CR-220 with blown left channel - Thoughts on fixes?

ameobob

New Member
I recently accidentally shorted out the left channel on my CR-220 and didn't realize it until my nose told me so, followed by major distortion of the high end on the left channel. It also seemed to be getting very hot :/ Now, if I only have headphones plugged in it sounds okay at low volume, but when I switch the speakers on it's very bad. I've scoured the schematic diagram and board layout and am thinking about re-capping all of the electrolytics on the left channel (I think about five caps total.) What is the likelihood this is the cause of the fault? Should I plan on replacing any other components (such as ceramic caps, transistors and/or diodes, or even resistors or that little inductor?) If I replace components on the left channel, should I do the same to the right channel just so they're identical? I've done some work on this amp already, but only to replace the lamps on the tuning indicators (which have subsequently burnt out again.) Also I've re-capped and restored several old computers, so I'm excited to get my hands dirty on some audio equipment. Thanks in advance!
 
Start by removing the output transistors on the bad channel and checking them out of circuit with your multimeter or transistor checker.
 
Hifiengine....Not sure of the quality of the scan. Probably better than nothing.
http://www.hifiengine.com/manual_library/yamaha/cr-220.shtml
Thanks for the links, I've already been looking at them to locate the likely suspects. How would I go about testing the transistors? I've got a nice multimeter and o-scope, what am I looking for?

Edit: Ah, I guess I'll check to see if any of them are reading low resistance between the leads. If it's a blown transistor should I just be able to find a direct replacement from my favorite online retailer or are these components more hard to come by in this day and age?
 
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Awesome, thanks for the tip! Turns out I have a blown Sanken 2SC1986. ( Drain-Source resistance like 2 Ohm >.< ) There was another identical transistor for the right channel that measured out just fine. I've bought a new one online (same manufacturer even) so hopefully it works out. I'll let you guys know the results!
 
Check the .33 Ω / 2 watt emitter resistors associated with the blown output transistor as well.
 
I measured those resistances with the transistors de-soldered, but i'll measure those .33Ohms as well just to be sure :)

Edit: Yeah, the 0.33 are all good to go.
 
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So, I ordered a transistor but it doesn't seem to be exactly right. The blown one is an "Sanken C1986 7NY" and the one I ordered was advertised as just "Sanken C1986" but turned out to be a "40L". I put it in regardless and started her up, but the left channel was still hosed. The new transistor measures 22.8 MOhm Gate-Source and 22.63 MOhm Gate-Drain, whereas the good 7NY on the right channel measured 17.45 MOhm G-S and 13.00 MOhm G-D. I'm having difficulty searching for a replacement for this transistor, is that three digit code at the end just a date code, or something more significant about the characteristics of the component? What should I search for to find a functional replacement that perhaps isn't exactly the same manufacturer and model code?
 
The new transistor measures 22.8 MOhm Gate-Source and 22.63 MOhm Gate-Drain, whereas the good 7NY on the right channel measured 17.45 MOhm G-S and 13.00 MOhm G-D
Drain,source and gate refers to FET's.(2SKxxx or 2SJxxx) Emitter,base and collector refers to normal bi-polar juction transistors (BJT). The Sanken is the latter. Where did you get the replacement Sanken ?
 
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The blown one is an "Sanken C1986 7NY" and the one I ordered was advertised as just "Sanken C1986" but turned out to be a "40L".
The last 3 digits are Date/lot codes (40L & 7NY) or something along those lines.
 
Alright, so it's been a while!

I did a couple pokings-around this evening and was looking for the source of the distortion, working my way back from the output transistors. I hooked up a cheapo square wave generator to the aux input and found an odd distortion as early as the base of TR701... I took TR701 and TR702 both out and compared them, and the resistances are all identical. Scope screenshots: Left channel TR701 [Image] Right channel TR702 [Image]. TR703 and TR705 have identical signals as the analogous ones on the right channel, as I would expect if all is working properly. At this point I'm thinking of just methodically swapping transistors between Left and Right channels to see which one carries with it some problem, but I don't want to cause collateral damage to any other components in case there are multiple baddies and they fall like dominoes once a bad one is put in. any other suggestions for a method or approach would be appreciated!

I also noticed the replacement tranny I got when placed in the right channel doesn't sound good, so I'm wondering if it's out of spec, such as perhaps it got blown again when I replaced the known bad Left channel one... but last I recall it measures okay, and I hate removing and replacing that heat sink...
 
I'd start where the signal gets wonky and pull and check any component in the path. It's possible a diode or zener could have been overloaded or you could have another shorted device in there somewhere.
I've never worked on a 220 but having gone though other models there's usually a few more stressed or damaged components when an output gets shorted.

I really like those little eBay transistor checkers. They are nice in that they give you a reading of what the transistor is but also gives you a gain number. That way you can check suspect devices against known good ones and see if they are way off. I've found faulty but working transistors like that several times.
 
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