Yamaha rs202d suspicious DC offset

[Aveit]

Hi all
I have I decided to meter the terminals on the Yamaha after blowing a pair of mission mx3s on all four drivers.
The law of averages has me suspicious as for all four to go at the same time is abnormal although I suppose not impossible.

The usual paranoia ensued as usual and the late night OCD head scratching commenced.
I decided to measure the DC offset expecting around 20mv as usual but found a uniform 40mv across all the speaker terminals.

It's late and I've just popped the lid back on after finding it's a non adjustable amp whilst fishing the speaker terminal spring out from inside the amp that I'd pushed in with the multi meter probe

I know that up to 50mv is usually deemed acceptable and that the higher quality amps are much lower.
I tested my 30 plus yr old pioneer a400 and got only 4.2mv left channel and 2.5mv right.

That caused me to track down the service manual for the Yamaha RS202d and from what I can make out 27-136mv is normal but it's not exactly clear.
(I've heard more DC offset is found with class a/b amps)
I find it on the high side which has me questioning things.

I have attached a picture of the service manual and would really appreciate your insight.
Am I reading it correctly?
It's late and it's possible I'm reading it wrong.

Is 40mv considered ok on such an amp.

As some of you know the Yamaha is within my second set up with the pioneer a400 via an amp/speaker switching box but after purchasing the new speakers (fayne audio) for my primary hifi as mentioned on another post I'm planning on moving the QA 3050i they've replaced into this system thus replacing the blown mission mx3s.

I'm now pondering even though my common sense dictates otherwise that 40mv DC offset could of contributed to early failure.

Anyway please see the attached screenshot, any advice would be helpful.

 

[AnalogSteph]

Is 40mv considered ok on such an amp.

Absolutely. As a rule of thumb, a value within +/-100 mV is perfectly fine.

I've had a look at the SM, and even the schematic states 0.048 V. This is odd, given that DC balance and input stage symmetry seem good on paper. Ah, looks like the amp was originally designed for +/-50 V but actually runs on +66/-61 V at idle, which would throw off the input stage tail current (which is defined only by +Vs and 3x 8.2k in series) and hence symmetry, as current in one leg is kept from changing very much by the following VAS transistor (maybe 1.1 mA instead of 1.0 mA, but that still means the other leg would see 1.55 mA, a substantial imbalance).

This thing has a quasicomp output stage with parts kept to a minimum. I guess that's kind of to be expected in a very inexpensive receiver, but still feels jarring (I think Japanese mass-market amps had generally dropped quasicomp output stages by the late '70s).

The residual noise spec of 70µV(A) for this model has to be inaccurate, typical output noise for the electronic volume IC is spec'd at 5 µVrms and power amp gain computes to 26.7, so that would be a good 133 µVrms and maybe 110 A-wtd. Amir noised higher than average noise but also had his cranked up a fair bit. Mind you, the chip was aimed at "mini or micro components" and only runs on 9 V single-supply.
(BTW, the BT input makes use of an NJM2904 opamp, an LM358 grade job, run on the same +9 V. This thing is truly an exercise in how cheap you can possibly make a receiver. Mind you, it looks like the audio is coming in around +1.33 V, so being able to go near the negative rail would have been a desired feature. Still, this doesn't feel ideal...)

Your old A-400 has much lower DC offset since it sports current sources for both tail current and the VAS. While not as powerful, it is definitely built to a higher standard. (That includes the phono stage.) I assume the protection relays would appreciate some TLC in this day and age.

That caused me to track down the service manual for the Yamaha RS202d and from what I can make out 27-136mv is normal but it's not exactly clear.

That's 27 to 136 out of a scale of 0 to 255 that spans 3.3 V. Or 0.36 V to 1.75 V. Heaven knows how this maps to actual DC offset, but something like +/-0.7 V seems in line with where conventional DC protection would trigger.

 

[Aveit]

Absolutely. As a rule of thumb, a value within +/-100 mV is perfectly fine.

