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  #1  
Old 02-11-2012, 09:05 AM
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Transconductor Transconductor is offline
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Yamaha CR-820 repair

Wonder if someone with experience on this receiver could help with something? I have been following some of the suggestions I have seen posted in various threads, but since there is no service manual available, I am finding that identifying some of the components is a little tough.
Someone else had replaced the "driver transistors" with TIP31C and TIP32C types. I am assuming this means there are a total of 4 (2 @ of the above). I have these ready to go in but I am having trouble identifying the "drivers". Are two of them TR719 and TR720? Others?
Initially, there was a signal in both channels, but with hum. I have replaced all suspect caps and this took care of the hum. There currently is no output in either channel, but I think this is going to be the "driver" transistors and idle adjustment. The output transistors check okay, but I have modern replacements just in case.
Any other suggestions?
Thanks for any assistance!
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Old 02-11-2012, 09:55 AM
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merrylander merrylander is offline
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A schematic is on the AK database

www.akdatabase.com/AKview
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Old 02-11-2012, 12:56 PM
ecluser ecluser is offline
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Quote:
Originally Posted by Transconductor View Post
There currently is no output in either channel, but I think this is going to be the "driver" transistors and idle adjustment.
The driver transistors are TR719, TR720, TR721 and TR722

Do you hear the click of the relay a few seconds after turn on?
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Old 02-11-2012, 01:04 PM
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Merrylander,
I do have the schematic, but am still unsure which are the driver transistors.
AK member JeremyK posted some information on his CR-820 restoration and this is his comment on the driver transistors:

"Remove the existing power output transistors and drivers. Then replace the drivers with TIP31C and TIP32C. These are pretty cheap."

I have purchased two @ of the above but I need guidance on which specific transistors need to come out in place of these two? Which are the driver transistors?

Now I'm starting to feel dumb...
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Old 02-11-2012, 01:30 PM
ecluser ecluser is offline
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TIP31C is a NPN transistor. For replacing the TR719 and TR720

TIP32C is a PNP transistor. For replacing the TR721 and TR722
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Old 02-11-2012, 01:43 PM
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WHich is much better than the previous information, that was rather like the announcer who said "Here are some baseball scores, 9 - 6, 5 - 3, and 8-4.

At the front of the circuit board are two chokes (coils of copper wire) with the power on check for DC voltage between one end of the choke and the chassis - does not matter which end.
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Last edited by merrylander; 02-11-2012 at 01:46 PM.
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Old 02-11-2012, 03:28 PM
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I have replaced drivers TR719-722 and on power up I do hear the relay click after about 1sec or so. I now have a signal out of the left ch only. When I increase the volume even slightly, there is loud crackling in the right ch.
I have verified that all my caps were installed in the correct polarity.
Are there some voltage checks I can do to start ruling out components?
In the mean time, I can double-check my soldering.
Thanks for your help guys.
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Old 02-11-2012, 06:15 PM
ecluser ecluser is offline
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Double check your soldering and pass the receiver to the deoxit bath
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Old 02-11-2012, 08:12 PM
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Crackling turned out to be the most obvious...dirty vol control.
Didn't hear that early on so mistakenly ruled that out.
A couple of questions:
Is it a good idea to replace the output transistors as preventative maintenance? Will this improve the sound or best to leave the original Toshiba's in place?
Also, FM sounds good but seems a little on the weak side, sensitivity-wise. Is there something easy to check?

Thanks for everything! AK rules.
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Old 02-11-2012, 08:31 PM
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ryuuoh ryuuoh is offline
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If your outputs are good, no need to change them.
Are you using an antenna?
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Old 02-11-2012, 08:36 PM
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Quote:
Originally Posted by ryuuoh View Post
If your outputs are good, no need to change them.
Are you using an antenna?
Yes, just a simple indoor dipole. I was thinking that there may be some lytics in the tuner section that might be the cause. Possibly some leaky that might be affecting some voltages. Possible?
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Old 02-11-2012, 08:39 PM
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I'd check the tuner supply voltages if they're available on the schematic. It might be needing an alignment.
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  #13  
Old 02-11-2012, 09:03 PM
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4seatPilot 4seatPilot is offline
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I found this thread: http://www.audiokarma.org/forums/showthread.php?t=62075

I have taken the instructions from Merrylander and combined them into one document, saved for future reference. I have two 820s in the midst of being tuned up.

I haven't tried the instructions yet, but considering the source, I'd say they're worth a shot!
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Old 02-12-2012, 09:18 AM
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Thanks to all and 4seatPilot for the link to FM alignment/troubleshooting thread.
I hadn't seen that on my prior searches. I followed Merrylander's instructions on tweaking the oscillators, etc. and was able to improve reception just slightly. Also improved dial alignment and tracking. Checked the voltages on IC 103 and all are good there. I do have a scope and just for completeness checked the 19khz osc-that was right on. Interesting that most FM stations at the lower end of the dial come in quite strong and most others are fairly weak as I tune up the dial. I can say that this receiver just seems on the weak side when compared to so many others I have had on the bench. Maybe this is just how this receiver is? It seems like it would benefit from an additional RF amp or maybe the mosfet referenced in the other thread gets weak after so many years.
Dial lights are next and that should finish this on for now.
Thanks again!
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Old 02-12-2012, 11:39 AM
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If you were doing the alignment tweak, note that the coils with the slugs affect the low end of the dial and the trimmer screws on top of the tuning capacitor affect the high end.
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