[sdiy] Help trouble shooting mfos echo rockit

[Steve Lenham]

According to p2 of the schematic, this circuit is operating from a 
single 9V battery, with a half-rail "ground" generated by a potential 
divider.

Joe's pin13 reading indicates that half-rail is only at 3.76V rather 
than 4.5V, but perhaps that's normal for this circuit.

What looks most concerning to me are the DC readings around the output 
of U4A (the first half of the filter). The outputs of the OTAs are 
buffered by a high-impedance JFET and then again by the internal buffers 
of the 13600.

What we see is 8.4V at the output of the first OTA (p5), i.e. saturated 
high, but nothing (0.3V) coming out of the JFET buffer. This goes to the 
input of the 13600's Darlington buffer at p7. The output of that buffer 
is sitting at 1.28V, but this is coming via the various external 
resistor networks - the buffer itself is biassed off.

In a nutshell, I would point the finger squarely at JFET Q1, either the 
device itself or the soldering around it.

Hope this helps!

Cheers,

Steve L.
Benden Sound Technology


On 21/11/2022 10:34, Tim Stinchcombe wrote:
> Hi JoeF,
>
> These three readings show that your 13700 isn't getting a proper negative supply:
>
>> 1.25-1
>> 0-6
>> 1.25-16
> (i.e. pin 6 is at ground, and this is reflected in the pin 1 & 16 readings too).
>
> >From the pin 11 reading I am assuming you are trying to run off +9, -9V split supplies:
>
>> 9.14-11
> Thus you should see -9V at pin 6, and approx. -9+1.2 = -7.8V at both pin 1 and 16.
>
> So check your negative supply rail is getting around the board properly.
>
> Tim
> __________________________________________________________
> Tim Stinchcombe
>