I've had a look at the SM, and even the schematic states 0.048 V. This is odd, given that DC balance and input stage symmetry seem good on paper. Ah, looks like the amp was originally designed for +/-50 V but actually runs on +66/-61 V at idle, which would throw off the input stage tail current (which is defined only by +Vs and 3x 8.2k in series) and hence symmetry, as current in one leg is kept from changing very much by the following VAS transistor (maybe 1.1 mA instead of 1.0 mA, but that still means the other leg would see 1.55 mA, a substantial imbalance).

This thing has a quasicomp output stage with parts kept to a minimum. I guess that's kind of to be expected in a very inexpensive receiver, but still feels jarring (I think Japanese mass-market amps had generally dropped quasicomp output stages by the late '70s).

The residual noise spec of 70µV(A) for this model has to be inaccurate, typical output noise for the electronic volume IC is spec'd at 5 µVrms and power amp gain computes to 26.7, so that would be a good 133 µVrms and maybe 110 A-wtd. Amir noised higher than average noise but also had his cranked up a fair bit. Mind you, the chip was aimed at "mini or micro components" and only runs on 9 V single-supply.
(BTW, the BT input makes use of an NJM2904 opamp, an LM358 grade job, run on the same +9 V. This thing is truly an exercise in how cheap you can possibly make a receiver. Mind you, it looks like the audio is coming in around +1.33 V, so being able to go near the negative rail would have been a desired feature. Still, this doesn't feel ideal...)

Your old A-400 has much lower DC offset since it sports current sources for both tail current and the VAS. While not as powerful, it is definitely built to a higher standard. (That includes the phono stage.) I assume the protection relays would appreciate some TLC in this day and age.

That's 27 to 136 out of a scale of 0 to 255 that spans 3.3 V. Or 0.36 V to 1.75 V. Heaven knows how this maps to actual DC offset, but something like +/-0.7 V seems in line with where conventional DC protection would trigger.

Thankyou very much for taking the time to answer fully.

I'm admit there's a lot I do not understand but it's very interesting to me so I'm endeavouring to school up accordingly.

Answers like that I find both educational and very very interesting.

I realise that a 250 £ amplifier isn't anything but budget, reflective of it being in a second hifi that's only sporadically used.
My main hifi has just had the speakers upgraded from QA 3050i to a pair of fyne audio f502 that I fell in love with.

What's strange is the aforementioned Yamaha rs202d is despite it being somewhat badly and cheaply designed a shocker in sounding way better than perhaps it should.

Upgrading my speakers for my main set means I was going to swap the said Q acoustics to the Yamaha amp within the second hifi.
Suffice to say I've decided instead to revive the pioneer a400 (a dear old friend) both sentimental and for its sound.

I've only had to replace the power led and clean it internally, apart from that it's fine and sounds very good.

Im guessing, but I'm thinking I've clipped the missions at loud volume streaming from a phone what is low quality (320 kbps) music via Spotify premium.
It's a guess but four drivers failing at once has never been my experience.

Despite the Yamaha boasted 100w the pioneer is a much louder amp and achieves comparable volumes at 5/10 than the Yamaha's 90/100 yet has never in 26yrs damaged anything other than old pair of mission 763i that after 26yrs gave in mechanically due to wear.

I'm afraid I do not trust the Yamaha now as my preference in music is loud to most.
Meaning I'm probably pushing it too hard.

That model is known for being over boasted in wattage.
I always as a general rule like to have plenty of headroom as with my main amp not needing to go past 52 of it's 100 and the a-400 not needing more than 6/10.

Maybe I'm wrong in thinking this helps prevent clipping of the audio.

I'm willing and somewhat eager to learn as hifi is a passion of mine.

I'm currently after much box swapping, demos, prices, types, formats and everything that goes with it happy and get to experience genuine frisson within my current main hifi.
Acram sa30
Marantz CD 60
Ifizen blue
System deck iix
Fyne audio f502
Wharfdale SW sub woofer
Austrian audio HI-X15

But being very interested want to know more as things develop.

I have diagnosed severe OCD amongst other things which causes me to obsess over things that do not bother most learning things as I go.

Ps
Is there a simple way to check for an amplifier going into clipping other than distortion just to rule that out.
A visual way would be good, I've seen it done with an inexpensive hand held oscilloscope but maybe you know an easier way.

Thanks again