Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Problem?

[JimBanville]

So I didn't realize there was a tiny written warning on the speaker impedance switch on the back that said moving the switch with the power on "may damage the unit". I actually switched it multiple times, listening for any possible audible changes. It wasn't until later I saw the tiny warning. I haven't noticed any issues though. Can someone please look at the schematic and tell me what damage I may have caused? Thanks!!

[R_J]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

If the amp is still working, you are ok, just don't switch it when the amp is on because you could damage the output transistors and maybe more.
in this case you got lucky. The service manual is too blurry (low res.) to read. I don't know why people do this to service manuals.

[JimBanville]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

If the amp is still working, you are ok, just don't switch it when the amp is on because you could damage the output transistors and maybe more.
in this case you got lucky. The service manual is too blurry to read

Thanks! At work now so I can't get a good look at the service manual. I'm on my phone. I was going to find the specific pages with the power supply, etc and post them. I'll try when I get home
Gotta say tho...hey, Yamaha...why not make the switch recessed like the voltage selectors often are? A switch projecting out could get bumped while moving or cleaning the unit. At least then you would have to be looking at the back of the unit to change it and are likely to see a BIG warning! Haha

[Khron]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

You might be able to see a bit more in a non-mangled version of the service manual:



There's a slide switch on the bottom-right of page 47 (containing the power amps), that goes to a signal called "8/4" going to the microcontroller, and a "/4ohm" signal coming back to the power board. That seems to drive some opto-controlled triacs(?).

I've got a hunch those just alter the amplitude of the supply rails (lower voltage for the lower impedances, higher voltage for the higher impedances), so as to keep the required current about the same.

I doubt flicking that switch would do much (or anything) if you weren't passing any signal through to the speakers.

[JimBanville]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

You might be able to see a bit more in a non-mangled version of the service manual:



There's a slide switch on the bottom-right of page 47 (containing the power amps), that goes to a signal called "8/4" going to the microcontroller, and a "/4ohm" signal coming back to the power board. That seems to drive some opto-controlled triacs(?).

I've got a hunch those just alter the amplitude of the supply rails (lower voltage for the lower impedances, higher voltage for the higher impedances), so as to keep the required current about the same.

I doubt flicking that switch would do much (or anything) if you weren't passing any signal through to the speakers.

Thanks! I was listening to music as I flipped switch to see if I could hear any difference. That didn't seem unreasonable at the time. Lol. Had I heard a "pop" or anything out of the ordinary, I'm sure I would have stopped. I heard nothing when flipping the switch. The music just continued as normal. I expected "something" to change, but heard nothing.

[R_J]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

In this model, the switch tells the microprocessor input to change between 4/8 Ω which changes the voltage supplied to the output transistors via the circuit on page 82 (pdf 47) so switching the 4/8 switch will change the output supply voltage from around +/- 55vdc to +/- 80vdc

[JimBanville]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

In this model, the switch tells the microprocessor input to change between 4/8 Ω which changes the voltage supplied to the output transistors via the circuit on page 82 (pdf 47) so switching the 4/8 switch will change the output supply voltage from around +/- 55vdc to +/- 80vdc

Thanks! So you think changing that voltage while powered-on would be bad for the transistors? Seems unlikely to me but you all are the experts. Haha

[R_J]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

Thanks! So you think changing that voltage while powered-on would be bad for the transistors? Seems unlikely to me but you all are the experts. Haha

Normally the impedance switch is right on the outputs and switches the actual output. but not in this case, it just increases the voltage to supply more current with the lower impedance speakers. Changing the switch in this case could damage the scr's SR121 especially if the unit was operating at a high volume.
Keep switching it and let us know what blows up!

[JimBanville]

Re: Switched speaker impedance selector w/ Yamaha HTR-5740 RX-V450 powered on!! Probl

Normally the impedance switch is right on the outputs and switches the actual output. but not in this case, it just increases the voltage to supply more current with the lower impedance speakers. Changing the switch in this case could damage the scr's SR121 especially if the unit was operating at a high volume.
Keep switching it and let us know what blows up!

Thanks for the info!!!
Lol. I assume you are joking about continuing to flip the switch when powered on. Why would I? I was simply curious "what" I could have potentially harmed and "why". I was also slightly hoping a knowledgeable person might look at the schematic and see no issues to the point of even "questioning" the warning in the first place. Haha. I had read about those switches in the past, but this was my first receiver with one. Nothing I ever read about them ever warned about adjusting them when powered on. Fortunately I picked this yamaha up for $6 at a goodwill type store last week, so it wouldn't have been a total disaster had I damaged something