Yamaha MSR400 - Powered PA Speaker

[sukaht]

Hello everyone.
I need help with my Yamaha MSR400 - Powered PA Speaker that I bought it used recently.

It is power on, and sounding good / working perfectly fine. However after 3 - 5 minutes, the heatsink is starting to get warm, and it smells burning a little bit.
I did tried multiple times on different day, event the speaker is not playing any song (input does not connect to any source, and the volume level is zero), after it turns ON, 5 minutes later, the heatsink is starting to get warm, and it smells burning a little bit. I am afraid keeping it ON longer than 5 minutes because it might damage it further.

What is possibility causing this? The Power Supply, or the Amplifier, or something else?
Thank you in advance for help and suggestions

[sukaht]

This is the diagram for my Yamaha MSR400 - Powered PA Speaker
I highlighted in yellow are component that running hot in just 3 minutes.
I also attached Service Manual for Yamaha MSR400, see page#29 - (component Q225N, Q227N, and R273) or (component Q225P, Q227P, and R274).

Please help, which component that is failing, and causing hot?

Like I said before, this speaker is still working fine and sounding loud when playing music, the bass and tweeter are in good sounding condition.
This is Bi-Amp class AB amplifier, which it power the woofer and tweeter separately.
The problem is just those components (2 transistor and 1 resistor for each Amp), they are running hot in just 3 minutes.
I think this is just an early state, so I am hoping I can replace that component my self.
I don't have a huge knowledge in checking fail component, but when I was in High School, I was familiar with making small amplifier/radio, so soldering and replacing electronic component will not be a problem.

Please help, and thank you so much in advance for all help from senior member here.

[CapLeaker]

Check the idling current. If not correct, it will cause hot transistors. It should say in the service manual on how and where to adjust it. There are two of them. I see it your 3rd picture.

[sukaht]

CapLeaker, thanks for replying.
I check the idling current VR201 and measured CN206 per service manual, and the result is Over Limit (OL). This is the one close to the white ceramic resistor R273 and R274, that very HOT in just 1 minutes. So what is the next step to check? I don't mind buying 1 or 2 component, because it is still cheap compare to bring it to service technician. So please let me know what is the suspected component that fail?

The other one, idling current VR202 and measured CN207 per service manual, and the result is 0.2mV.
Again, thank you so much for anyone reply and help.

[CapLeaker]

I don't know what the idling current is supposed to be neither I read the service manual. 0.2mV doesn't sound right either, more like 10 or 15mV...
Cording to Google it is supposed to be 10mv +- 0.2mV. You sure the service manual sez 0.2mV idling current?
If one side is good and the other is off the chart, not adjustable or not in range of adjustment, my first bet would be the trimmer resistor may be bad. Take it out and check it. If it is within spec, look at the biasing circuitry. Something is off somewhere. If a component tests funky, take it out or lift a leg and measure it again out of circuit.

[sukaht]

CapLeaker Thanks for replying.
Service manual saying idling current should be 1±0.2 mV.
I have 2 speaker, the same model, the other one is working normal, without overheating problem.

When I compared the idling current on the good speaker, it is CN206 (VR201) =0.8mV, and CN207 (VR202) =1.1mV.
Compared with the bad speaker, CN206 (VR201) =OverLimit, and CN207 (VR202) =0.2mV. So it is really off.... I will check trimmer resistor later, because it located under the board, I have to remove the heatsink.

FYI, I checked connector CN209 per service manual (see page 14), the good speaker read 7mV, and the bad speaker read 21mV.
Again, thank you so much for anyone reply and help.

[CapLeaker]

I downloaded that service manual and had a look, The idling current cording to the service manual is 1 +-0.2mV. That means it should be set between 0.8 and 1.2mV

[sukaht]

CapLeaker Thanks for replying and checking the service manual.
Trimmer resistor is not bad, I have the same measure for both good and bad speakers. But I don't know how to look at the biasing circuitry.
Maybe it is time to bring it to professional technician. I was just hoping you can pint point which component to replace, after you see the service manual. But if you busy/not sure, it is ok, and thank you anyway for trying to help me.
I thought this is an easy fix, because the amplifier is still working fine, but only the low value resistor (R273 and R2734) are getting too hot to touch. Maybe the transistor (Q227P and Q227N) is about to fail?

[CapLeaker]

You got resistors and transistors in the whole bias circuit that needs to be checked and see why you can't adjust the bias to what it should be. There is a parallel resistor with that trimmer pot if I remember correctly, was that o.k?

[sukaht]

CapLeaker Thanks for replying.
Parallel resistor with that trimmer pot is ok and I compared it with the good speaker too, the same measure.
I spent all day today to check related transistors and resistors, they looks ok, but I found the voltage to ground is over 2 volt all around the bad amps-speaker, it is 3v compared to good amps-speaker is 1.4v. So I think this over 2 volt causing the transistor (Q227N and @227P) turn on (while idling) and make low resistor (R273 and R274) got too hot.
How do I find out which component that causing extra 2 volt ? Maybe one of resistor is short?
Again, thank you so much for anyone reply and help.

[check12]

First thing I would do with that amp is loosen the heatsink screws & remove the pcb with all devices attached, turn it over and check the whole underside for dry/bad solder joints... then while it's off reflow the solder on at least all the legs of the components that attach to the heatsink. Then replace pcb on heatsink & check idle current again....

I'm taking it you've already checked all voltage rails etc....

[sukaht]

check12 Thank you for replying.
Bingo 👍, you are correct, It was loose/bad solder joints on bias transistor 2SC4793.
Yesterday, with the unit powered on, I checked all the voltages and compared it between channel 1 (bad channel) and channel 2 (good channel), and found out 1 bias transistor Q215 (2SC4793) had weird voltage result, very low, B-E=0.0001v and C-E-0.0005v (in fact, this was because of Emitter leg was not connected to the circuit / had loose solder joint). I did not know about it until I was about to remove it to check off the circuit. After checking, that transistor 2SC4793 is still good, I soldered it back, and powered up the Amplifier. I was so happy, no more overheat on those low resistors (R273 / 0.5 Ohm and R274 / 0.5 Ohm). I can touch it now 😀.

Big thanks to CapLeaker for narrowing the problem to check biasing circuitry, you are correct 👍 that was causing the problem, and check12 for your respond to my post, probably I will follow your advise if I did not find that weird transistor. I am so lucky having support from all of you on the internet.

Special thanks also to Richard from Learn Electronics Repair (I don't know his user-id in this forum), I watched his videos on YouTube, on how to check transistors, so it make me possible to identify which suspected component. I shared this video for everyone to watch= https://youtu.be/GDDwTbM9DLw?si=MmyAk4ccJ2xK4UDE

Now, the speaker is on testing, playing loud music for almost 1 hour, and the heatsink is normal room temperature.
Thank you everyone.

[CapLeaker]

Awesome! Good to hear you got her sorted out and found the problem.