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Author Topic: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning  (Read 481 times)

substatica

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Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« on: August 25, 2019, 08:54:07 PM »
Working on a Yamaha YTA-95. I've got everything functioning except that the Speed/Intensity controls for the Wah/Tremolo have no effect. I can hear the Wah filter, but it doesn't oscillate or vary in intensity with controls.

I've re-capped the effect control board and the wah board. Tested the two transistors and JFET on the control board. Any ideas?
« Last Edit: August 25, 2019, 09:09:36 PM by substatica »

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #1 on: August 27, 2019, 10:38:14 AM »
Anyone have any insight into what part of this circuit, or component, is mainly responsible for the effect oscillation?

mandu

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #2 on: August 28, 2019, 03:12:18 AM »
The tremolo oscillator is on the right side of your snap photo, called as the phase shift oscillator . You can test if the output voltage on the collector is changing using an analog multi meter. The 3 nos. 1 mfd capacitors, 3 resistors (two are variable potentiometers 3.3K+50K pot) and the transistor form the oscillator. The transistor gain must be greater than 29. The phase shift happens from the collector to the base by 3 x 120 deg. with the 3 capacitors and 3 resistors. Varying one or two resistors changes phase shift and thus the frequency. If the phase shift fail, ex. dried capacitors or high resistance, the oscillation will stop. Regards.
« Last Edit: August 28, 2019, 03:21:47 AM by mandu »

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #3 on: September 01, 2019, 02:12:09 PM »
How might I measure the gain in circuit? I believe I've tested all the components on this board now and found no faults.  The collector of the left-most transistor fluctuates between 2.675 VDC and 2.685 VDC.

Not sure where to look next.


g1

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #4 on: September 01, 2019, 02:23:56 PM »
Capacitors are very difficult to test accurately.  Especially for minor faults that would stall an oscillator.
Cap replacement is the only reliable troubleshooting method for trem circuits.

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #5 on: September 01, 2019, 03:08:47 PM »
Hrm, I've replaced all the electrolytics, and all the tantalums read fine on a capacitance meter out of circuit.

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #6 on: September 01, 2019, 03:49:21 PM »
Replaced all but one, I don't have a 2.2uf kicking around. No change.

Though I have noticed that both when the amp is powering up, and when it's powering down I hear the effect change as if it starts to oscillate as components drain.

It's got me stumped, because it's not just the missing oscillation, the intensity control has no effect either -- but I can hear the wah effect when it's turned on, it just doesn't change in intensity and it doesn't oscillate. It's like this whole control board is being ignored.
« Last Edit: September 01, 2019, 04:47:28 PM by substatica »

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #7 on: September 01, 2019, 06:44:56 PM »
Also, the schematic calls for 24 VDC onto that board and it's getting 26 VDC, would that present a big issue for the circuit?

mandu

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #8 on: September 01, 2019, 10:08:04 PM »
Did you check the 470 mfd emitter bypass capacitor? If low value, then less gain at the very low frequency setting and the oscillator will start and stop at power up. You can temporally place a similar capacitor to test. Regards

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #9 on: September 02, 2019, 12:03:36 AM »
Did you check the 470 mfd emitter bypass capacitor? If low value, then less gain at the very low frequency setting and the oscillator will start and stop at power up. You can temporally place a similar capacitor to test. Regards

Yeah I've replaced all the caps except for the 2.2 mfd, though I can re-check the 470 mfd tomorrow.

I swapped out the 2SC828 with a 2N3904, which still doesn't solve the issue but I can get some variation in the effect messing with the 500K trimpot and the 270k resistor beside it. At this point the only items I can't fully test are the Transistors and JFET. They pass the basic tests. I don't have drop in replacements for the 2SC644 or the JFET on hand.

phatt

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #10 on: September 02, 2019, 08:23:09 AM »
Maybe check the trim and pot wipers are actually in contact with tracks.
That can stop it from working.
Phil.

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #11 on: September 02, 2019, 08:30:51 AM »
Maybe check the trim and pot wipers are actually in contact with tracks.
That can stop it from working.
Phil.

Yeah they are, I took both of the off, checked/cleaned them and remounted them.

While I'm replying, here's the layout diagram for this board spruced up with a bit of color.

substatica

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Re: Yamaha YTA-95 Wah/Tremolo Speed/Intensity not functioning
« Reply #12 on: September 02, 2019, 08:54:26 AM »
Here's a video of the current behavior,

https://youtu.be/9_6zlbHQyQo

And here's a shot of the board right now, after replacing all but one of the caps and one transistor,
« Last Edit: September 02, 2019, 09:30:10 AM by substatica »

 

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