mxr distortion + repair question

[9 volts]

Hi there, I'm having troubles fixing my mxr distortion. It works for about 30 seconds then begins to get blatty. I have replaced the 1uf caps and tried different ic's. Weirdly if I reduce what is r3 on the ggg scheme from 1m to 500k it sounds great.....which gets closer to the resistor used in the 250 dod. Any thoughts? Theres not too many other components in there....
Thanks

[anotherjim]

If the caps you replaced are polarized electrolytic then be certain you put them in the right way around. C1 in particular.
For a time delay fault like described, I suspect the 1M+1M rail-splitter and its cap (R2,R3,C1). To test, measure the DC voltage on the 741 output pin6. It should be stable at half the supply voltage.

[9 volts]

Thanks, I'm getting 8.4 volts on pin 6 (output). Will check the 1m resistors.

[9 volts]

No luck so far. I've changed all 1m resistors. Still have the same voltage on output of ic. I've tried a few ic's with the same result. So weird.. this is an original pcb so shouldn't have track issues.

[Slowpoke101]

Please post the voltage readings on all the pins of the IC. This will help greatly.
Pin 3 should be at about 4.5V DC. It is fed via a 1M resistor going to the junction of 2 1M resistors. That junction forms the voltage divider which provides the virtual ground voltage (4.5V DC ). Please note that there is usually a 1uF capacitor connected to that junction and ground. If the capacitor is shorted there will be no 4.5V available.

Also measure the resistor between pins 2 and 6. It should read about 1M but remove to IC first (if possible ). Also make sure that power is removed when measuring resistance.

[antonis]

I should say that IC is permanently hitting positive supply rail but I'm not sure if 741 output can go so close to 9V..

Measure pin6 voltage at various Distortion pot settings - in case of different DC readings you've shorted C3 (cap between pin2 & 4k7 resistor)..

In case of permanent 8.4V reading you probably have open NFB loop.. (R4 disconnected either from pin6 or pin2..)

[9 volts]

Awesome! It was a shorted c3. I measured pin6 and got different dc reading when turning distortion. I did a quick substitute of c3 the cap between distortion pot and pin 2. All systems go!
Thanks again. You guys are great!