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Missed it by that much, a Yamaha RX-300U struggles to rise again

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FrankyLttrs

Active Member
I've owned this Yamaha RX-300U since new. It's been sitting in a cabinet for 20 years and I thought I might resurrect it. It's a model no one cares about, but it is dear to me because it is the first piece of non-essential electronics my wife and I bought when we were starting out. On powerup the left channel would take a long time to come in, and then if you turned the volume up beyond 50% it would go into protect mode.
Popped the hood and I actually was happy to see it had discrete finals, I assumed being late '80s it would be STKUnobtaniums. Given the discrete finals, I thought I might expend some effort on it.
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On closer look, uh oh, corrosive glue around the power caps.
Not only corrosive glue, but someone missed with the glue gun and nailed a driver. I don't have a schematic, but I'm guessing that's the left channel.
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An even more gruesome shot. The stuff was all over a bunch of bare jumpers and got so hot in some places it discolored the board. I guess the glue had enough conductivity that it was working as a heating element.
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Removed the power caps and cleaned it up as best I could. Fun fact, there was NO glue under the power caps, it was just glopped on the side. 34 years ago, whoever was running the glue gun had one job...
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So on reboot, it comes on properly and will run at full power...but the left channel is weak and distorted. Two out of three ain't bad! Given the amount of corrosion on the driver transistor, I'm not surprised. I don't have any spares for the transistors so maybe I'll get some and give it another go, or maybe not, RX-300Us go for like $50 on eBay.

I learned a lot (they were still using the corrosive glue in the late '80s), developed some new skills (dental tools work great to clean the glue off), and I got to try out my new desoldering gun (sweet!!).

Final note, what is up with Yamaha and their signal routing? This thing has terrible crosstalk between the receiver stage and the other inputs. It's at the board level. The receiver stage is in the back next to the AUX inputs, they run both signals in parallel traces from front to back, and the receiver is never off, so at maximum amplification on the AUX and no AUX signal, you hear the radio. Been that way since day one and is part of the reason it's been in the cabinet for 20 years. And what about routing the un-rectified output from the transformer in parallel with the output signals (first picture)? Isn't that like, rule #1 to not run unshielded DC signals in parallel with AC power lines? A CA-610II I'm working on does the same thing.
 
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Asmodeus2112 said:
Dirty protection relay, volume pot or switches?
Click to expand...
Deoxed the switches and pots. Relay is sealed, but if I go back in I'll probably do something with it. Should have reflowed the affected components and jumpers when I was in there as well, but I was so excited to try it out with the glue removed I jumped the gun, I need to set up a checklist system. Another thing I learned, when you open it, just be prepared to replace everything rather than diagnose, then order parts. It's a lot different than when I work on cars, you don't have to drop the engine just to replace the spark plugs. I also learned that for this model, you have to half-disassemble the front plate and fully disassemble the back plate in order to get the main board out, what a pain.
 
avionic said:
What part # is printed on the relay ? Mouser likely has an Omron drop-in replacement.
Click to expand...

Hi @avionic , I hope you're doing well . . . I'm working on an RX-300U that has also has a relay issue so I figured I'd reply here in case it helps others.

On mine, tapping on the relay brings sound back when it cuts out. I wonder if we can get these relays. Or if possible, I'd like to pull the cover off of it and clean the contacts - if the cover comes off . . . It has little tabs on it (does it?). I've tried gently squeezing it with pliers and pulling but I'm afraid to pull hard.

Thanks for any advice!

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You need to de solder it from the board to be able to clean it properly. But new relays should be available. Dave should be able to steer you to the right one.

Athanasios
 
avionic said:
What part number is printed on the top of the relay? A photo of the footprint would be handy.
Click to expand...
Thanks - one side has some info (had to use a mirror so it's backwards!) I do think the de-soldering suggestion is good, thanks @Nashou66
of course on this unit that requires major disassembly but that's ok.
If I'm de-soldering it I might as well get a new relay in there if it's available.
Thanks very much! I appreciate it.

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I guess that it may look like this on the bottom, based on some for sale in China.
Hoping there's a new (current) equivelant.
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I have an RX-300U too. I have been away from my bench for a while, reading this thread seemed like a good opportunity to get back into the hobby. Certainly corrosive glue, and more black plastic than higher end and older Yamahas, so have to be careful not to break anything. The relay that @avionic recommended looks like a good match, same circuit etc., I'm going to order one. Mine has intermittent channels etc. Definitely have to clean the pots etc.
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Great! Thanks for the relay pic. The plastic is disappointing but don't these sound good? I've only been listening with headphones but when the signals are making it through all the circuitry it is really fun to listen to. Maybe we can compare notes.

Just curious: Did you find some replacement 5600 uF caps? I looked a little but so far haven't found any I thought looked suitable . . . I don't want to change the sound as I think it's awesome as-is. The glue on mine is making the jumper wires corrode.

I cleaned the controls on mine once and evidently just F5 isn't enough for the pots. So I need to take it apart and try again, maybe D5 first then F5. These pots seem to be more difficult to get working than other amps or receivers I've fixed up. Maybe they are just low-cost pots (probably). It makes me curious about finding replacements for those too maybe.

One other small issue I have: I washed the plastic face and got water in the clear lens which, of course, dried with water spots. Now I'm debating trying to get the plastic lens off to really clean it and then glue it back on.
 
fromnowon said:
Great! Thanks for the relay pic. The plastic is disappointing but don't these sound good? I've only been listening with headphones but when the signals are making it through all the circuitry it is really fun to listen to. Maybe we can compare notes.

Just curious: Did you find some replacement 5600 uF caps? I looked a little but so far haven't found any I thought looked suitable . . . I don't want to change the sound as I think it's awesome as-is. The glue on mine is making the jumper wires corrode.

I cleaned the controls on mine once and evidently just F5 isn't enough for the pots. So I need to take it apart and try again, maybe D5 first then F5. These pots seem to be more difficult to get working than other amps or receivers I've fixed up. Maybe they are just low-cost pots (probably). It makes me curious about finding replacements for those too maybe.

One other small issue I have: I washed the plastic face and got water in the clear lens which, of course, dried with water spots. Now I'm debating trying to get the plastic lens off to really clean it and then glue it back on.
Click to expand...
I was buying another amp and asked the guy if he had anything else for sale. He had this RX-300U for almost free. Since I like most Yamaha with variable loudness I bought it, at least it could be a doner for screws and other bits. This thread made me decide to spruce it up. The filter caps on my unit seem fine, so i don't think I am going to invest in new ones at this point. I'll do the same as you, clean the pots and the glue, replace the relay if it seems faulty. Most of my channel cut outs are probably the pots, we'll see.
 
Those Nichicons look like a good choice also. Thanks for sharing!
You're ahead of me; I have responsibilities for another week or so before I'll have any time for fun hobby stuff. (end of semester)

Are you going to replace any other smaller caps while you're at it? (I can't find a schematic anyplace, by the way. Hoping not to need it though but it would be nice to have.)
 
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