Xotic BBpreamo bass no Gain

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shiflet
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Post by shiflet »

Good afternoon.
I have a xotic bb preamp bass that when activated does not generate any distortion. At the maximum travel of the potentiometer it is as if it were in the 1/4 of the same

I have changed the two ICs for testing, also change the 1N4148 diodes, but still the problem persists.

I have noticed that in the Volume potentiometer the voltage fluctuates between 4.5V and drops to 2V, without moving the potentiometer.

In the 10uF capacitor connected to the potentiometer, the voltage also fluctuates between 4.5V up to 2V, change it but the problem still persists.

The footswitch works smoothly.

I am following this scheme, but I cannot detect the problem.

Regards
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plush
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Post by plush »

Measure voltages at each semiconductor pin and post them here or mark on the schematic

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shiflet
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Post by shiflet »

Thanks for answering, these are the voltages:

Q1

E 3.08
C 9.10
B 3.56

Q2

E 3.09
C 9.10
B 3.67

IC1

1 4.48
2 4.50
3 4.48
4 0
5 4.50
6 4.69
7 4.48
8 9.10

IC2

1 4.51
2 4.55
3 4.51
4 0
5 4.50
6 4.52
7 4.51- 4.52
8 9.09

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Manfred
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Post by Manfred »

Does the base voltage at Q2 also change when you move the volume potentiometer?

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Post by shiflet »

Manfred wrote: 03 Jun 2020, 13:49 Does the base voltage at Q2 also change when you move the volume potentiometer?
When moving the volume pot, the voltage of Q2 does not change

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Post by plush »

shiflet wrote: 02 Jun 2020, 21:44
I have noticed that in the Volume potentiometer the voltage fluctuates between 4.5V and drops to 2V, without moving the potentiometer.

Volume pot is tied to bias voltage and AC coupled with nearest stages.

Fluctuation means that bias is not stable or/and coupling caps are shorted or defunkt.

Check C14 and C15 for possible shortages (desolder at least one pin of each and measure dc resistance)

Check resitances of R25-R26, check C17, C18 and D6 for possible shortages or malfunction.

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Post by shiflet »

plush wrote: 03 Jun 2020, 14:25
shiflet wrote: 02 Jun 2020, 21:44
I have noticed that in the Volume potentiometer the voltage fluctuates between 4.5V and drops to 2V, without moving the potentiometer.

Volume pot is tied to bias voltage and AC coupled with nearest stages.

Fluctuation means that bias is not stable or/and coupling caps are shorted or defunkt.

Check C14 and C15 for possible shortages (desolder at least one pin of each and measure dc resistance)

Check resitances of R25-R26, check C17, C18 and D6 for possible shortages or malfunction.

The resistance of C14 (10uF), depending on how you measure, positive-negative, or negative-positive, ranges from 500K to 6M. C15 (100nF), measures infinity (1)

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Post by plush »

shiflet wrote: 03 Jun 2020, 15:25
The resistance of C14 (10uF), depending on how you measure, positive-negative, or negative-positive, ranges from 500K to 6M.
500k is kinda low, I'd replace it.

Please check the rest

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Post by shiflet »

plush wrote: 03 Jun 2020, 15:35
shiflet wrote: 03 Jun 2020, 15:25
The resistance of C14 (10uF), depending on how you measure, positive-negative, or negative-positive, ranges from 500K to 6M.
500k is kinda low, I'd replace it.

Please check the rest
I have changed the 10uF capacitor, and checked the other components (the ones by the source, they are all ok) but the problem persists

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Post by Intripped »

Perhaps the potentiometer itself is bad. Sometimes one or more pins loose the mechanical connection with the internal carbon trace, creating an intermittent contact.
This could explain the fluctuating voltage.

Edit
You should have noticed that also the volume of the pedal fluctuates though...

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Post by shiflet »

Intripped wrote: 04 Jun 2020, 04:36 Perhaps the potentiometer itself is bad. Sometimes one or more pins loose the mechanical connection with the internal carbon trace, creating an intermittent contact.
This could explain the fluctuating voltage.

Edit
You should have noticed that also the volume of the pedal fluctuates though...
Yes, you are absolutely right! The volume also changes, when it is at 0, the signal continues to pass ...

What I don't understand now is, how could it be that when putting the audio probe on Pin 1 of IC1, no distortion is heard, since there should be have the greatest distortion, but it is not ...

Sorry for my Bad english

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Post by Intripped »

Mmm, nothing changes if you turn the gain pot?
... Another potentiometer gone bad?
Last edited by Intripped on 04 Jun 2020, 05:10, edited 2 times in total.

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Post by shiflet »

Intripped wrote: 04 Jun 2020, 04:57 Mmm, nothing changes if you turn the gain pot?
... Another potentiometer gone bad?
It is heard as the sound changes, but it gives little effect, as if the Gain was between 1 and 2 (assuming that the pot goes from 1 to 10)

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Post by Intripped »

No, probably not the pot here.
maybe a solder bridge somewhere around the clipping diodes, or pin 1 and 2 of the ic. In the feedback path anyway

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Post by shiflet »

Intripped wrote: 04 Jun 2020, 05:06 I've just edited my previous message
I have verified that there is no bridge between pads, which may be causing a problem, but tomorrow I will check again more thoroughly

Thx!

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Post by Intripped »

:thumbsup

... Wrong diodes orientation?

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Post by Manfred »

Hi Shiflet

The error is difficult to find without measurement, what measuring equipment do you have?

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Manfred wrote: 04 Jun 2020, 20:29 Hi Shiflet

The error is difficult to find without measurement, what measuring equipment do you have?
Hello! I have a multimeter (with which I measure resistance and voltage) and a home audio probe to look for the signal

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Post by Manfred »

With an normal multimeter you can also measure AC voltages in the lower frequency range.
The output signal of the guitar generated with low E-string or A-string should work.
Up to which output of the OP-amps you can detect the signal with the audio probe?

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