Yamaha M70 ZDR

[Yamaha5000]

I do have a sincere concern about the harmonic spikes, what to do about them?

 

[Yamaha5000]

I have a 2nd M70 and ran the same test on it. Results shown below:
Notice reduced value of Harmonic Spikes

 

[Sparkplug]

I do have a sincere concern about the harmonic spikes, what to do about them?

Usually the reason is due to poor grounding, different transistors having different stray capacitances, different gains, etc. . You can try the procedures you have to see if pots adjustments make any visible difference.

 

[Yamaha5000]

different transistors having different stray capacitances, different gains, etc

Are you referring to the output transistors?

 

[Yamaha5000]

Would it be correct that HB+, HB-, LB+ & LB- would be a constant DC voltage with no AC component?

 

[Yamaha5000]

V at Yellow and White

 

[Sparkplug]

Are you referring to the output transistors?

Any transistor in the audio path, from the input to the output. Couple years ago I replaced the OEM op-amps in my Technics tuner with ultra high grade op-amps and ended up with a very annoying and constantly present high pitch howling. Although at higher volumes it wasn’t audible, it was there. It turned out the super op-amps had 110dB gain as opposed to 100dB for the originals. I took the last one in the line up out and gone was the noise. In effect I reduced the gain by 10dB.

 

[Sparkplug]

Would it be correct that HB+, HB-, LB+ & LB- would be a constant DC voltage with no AC component?

There’s the ripples due to rectification but with proper filtering, yes, those are practically DC voltages. Like anything else, however, if you look at them under a microscope, you’ll see things you didn’t know were there, but they’re generally inconsequential. As long as the harmonics are down by the amount the standards say, all is good. I don’t know about audio standards but for cellular band, for example, the harmonics (second, third, and their first order intermods) must be down by 42dB or so wrt the main signal.

 

[Yamaha5000]

First harmonic is around 34db, second harmonic 48db, third harmonic 52db

 

[Yamaha5000]

AC ripple= AC voltage.

Just set your DMM to AC Volts (or rather MilliVolts) and measure between Ground and rails: you should get less than a One Volt AC.

I measure 0.000v with the DMM
I measure 2vpp with scope

HB+



LB+

 

[mbz]

Missing attachements of HB+ and LB+ ripple waveforms edit they are ok, problems my end
I've got a M-70, will open up and measure same.
The HB+ and LB+ traces are gibber-ish, set time scale to about 1-5mS, looking for a 120Hz "sawtooth sine" waveform


The ripple is caused by the amp(load) running down the stored energy in the caps when the input (rectified) charging voltage is low. Generally class A amps will have higher bias and run down the stored cap energy quicker so higher ripple.

First harmonic is around 34db, second harmonic 48db, third harmonic 52db

I don't see a connection between ripple and audio harmonics if that is what you are suggesting.

 

[mbz]

Ignore noise, oscilloscope was close to laptop/psu and surrounded by fluro...
Amp has not been refurb'd, put on hold. The alternating ripple on LB+ probably points to an issue.

I noticed your timescals was set to 2mS, your traces don't look anything like ripple.

 

[Yamaha5000]

he HB+ and LB+ traces are gibber-ish, set time scale to about 1-5mS, looking for a 120Hz "sawtooth sine" waveform

The photo was not good but it was set to 2ms

 

[Yamaha5000]

I measure 2vpp with scope

Error should have stated 0.2vpp

 

[Yamaha5000]

On the LB I can see a degree of a saw tooth but it is buried in that rubbish

 

[Yamaha5000]

The ripple is caused by the amp(load) running down the stored energy in the caps when the input (rectified) charging voltage is low. Generally class A amps will have higher bias and run down the stored cap energy quicker so higher ripple.

My stupidity! I don't have a Main IN or Spkr load connected!

 

[mbz]

My stupidity! I don't have a Main IN or Spkr load connected!

No, I did not mean a spkr load. I was referring to the amps circuitry as being the load on the rectifier/main filter caps.
My traces were obtained without a speaker load.

Error should have stated 0.2vpp

For an unregulated supply I previously would have thought that is ok. I'm surprised the ripple on my unit was so low, might be a measurement error on my part.

 

[Yamaha5000]

But shouldn't I see this sawtooth wave?

I am not seeing anything like you have shown on your scope.

 

[mbz]

But shouldn't I see this sawtooth wave?

I am not seeing anything like you have shown on your scope.

Remove the base plate and measure at the solder joints for the main caps. Larger caps are +/-HB. Smaller caps are +/-LB, as normal, cap common pin is GND. The "sawtooth wave" is there, if you can't see it clearly then you have an oscilloscope/tools problem, don't blame the amp.

Try connecting the probe to the 1Khz lug on the front of the oscilloscope... 0.2-1mS time base

 

[Yamaha5000]

1Khz lug

I am not familiar with that terminology