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Yamaha cr2020 trouble

kenkirk

kenkirk

Active Member
Hi!

I restored this cr2020 a few years ago. Fully recapped, all service mods completed, better heat sinks on the PS transistors ( the PS board looks more point to point... lots of trashed traces ) new LED's etc. It has been running great for years since.

But now it has developed an issue in the preamp section. When first powering up, especially when cold, the right channel is thin and distorted. After a few minutes of warm up, it suddenly sounds fine. The mode switch will switch the trouble to the left channel. I have exercised the coupler switch. All sources present the same trouble. Exercising the controls has no effect on the trouble. I can hear the bass , treble etc having an effect. No noise on any pots. The tone defeat switch has no effect other than removing any additional eq. So this seems to be a problem in one of the preamp cards. I did recap the cards with Nichicon a few years back. I suspect a failing transistor in the right channel on one of the cards.

Does anyone have any suggestions of which transistors to suspect first. Also, when I pulled the cards years ago, I had the receiver torn down. What is the best and easiest approach to remove the cards when only the preamp cards need to be removed?

Thanks!

Ken
 
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kenkirk said:
When first powering up, especially when cold, the right channel is thin and distorted. After a few minutes of warm up, it suddenly sounds fine.
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Does rapidly cranking up the volume control bring the channel back to life?
 
avionic said:
Does rapidly cranking up the volume control bring the channel back to life?
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No, it makes it sound worse. The bass starts to rapidly break up. Like an amp that was switched off. I did notice this morning that after it warms up and seems to be back to life, the right channel output is less than the left. I feed both channels 800k and switched the mode selector from left to right with a speaker hooked up to the right speaker output, and I noticed when playing from the right, the level was reduced maybe 10 to 15% over the left. I could hear it and see it on the meters. I did measure 24+ and 24- on the B supplies at the pre board. All of the cards are getting good power.

Thanks!
 
Have you used a seperate pre-amp to drive the 2020's amplifiers?
 
avionic said:
You have and o'scope?
Click to expand...

yes: dual channel Tektronix, Leader SG and Leader A/C meter,Fluke mm.
I tend to focus more on reel to reel stuff. But I like this old Yamaha. It resides in my bench area to provide tunes while I do electronics.

The tone defeat has no effect on the distortion. While distorting the eq knobs do have an effect. Since I can bypass the eq section with the tone defeat but the problem remains, I am thinking it is either the "B" "D" or "E" daughter cards. I have read a ton of your posts over the years Avionic, so I am all ears. I know you know your way around these Yamaha's.

Ken
 
I have also carefully flexed the boards with a wooden stick and jiggled the wires around hoping to discover a loose connection or cracked solder joint. Nothing. No pops or crackles. It just needs a few minutes of on time and the channel comes in although at slightly less gain. And I bet there is higher distortion but I just can't detect it with my ears.

Ken
 
avionic said:
I usually unsolder them from the main board.
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I can test some from the board pins underneath. I think I was able to pull them when I had the amp heat sinks out. I do not think I removed the switches above the cards when I recapped them.
 
avionic said:
It's been a while since I have had one apart.
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I understand. Me too. I have been reading some other threads that suggest there may be some suspect transistors on the daughter cards. I do have a Super Cricket to help identify a weak transistor. But replacing them with matched pairs would be my choice once the cards are out.
 
Well the receiver is working perfectly now. I did nothing. I can speculate all day, maybe a coupling cap needed some time to reform? The receiver had sat for a few months with no activity in an unheated room above my garage and was subjected to temps in the mid 30's for a few days when I discovered the trouble. It's working perfectly again. I just wanted to update the thread. Thanks.

Ken
 
Well I hate to reactivate this old thread, but my CR 2020 is acting up again. I have isolated the issue to the right channel of the preamp. The deck was fully recapped a while back and worked perfectly for a few years.I have had the cr2020 in climate controlled storage and decided to get it out and put it back to work. The old problem has reappeared. I have a noise issue in the right channel preamp. Sometimes the noise includes a loss of gain so the channels are out of balance, sometimes its just a scratchy noise like a tube going out.And occasionally it will work perfectly until I power cycle it. My best guess is a transistor on one of the daughter cards in the right channel is failing. Before I start digging out the cards I thought I would ask if anyone has experience with these preamp cards. I did recap them I guess 6 to 8 years ago. All the pots have been hit with Deoxit and seem fine. Same with the switches. Switching in and out the tone controls makes no difference. The noise does track with the volume control. Thanks for any suggestions!

Ken
 
Transistors either work or they don’t, there’s no in-between state.

The fact that sometimes the amp just works fine indicates, your problem is very likely poor solder joints and connections, I’d start there instead of pulling any parts.
 
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