Strange problem with Boss SD-1  [SOLVED]

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InTelejens
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Post by InTelejens »

I have a vintage Boss SD-1 that works when engaged but no sound when bypassed.

I’ve searched online to see if anybody else has experienced this same issue. Only common thing that I found was the opposite problem- works in bypass mode but no sound when engaged.

Anybody have any ideas?

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Manfred
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Post by Manfred »

First bridge drain to source of Q1 and see if the signal comes out.
BossSD-1_Schematic.jpg

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CheapPedalCollector
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Post by CheapPedalCollector »

Bad Q2 maybe

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InTelejens
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Post by InTelejens »

Manfred wrote: 05 May 2021, 15:58 First bridge drain to source of Q1 and see if the signal comes out.
BossSD-1_Schematic.jpg
I’ve bridged the drain and source of Q1 and the signal came back.

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Post by Manfred »

I’ve bridged the drain and source of Q1 and the signal came back.
Ok, then measure the voltage at the gate of Q1 in both switching states.
Is there a noticeable voltage difference between the states?

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Post by InTelejens »

Manfred wrote: 07 May 2021, 16:34
I’ve bridged the drain and source of Q1 and the signal came back.
Ok, then measure the voltage at the gate of Q1 in both switching states.
Is there a noticeable voltage difference between the states?
I measured the voltage of Q1 in both switching states- no change, 0V both states.

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Post by Manfred »

Measure the voltages on the collectors of Q3 und Q4 in both switching states.
Does the illumination of the LED change between on and off when switching?

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Post by InTelejens »

Manfred wrote: 08 May 2021, 06:27 Measure the voltages on the collectors of Q3 und Q4 in both switching states.
Does the illumination of the LED change between on and off when switching?
Q3 collector reads 6.29V bypassed and 0V engaged.

Q4 collector reads 0V bypassed and 6.3V engaged.

LED illumination doesn’t change between on and off when switching.

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Post by Manfred »

The switching control circuits works.
Measure the voltage on the cathode of the diode D7 for both switching states
and check if pin1 of the Level pot ist still connected to ground.

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Post by InTelejens »

Manfred wrote: 08 May 2021, 10:22 The switching control circuits works.
Measure the voltage on the cathode of the diode D7 for both switching states
and check if pin1 of the Level pot ist still connected to ground.
Voltage on the cathode of D7 reads 0V bypassed and 5.75V engaged.

Pin 1 of the Level pot is connected to ground.

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Post by phatt »

I'm confused? :scratch
The Schematic above shows pin 1 of the Level pot is connected to V bias not ground/Common.
Phil.

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Post by Manfred »

phatt wrote: 09 May 2021, 11:20 I'm confused? :scratch
The Schematic above shows pin 1 of the Level pot is connected to V bias not ground/Common.
Phil.
You are right, my fault.

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Post by Manfred »

Voltage on the cathode of D7 reads 0V bypassed and 5.75V engaged.

Pin 1 of the Level pot is connected to ground.
Sorry, Phatt is right, my mistake.
At pin 1 of the potentiometer must be about 4.5 volts to measure.
if you can measure this 4.5 V there, then D7 or Q1 can be defective.
You can check the diode by ohmmeter.
Without operating voltage you can also check the JFET with the ohmmeter, there should be a resistance in the ten ohm range between drain and source.

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Post by InTelejens »

Manfred wrote: 09 May 2021, 15:36
Voltage on the cathode of D7 reads 0V bypassed and 5.75V engaged.

Pin 1 of the Level pot is connected to ground.
Sorry, Phatt is right, my mistake.
At pin 1 of the potentiometer must be about 4.5 volts to measure.
if you can measure this 4.5 V there, then D7 or Q1 can be defective.
You can check the diode by ohmmeter.
Without operating voltage you can also check the JFET with the ohmmeter, there should be a resistance in the ten ohm range between drain and source.
I measured 4.6V at pin 1 of the potentiometer. Diode didn’t have any resistance at the drain and source.

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Post by Manfred »

I measured 4.6V at pin 1 of the potentiometer. Diode didn’t have any resistance at the drain and source.
The diode does not have source at drain, ich meant the JFET Q1.

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Post by InTelejens »

Manfred wrote: 09 May 2021, 20:57
I measured 4.6V at pin 1 of the potentiometer. Diode didn’t have any resistance at the drain and source.
The diode does not have source at drain, ich meant the JFET Q1.
Sorry, I meant to say JFET Q1 has no resistance at drain and source.

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Post by Manfred »

As already mentioned, without power supply or battery the resistance between drain and source must be low.
If in your SD-1 the 2SK30A type for signal switching are used, this resistance is about 400 ohms.
I read this resistance value of the 2SK30A on one of my effect units with identical switching circuit.
This value is higher than expected, with the typical JFET for switching application this value is in the range of some tens of ohms.

If the measured value has a very high resistance or is not measurable then Q1 is probably defective.

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Post by InTelejens »

Manfred wrote: 09 May 2021, 22:17 As already mentioned, without power supply or battery the resistance between drain and source must be low.
If in your SD-1 the 2SK30A type for signal switching are used, this resistance is about 400 ohms.
I read this resistance value of the 2SK30A on one of my effect units with identical switching circuit.
This value is higher than expected, with the typical JFET for switching application this value is in the range of some tens of ohms.

If the measured value has a very high resistance or is not measurable then Q1 is probably defective.
I used an audio probe and traced the input signal path and noticed that the signal got quiet after R1 into C1.

I noticed that R2 had been replaced with a 2.2K resistor, looks like it’s the mod value for R6- botched mod by the previous owner!

I removed the 2.2K resistor and replaced it with the stock value 470K- the pedal works in bypass mode again.

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