Something is pulling down the 4.5v bias voltage...

[drummer4gc]

Schematic for reference on the last page of this pdf: http://www.synfo.nl/servicemanuals/Boss/DD-3T_SERVICE_NOTES_3rd.pdf

I've got a Boss DD-3(t), the newest version of the pedal, that outputs no sound. I've audio traced it and discovered the signal disappears in IC1A. Voltage on the chip shows proper 9v and 0v on pins 8 and 4, but the bias voltage on pin 3 is 0.92v.

I replaced C1 and C46 in the power filtering/voltage divider section to no avail. I then removed C46 completely and tested the voltage on its solder pad - 0.92v. Something is pulling down the voltage from the expected 4.5v at the junction of R63 and R59, and I'm looking for ideas on where to begin looking. Are there likely culprits for something that's developed a short? Seeing as the pedal is almost entirely SMD, I want to avoid pulling components and chips on a witch hunt as much as possible, so any educated guesses are welcomed and appreciated - thanks!

Matt

[antonis]

If you could disconnect pins 3 & 5 you should find if IC1 is defective or not..

For now, check R63 & R59 (mainly R59) for any degradation..

[duck_arse]

to the list provided by Mr. Antonis, please add: R60 & Q11, R58 & Q10, R62 and probably VR1 and 2. have you measured the resistance from the R63//R59 point to supply and ground?

[anotherjim]

I see no limit resistor on the +9v input. If a strong reversed polarity power supply has been used, the reverse protection diode could have blown open circuit leaving no protection for everything else. Test that diode. If it has blown, any number of IC's could be damaged. With SMD, use a fine tip iron and a sharp craft knife to lift all the op-amp legs directly connected to Vref.

[drummer4gc]

I was able to use a safety pin to pry up the legs for pins 3 and 5 from IC1, and my bias voltage is back! Don't have any SMD op amps lying around to complete the test, but I assume replacing IC1 will take care of it.

Thanks for the help!