Yamaha G5 Sounds Fizzy Distorted

[misterc57]

Thank you to all for the schematic (attached). Now to my problem.

I have had this amp sitting on the shelf for years waiting to get to it. I thought it needed the speaker replaced but after some basic diagnostics the problem is not the speaker.

This amp breaks up too quickly and sounds fizzy, even at low volumes and on either input jack. Same issue happens if I route the output to a different "known good" speaker.

All pots and inputs have been cleaned. Now that I have the schematic I plan to trace a signal through the circuit and hope to find where it is breaking up.

Any suggestions from anyone before I get started? I will not get to this until the end of the week.

Looks like the 4558 chip is strictly for reverb processing.

There is a bias pot on the board, can anyone tell me how to properly set that?

Thank you! Mark

[eschertron]

There are voltages listed on the schem, so start there. The power supply is putting out a single-ended 25.5V, so basically this thing is a stomp box with a built in monitor. Once all the voltages are OK (if not, document the discrepancies and post them) then put a signal into the input and trace stage by stage. Use a cap to decouple the 6+VDC from the signal. A probe taking the signal to a clean guitar amp, with the volume appropriately low, should give good indication of the unit's sound. If you get good results up to the gate of TR4, then suspect something in the power section; yes, even the bias.

[Enzo]

Set it so crossover notch just disappears.

Before adjusting anything, scope the output for crossover distortion. That would be my first suspect.

Otherwise isolate the problem. Sample/listen/view the signal at the master volume. You could also inject signal at that point. These tests would identify the problem as either preamp or powr amp related.

Of course no circuit works right without good power. You have what appears to be a single sides 25v powr supply for the power amp, is it up to voltage and clean?

preamp? the print even is nice enough to have voltages around the transistors. (as does the power amp) How do they stack up?

[g1]

If you have no scope to check for crossover, proper bias setting would give the 3.2mV at TR9 emitter. (works out to about 14mA current through TR9)
You will need a fairly accurate meter to measure that though.

[misterc57]

Looks like I'll get to chalk up more time on my scope on this project!!!

[J M Fahey]

And donŽt expect too much volume out of it.

Inject 100mV 1kHz a the High input, set Treble and Bass to 6 (on a 0-10 scale), Master to 10 and rise volume slowly until output starts to clip, then back slightly.
If you get between 4 and 5V RMS at the speaker terminals .... thatŽs the official spec.
This is strictly a bedroom amplifier.

If no scope available, connect a piezo tweeter straight in parallel with the speaker and place piezo near your ear, you will clearly hear when amp starts clipping, go back and forth between pure, flute like tone and added buzz/grit until you find the exact clipping edge.

EDIT: beautiful amp:

[misterc57]

Inside pics...

[misterc57]

TR4

Trying to make sense if this.

I am getting confused at TR4 which is a 2SAB41. I am confused on what legs are the B C and E.

I have the follwing voltages at TR4 4.9, 0.58, 5.5 all VDC going left to right as viewed in the photo.

Bias was adjusted to read 3.2 mV at the TP (test point). It was real close before I adjusted it. Where should my vol levels be when adjusting this?

Master Vol at 10, input Vol at 3, readings at TR4; With a 1K signal at the input of the amp I see a clean wave form on outer legs, on the middle leg (the top of the wave looks like a cowboy hat).

Master Vol at 10, reading at positive output wire; With a 1K signal at the input of the amp wave form is clean, as Vol goes above 3 the top and bottom starts to flatten. Flattens more as volume is turned up.

Input Vol at 10, reading at positive output wire; With a 1K signal at the input of the amp wave form is clean, as Master Vol goes above 7 the top and bottom starts to flatten. Flattens more as volume is turned up.

Update: Bumped up my input signal to 100 mV (it was far too low on the above readings).
What I am pretty sure of right now is that there is a clean signal on the left leg of TR4 and distorted signal on the other two legs.

