Boss - TW-1 issue

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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Stevenrb718
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Post by Stevenrb718 »

I have a Boss TW-1 that doesn't power up. The bypassed signal is always present. The LED doesn't light, but the momentary switch is tested good. The thing that confuses me is when checking voltage throughout the pedal, the points show no voltage in relation to the ground. 9V is on many points in relation to the negative battery terminal however. I'm not sure if this is a normal feature of an 80s Boss ACA pedal.

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x-tn
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Stevenrb718
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Post by Stevenrb718 »

I'm not looking to mod it to PSA specs, I'm looking to diagnose why the pedal isn't working.

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plush
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Post by plush »

Stevenrb718 wrote: 20 Jan 2021, 17:05 The thing that confuses me is when checking voltage throughout the pedal, the points show no voltage in relation to the ground. 9V is on many points in relation to the negative battery terminal however.
Are you measuring it with the input jack connected? Disconnecting input jack terminates power supply's path to ground (this is made to stop battery from draining when there's no cord plugged in). Plug input jack and re-measure.

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Stevenrb718
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Post by Stevenrb718 »

plush wrote: 23 Jan 2021, 17:17
Stevenrb718 wrote: 20 Jan 2021, 17:05 The thing that confuses me is when checking voltage throughout the pedal, the points show no voltage in relation to the ground. 9V is on many points in relation to the negative battery terminal however.
Are you measuring it with the input jack connected? Disconnecting input jack terminates power supply's path to ground (this is made to stop battery from draining when there's no cord plugged in). Plug input jack and re-measure.
Yes I am, and the wiring for the plug detection seems to be correct according to the schematic. Now on-board function of the detection circuit I'm not sure.

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x-tn
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Post by x-tn »

Stevenrb718 wrote: 23 Jan 2021, 17:07 I'm not looking to mod it to PSA specs, I'm looking to diagnose why the pedal isn't working.
The page I linked says:
"some of the older Boss pedals (those with an ”ACA” sticker by the adapter jack) won’t work right with a normal 9vDC power supply. Keeping them on a daisy chain supply, with power and signal cables connected to at least one normal (PSA) pedal makes them light up, but in all other instances they need 12 volts DC on the adapter jack."

And you talk about 9V in your initial post. I thought you try to power it with 9V and it doesn't work because of that.

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Stevenrb718
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Post by Stevenrb718 »

x-tn wrote: 23 Jan 2021, 21:54
Stevenrb718 wrote: 23 Jan 2021, 17:07 I'm not looking to mod it to PSA specs, I'm looking to diagnose why the pedal isn't working.
The page I linked says:
"some of the older Boss pedals (those with an ”ACA” sticker by the adapter jack) won’t work right with a normal 9vDC power supply. Keeping them on a daisy chain supply, with power and signal cables connected to at least one normal (PSA) pedal makes them light up, but in all other instances they need 12 volts DC on the adapter jack."

And you talk about 9V in your initial post. I thought you try to power it with 9V and it doesn't work because of that.
I got ya. No, my reference to 9V was knowing that the ACA pedals take a 12V supply and regulate it internally with the resistor/diode. I actually have an ACA spec adapter. Unfortunately, the pedal has the same problem regardless of power supply. I was just questioning why voltage tests were showing relative to negative circuits rather than ground. I do appreciate the ideas.

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CheapPedalCollector
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Post by CheapPedalCollector »

Post some pics of the circuit board.

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Post by x-tn »

Stevenrb718 wrote: 20 Jan 2021, 17:05...the points show no voltage in relation to the ground. 9V is on many points in relation to the negative battery terminal however.
Stevenrb718 wrote: 23 Jan 2021, 22:07I was just questioning why voltage tests were showing relative to negative circuits rather than ground. I do appreciate the ideas.
There is a 470ohm resistor and a diode between the GND and the power input negative. The schematic is not so clear but I think they are R48 and D8. One or both of them should be failing. You can try to measure them without removing. Just don't plug a jack on the input while measururing them.

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