've seen exploding caps on youtube, and it wasn't like a high pop. It was deeper.
 
Tap on the relay see if your missing channel intermittently cuts in and out.
 
Just went and wacked on it with a pen, and no change, but thanks!

I presume we mean the clear plastic-housed power on relay with the pair of contacts visible inside (the existence of which I learned of just yesterday on this forum!)

Thanks,
Steve
 
OK– I just tested the current at the speaker connectors with speakers disconnected. Phono selected, no inputs.

The right side reads .006 volts (another reading later was .013v)

The left side reads .42 volts. When switching the unit on, when cold, the relay kicks in and the voltage begins rising to this reading slow and steady over two minutes. If I test again soon after, the voltage starts around .20v and climbs quickly to .42v. (.39v on another later test)

I'm hoping this info will indicate what's wrong.

Thanks,
Steve
 
OK– I just tested the current at the speaker connectors with speakers disconnected. Phono selected, no inputs.

The right side reads .006 volts (another reading later was .013v)

The left side reads .42 volts. When switching the unit on, when cold, the relay kicks in and the voltage begins rising to this reading slow and steady over two minutes. If I test again soon after, the voltage starts around .20v and climbs quickly to .42v. (.39v on another later test)

I'm hoping this info will indicate what's wrong.

Thanks,
Steve

Check it with the TAPE or AUX input selected. There will be to much noise on the PHONO. 420mv is way to high- I'm surprised it even comes out of protection.
 
I'm getting 436mv on the left and 6mv on the right. Makes no difference in the reading whether on tape, aux, or phono.

Thanks,
Steve
 
The service manual tells how to adjust the idle current, try that for starters. 420mV is high but not as high as a blown output would be. If you find that you cannot adjust the idle current on the left channel replace the 2SA844 transistor that is righ under the variable resistor.
 
OK.If adjusting the idle has minimal effect.As Rob suggested . Plan B... To get the offset down under control.
TR702 & 704 need to be replaced with a "matched pair" of new transistors (2SA872)
I'd replace TR706 as well (2SC1981) and D702 ( 1N4148 ) .
 
I have the owners, but not the service manual. It doesn't seem to be as easy to get. Can someone recommend a source for this?

Maybe I can enlarge the schematic on the owner's manual and it will help me identify those transistors. Steep learning curve here. Glad to have an approach laid out!

Thanks,
Steve
 
I have the owners, but not the service manual. It doesn't seem to be as easy to get. Can someone recommend a source for this?

Maybe I can enlarge the schematic on the owner's manual and it will help me identify those transistors. Steep learning curve here. Glad to have an approach laid out!

Thanks,
Steve

Post # 8 is a pretty good schematic scan link.
 
That link to the schematic on #8 is so much better than what I was trying to make work! And thanks for the link to Norman's site.

I'll have a look around the board and see what I can see with all this new info and a schematic that is legible. Not sure about how comprehensible it will be to me, I feel I have a toe-hold, now.

Thanks,
Steve
 
I think I found what I believe must be the variable resistors, (labeled IDL), and a couple of transistors nearby that say A844 on them. I'm not confident which orientation Merrylander was using to describe "below". I've pointed out what I think he means, (low tech, huh? )– the closest to the faceplate variable resistor and two candidates for the transistor in question. Left in the photo is the front of the unit.

And Dear Avionics,
I'm unable to email Norman to buy a service manual from the email link he provides without a "mobile me" account. Nothing to copy and paste there. Dang.

On the probably delusional notion that I just turn the slot in one of those resistors and see if I can bring the voltage down, perhaps a service manual isn't strictly necessary. It's worth it to me to get one, though, if it means fixing my darling, so if you can relay the actual email address...

Also, I located the additional transistors you recommended replacing, but I can't identify that diode. Is it the tiny blue and copper colored one or a black plastic with the metal tab? I'm not seeing that number on anything so far.

And tell me please, the idle current is adjustable, but DC offset is not? They could be different names for the same thing for all I know. I told you it was a steep curve.

Thanks so much for the help,
Steve



IMG_7488.jpg
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On the probably delusional notion that I just turn the slot in one of those resistors and see if I can bring the voltage down, perhaps a service manual isn't strictly necessary
Not a good idea. Adjusting the bias/idle current while monitoring the offset voltage is a very quick way to destroy output transistors. Should be test points within the unit to adjust the idle/bias (which will be across one or two emitter resistors per channel). Get a service manual. Or maybe Rob has the information you need.
 
I found a service manual for the CR 1020, at least, and this is what they have for the operation in question. Maybe someone has this page for the CR 820. I didn't see any other instructions than what is on this page.

Thanks!
Steve

Cr1020.jpg
 
Great! I'll hang around the house. The receiver is down at my workshop.

Steve
 
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