[g1]

None of that seems abnormal.
Have you tried using headphones?
The speaker is depending on switches inside the headphone jack.

[misterc57]

Yes. Same results. I can hear and see the bad wave form before and after the headphone circuit.

[J M Fahey]

Trying to make sense if this.

I am getting confused at TR4 which is a 2SAB41. I am confused on what legs are the B C and E.

You are not supposed to scope TR4 on its own (unless asked to do so for some specific reason) but power amp out to speaker at the 1000x50 output capacitor.
We expect +V to be around +24V, out cap positive leg about half that , so some +12V, negative leg almost zero volts, do you confirm that?

I have the follwing voltages at TR4 4.9, 0.58, 5.5 all VDC going left to right as viewed in the photo.

Ok, so E is +5.5V , B is +4.9V, C is +0.58V .
Circuit shows respectively +6.7 +6.1 +0.6 so we are real close.

Bias was adjusted to read 3.2 mV at the TP (test point). It was real close before I adjusted it. Where should my vol levels be when adjusting this?

Zero, no signal in, Bias is a *static* adjustment.

Master Vol at 10, input Vol at 3, readings at TR4; With a 1K signal at the input of the amp I see a clean wave form on outer legs, on the middle leg (the top of the wave looks like a cowboy hat).

You were not supposed to see that young fellow, now weŽll have to kill you

Master Vol at 10, reading at positive output wire; With a 1K signal at the input of the amp wave form is clean, as Vol goes above 3 the top and bottom starts to flatten. Flattens more as volume is turned up.

Congratulations, you reached clipping.
But you are maddeningly vague: what-was-the-voltage-you-measured????
strike 1

Input Vol at 10, reading at positive output wire; With a 1K signal at the input of the amp wave form is clean, as Master Vol goes above 7 the top and bottom starts to flatten. Flattens more as volume is turned up.

Congratulations, you reached clipping.
But you are maddeningly vague: what-was-the-voltage-you-measured????
strike 2

Update: Bumped up my input signal to 100 mV (it was far too low on the above readings).
What I am pretty sure of right now is that there is a clean signal on the left leg of TR4 and distorted signal on the other two legs.

see above

[misterc57]

You are not supposed to scope TR4 on its own (unless asked to do so for some specific reason) but power amp out to speaker at the 1000x50 output capacitor.
We expect +V to be around +24V, out cap positive leg about half that , so some +12V, negative leg almost zero volts, do you confirm that?

Confirmed... Cannot get to legs of 1000/50 cap but took following readings;

11.8 VDC at .22 resistor feeding + side of cap (and it looks clean on the scope, if that is what was asked earlier on)

varying from + .05 mVDC to -.14 mVDC at "O" (- side of cap)

[misterc57]

Am I close or am I off?

Been doing a lot of probing, reading, etc. I discovered that I really could not tell much with a signal generator pumping a 1K wave as I could not hear the breakup.

I ended up plugging a guitar into the HIGH input and listened to strumming at different points in the circuit. I hear clean strumming on both sides of the coupling cap leaving TR3. See highlight in picture. At the outer leg of the Master Volume pot I hear the awful fizz sound as well as at downstream components.

Now I could be completely wrong but I am hoping I have narrowed into the area with the problem.

There are two resistors and two diodes in that section between the two points. Components are mounted sideways so I can only get on one leg of each component. I am going to have to pull the board up to continue. Hoping to get some feedback before doing that.

To answer some earlier questions. All voltages I checked (all over the place) were very close to the schematic. I did not see crossover distortion on the scope. I have a very old CRO scope. All I can see is a waveform, it does not show me any voltages etc.

Thank you

[g1]

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

[J M Fahey]

you reached clipping.
But you are maddeningly vague: what-was-the-voltage-you-measured????

I really could not tell much with a signal generator pumping a 1K wave as I could not hear the breakup.

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

With a 1K signal at the input of the amp wave form is clean, as Vol goes above 3 the top and bottom starts to flatten. Flattens more as volume is turned up.

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.