Yamaha G5 Sounds Fizzy Distorted

[misterc57]

So it is good on the cap side of the 10K but bad on the pot side?
That is where the reverb return is wired in.
Is the bad sound there with the reverb turned all the way down?

Yes and yes. All tests have been done with reverb set to zero.

Does the wiper of the reverb pot give 0 ohm reading to ground when turned all the way down?

Not sure I can get to wiper until I pull the board. Will report back.

[misterc57]

No need to, you can SEE it which is even better:

So rise volume until top and bottom flatten, back down a little until top/bottom *just* flatten, set multimeter to 20VAC scale and measure VAC , all this with speaker connected and plugged straight into the wall, no bulb limiter.
Yes, it will be annoying s a siren, but all it takes is 30 seconds.

Post VAC measured here.

Will do. Am I reading at the .22 resistor feeding + side of 1000/50 cap (where I took the 12 VDC reading posted earlier)?

Thank you all.

[misterc57]

Master Vol at 10. 1K signal at 100 mv input. Reading 7.5 VAC when clipping starts (Input Volume about 1).

Master Vol at 5. 1K signal at 100 mv input. Reading 5 VAC when clipping starts (Input Volume about 4).

[misterc57]

Still plugging away at this. The board is out. Reverb pot (and all other pots) are working fine.

I was seeing the noise start at the Master Volume pot. I was told that noise could be working its way back from the power section. I am thinking of lifting the 39K resistor after TR4 to isolate the power section, and see if there is still noise at the Master Volume. Does that idea make sense?

Thank you.

[J M Fahey]

Surprisingly good results.
Or not surprising, considering it is Yamaha fter all

Master Vol at 10. 1K signal at 100 mv input. Reading 7.5 VAC when clipping starts (Input Volume about 1).

Suppose itīs 7V RMS with no clipping at all: 49/4=12 W RMS
Which is not surprising, classic practice amp IC TDA2030 puts out guaranteed and clean 12W RMS into 4 ohms with +/-14V rails, fully equivalent to what you have there.
Yet Yamaha sells it as a "5W" amp

Master Vol at 5. 1K signal at 100 mv input. Reading 5 VAC when clipping starts (Input Volume about 4).

Cool, thatīs what any MV does, lets you clip at less than full power, in this case 25/4=6W RMS ... and you can turn it down as much as you please, down to 0.

I was seeing the noise start at the Master Volume pot. I was told that noise could be working its way back from the power section. I am thinking of lifting the 39K resistor after TR4 to isolate the power section, and see if there is still noise at the Master Volume. Does that idea make sense?

well, YOU see it , we donīt.

Please post some scope screen captures or pictures showing the noise.

Even better, record a short video, rising and lowering volume so we can match real time the sound we hear and whatīs shown on screen.

[Enzo]

A ways back, your partial schematic shows right after the highlighted cap a 470 ohm resistor to a crossed pair of clipping diodes. Have you lifted that 470 ohm resistor to see if you are just hearing those diodes clip?

[misterc57]

A ways back, your partial schematic shows right after the highlighted cap a 470 ohm resistor to a crossed pair of clipping diodes. Have you lifted that 470 ohm resistor to see if you are just hearing those diodes clip?

You nailed it Enzo. I lifted one leg of the 470 ohm resistor and no more noise issues! So, what is going on here? Those diodes are meant to clip? If so they make it sound horrible. I have listened to a few videos of this amp and did not hear this terrible fizz sound.

Thank you!

[Enzo]

One man's horrible is another mans greeat distortion.


Like any matter of taste. Some people like feta cheese, but to me it smells like vomit.

We used to argue about tubes in a certain amp. one tube was dull and lifeless while the other was clear and defined. But to another listener, the first tube was smooth and mellow, while the second tube was shrill.

[misterc57]

Trying to decide where to go from here.

Questions;

Any harm in leaving this resistor lifted and not having those clipping diodes in the circuit?

Is it possible those diodes degraded and sound like crap, should I try to replace them with a pair of 1N4148, or will I end up with the same fizzy sound?

Thank you!

[g1]

It's funny that you didn't hear it when you said the signal was good at both sides of the coupling cap after TR3.
You could disconnect the 470, or put in a switch, or change it to a higher value variable resistor (pot).
Likely with master at full and volume set low, you would not hear the problem.

[misterc57]

>>>It's funny that you didn't hear it when you said the signal was good at both sides of the coupling cap after TR3.
Good point.

I was hearing the fizz start with Master at 3 and Volume at 3.

[J M Fahey]

Those diodes are there on purpose, namely to clip signal.

And if you use master volume on 10, you will never hear them, because power amp will clip before they do.
1N4148/1N4002/*any* Silicon diode will clip exactly the same.

You sent us off course with the claims of horrible fizzy sound ... when itīs the weakest Guitar distortion available, and, by being at the very end of the preamp, untouched so it clips exactly the same as the power amp.

This problem could have been solved 23 posts behind IF you had showed the "horrible fizz" waveform ... still no luck on that.

Would have helped a lot, because we ignored that when you scream "I am being attacked by a bloody savage beast!!!!!" ... now we know you mean:


Well, all is well that ends well, glad you solved it.