• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Yamaha CR-1020

Ray122

Ray122

Member
Hey everyone! Hope you're all enjoying the holidays!

I once again found myself in a repair pickle and asking for the sage wisdom of members much more experienced than myself.

Recently re-capped a Yamaha CR-1020 for a friend and first I ran into the issue with the misprinted polarity sign on the power supply sub-board, fixed that, and now I can't seem to adjust the bias on the right channel amplifier board. The amplifier works and comes out of protection mode when you fire it up, but there are some pretty nasty distortions, especially on low frequencies as you bring the volume up.

Naturally, I checked all of the usual problem components, and to me, there are no obvious issues I can find from checking the transistors, resistors, and diodes in the circuit board. The voltages start to drift from what they should be in the manual after TR606 and tracking down this issue by just looking at the schematic and comparing my measured voltages to what the service manual says is a skill I don't have at the moment. Before I start re-building the whole amp section, I'd love to get some troubleshooting advice from some wiser members.

I've attached a PDF of the recorded voltages for each transistor compared to what they should be according to the service manual, but let me know if is any other info you need from me in order to help me diagnose this.

Thanks in advance!

-Ray
 

Attachments

Register to hide this ad
Welcome aboard...Did the receivers right channel work OK before the recap ?
 
Remove one lead of R631 (emitter resistor) and check its resistance.. .47Ω / 5 watt
 
Have you cleaned any switches and controls yet ? Don't forget the coupler switch on the rear panel..It is always filthy..
 
Does this unit have a mains voltage selector on the rear panel ? What is your mains voltage ?
 
Both channels did work prior to the re-cap, and the issue is isolated to the right amplifier section.
Haven't checked the rear panel switch yet. I cleaned all the other switches, so I'm assuming it's not a dirt related issues, especially considering I can't adjust the bias like I can on the left. But I've learned to never underestimate a dirty switch.

Next time I'm in my shop I'll clean and check the rear panel switch, the mains voltage switch, and check that emitter resistor. Stay tuned! I'll try to get you that info tomorrow
 
Ray122 said:
the mains voltage switch,
Click to expand...
If it has a selector switch. Just check the setting. Your rails are running a little high. If your main voltage is 120vac . Select the closest setting. If its set to 110v and your voltage is actually 120v this would explain the slightly higher rails.

Note: It is not going to be the cause of your bias issue. That's something completely different.
 
Ray122 said:
Both channels did work prior to the re-cap, and the issue is isolated to the right amplifier section.
Click to expand...
Go back over your work.. Check solder connections. What all was replaced in the amplifier assemblies?
 
Alright, so there is no voltage switch, and the power supply aligns with the US/Canada model described in the service manual.

Cleaned the amplifier coupling switch in the back thoroughly, and unfortunately, no dice there. I've still got distortion on the right channel.

From there I double checked all of my work. I just replaced all of the electrolytic capacitors so I double-checked all of those with a multi-meter and I don't have any dead shorts or cracked traces anywhere. I took a nice long look at tall the other points on the board as well, and everything looks OK to my eye.

Finally, I lifted one side of R631 and 632 (just for good) measure and both are measuring 0.8 ohms. Not sure if that is enough of a variation to raise any red flags.
 
Short your test leads together and measure your test lead resistance. Subtract that from .8Ω ..
 
Check FR601 ( 68Ω) as well as removing VR601 and verify it is actually adjusting resistance from near zero to 4.7KΩ.
 
TR607e,c voltages are a bit low (+/-0.8V), need 2xVbe for TR611/612 and TR613/614.
TR608e(-0.158) should be the same as TR607c(-0.805) recheck.
Thinking slightly higher current through R614 causing lower TR607e voltage. Have
you replaced any transistors especially TR607, 608?
EDIT: Have you tried "resetting" trimmer VR601 to fully CCW and starting bias procedure again?
 
I haven't swapped any transistors, but I have noticed that some of the transistors don't align with what was specified in the service manual. There's a chance this was serviced in the past.

Let me re-check the double-check those voltages. I haven't tried re-setting it, but I did give it a few turns back and forth to try and clear any potential dirt while the unit was off. I'll give that a go as well.
 
  • Like
Reactions: mbz
You must log in or register to reply here.
Back
Top Bottom