Problem with Nucleus Overdrive

[Haigster]

I need some help with this pedal, I've made 5 in a row without any problems but this one has me stumped. The signal sounds great right up until the end of the circuit and then gets really quiet and noisy (ground hum). Here's a gut shot and where I believe the problem is in the schematic. I've replaced R25 twice (it's a 220k now) but that didn't make a difference.

Appreciate the help
Haig

 

[Barry]

Have you tried swapping the opamp

 

[Haigster]

Thanks Barry,
Yes, I've tried two or three different TL072's same result every time.

 

[PJS]

What voltages are you reading in the op amp pins? Especially 5, 6 and 7. Also, do a continuity check between pin 7 and ground.

 

[Haigster]

I checked the voltages on the TL072 opamp:
pin 5: 0.37v
pin 6: 3.02v
pin 7: 8.49v
pin 8: 9.13v

I don't seem to have continuity between pin 7 and ground. Are you sure that's necessary? the schematic doesn't seem to show that. thanks for helping out.

 

[PJS]

No, the opposite. Pin 7 to ground would in fact be a problem, and would mean you would lose all signal there. The fact that it does not connect to ground is good. On the other hand, the schematic says that pin 5 connects directly to Vref, which would usually be around half your power supply (pin 8, 9.13V. Vref so low is likely to be your problem. You should do a visual check first through the Vref line and see if there is an accidental short.

 

[Haigster]

Thanks for the help PJS,
I figured it out, there was a cold joint at R28 just before pin 5. Works fine now.
Appreciate it.
Haig

 

[Haigster]

I've had a bit of time to play through the Nucleus and don't think it's functioning properly quite yet. The sound is fuzzy and loose and not at all like the demos I've heard of the Atomic Overdrive, which has more of a hot-rodded Marshall tight distortion sound. I've probed around the circuit but there's no obvious place where the signal had degraded.

Any help is appreciated.
Haig

 

[Robert]

Where did you get your JFETs? Are you sure they're authentic?

 

[Haigster]

The JFETs are all from Tayda, I know, they may be questionable but I'm not sure how to test them.