Another Yamaha CR-2020 Rabbit Hole

sansevy

sansevy

New Member
I've had a CR-2020 on the bench for a few weeks now and I am chasing problems all over. I completed the service bulletin work and encountered smoke from the left side of the E. Cap board when I powered it on. Based on the thread where Avionic helped Bluhn321, I swapped out a bad transistor (TR714) and the unit no longer overheats on the E. Cap board. However, the relay does not click and I am measuring 2.1V on the -25V rail and 26V on the 25V rail. Any ideas of where to go next?

I also measured the voltages at the "T" posts on the main C board. I get 10mv on one of the boards but 0 on the other. Not sure if this is related to the other issue or it is signaling a second problem in the circuit.

I did my best to search for other threads that pointed to solutions, but I have struck out thus far.
 
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Well, I started checking transistors upstream/downstream of TR714 and found a few others that seemed to have failed (TR704 and TR708). I've had to resort to cannibalizing one from another receiver (CR-620) and one from the Pre 3 board on the CR-2020, just to see if that does the trick...Well, it seems to have gotten the CR-2020 out of protection.

The idling current on one of the two main circuit boards (the board on the heat sink closest to the protection board/transformer) is still at 0mv. Not sure where to go from here...

If you have any ideas, I would greatly appreciate it.
 
I pulled the main circuit board and found that it had two blown fuses. Figuring these were blown before I completed the recap, service bulletin work and transistor replacement, I decided to grab the fuses from the other board and try them out on the one with the blown fuses. Well, the fuses blew. Now I am trying to figure out what might be causing the fuses to blow. Figuring that the lack of idle current is due to the blown fuses. Any ideas?
 
I posted the play-by-play above, in case it is helpful in rounding out a picture of where I am with the project. Hope someone out there takes pity on me!
 
Sorry. Didn't know that. Having a hard time getting my .pdf of the schematic down to a file size that can be uploaded. I tried to crop and save as a .jpg. Hopefully, this is still legible enough.
 

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This might have attracted a bit more attention in the "Yamaha" forum. Hopefully Avionic wanders by shortly, he's the guru of all things Yamaha.
 
sansevy said:
Sorry. Didn't know that. Having a hard time getting my .pdf of the schematic down to a file size that can be uploaded. I tried to crop and save as a .jpg. Hopefully, this is still legible enough.
Click to expand...
I'd need a telescope to read that scat.:biggrin: Either that or I'm not opening it correctly. Anyway I have a hard copy of the manual.
 
Hehe. I was figuring you did, Avionic. Should we continue in PM or should we take it to this thread for the good of the group?
 
I had a similar issue with my CR-1020; intermittent popping from both channels and a bit of low level hum at initial turn on. Many many hours of troubleshooting, so wanted to share here in the hopes of saving others some heartburn.

After a full power supply recap (both pcbs) and power transistor upgrade to higher voltage TIP31C in the supply, the protection relay would not fire. The -25V output sat at -10V. Cleaned the flux and grime off the bottom side the PCB and the output went to +6V. Disconnected the two purple wires from the -25V post to eliminate loading. No improvement. Removed and retested all 4 transistors; junctions not shorted nor open.

Decided to look at the .01uF capacitors. In circuit across the terminals, the one on the -25V supply measured 6 kOhms whereas the one in the +25V supply measured 3 megohms. Pulled the caps; neither was leaky. Pulled the 4 transistors again. Still 6 KOhms...strange.

Used flux remover to clean the top side of the PCB around the .01uF cap locations, removing glue residue around the heatsinks. Resistance now 2 megohms. Apparently the glue that Yamaha used was not electrically benign and the -25V supply is extra sensitive to leakage current (check out the circuit topology).

Reinstalled all components. Reconnected all wires. Voltages perfect. Relay works.
—Ed
 
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