MOTM

I just completed my second 300 Oscillator. It appears that
everything works fine except the Sine and Triangle have no output.
The Saw and Pulse work fine.

Any suggestions as to where I should look? I have checked over the
board, things look fine. The output for the Sine and Tri are just
completely dead. I have not checked any voltages anywhere yet.

Please send any suggestions, I am not a EE, I do have a DVM, I do not
have a scope.

Thanks
Andy Miller

1) Be sure diodes D1 and D2 are in the right orientation.

2) Be sure there is +5V on both ends of ferrite L3.

3) Be sure you have 100pf caps in C26 and C23, NOT 104s (0.1uf)

Paul S.

----- Original Message -----
From: "gooboworks" <andy@...>
To: <motm@yahoogroups.com>
Sent: Sunday, January 27, 2002 1:44 PM
Subject: [motm] MOTM 300 problem


> I just completed my second 300 Oscillator. It appears that
> everything works fine except the Sine and Triangle have no output.
> The Saw and Pulse work fine.
>
> Any suggestions as to where I should look? I have checked over the
> board, things look fine. The output for the Sine and Tri are just
> completely dead. I have not checked any voltages anywhere yet.
>
> Please send any suggestions, I am not a EE, I do have a DVM, I do not
> have a scope.
>
> Thanks
> Andy Miller
>
>
>
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>

Hi,
I'm having a problem with one of my 300's : it occassionaly drifted a
lot and it was working fine before.

I had noticed that putting some pressure on certain locations on the
pcb could make it stable again. So I was hoping for a bad connection
somewhere. I used my multimeter to check the connections that I thought
had an influence on the oscillators pitch, but could not find anything
strange.
Then I started measuring voltages and compare them to one of my 300's
that is working OK. There is a lot of fluctuation in the voltage
controlling the pitchof the broken 300, whilst (as to be expected) on
my other 300 this has a stable voltage.
I kept measuring and I think I traced the error to the part of the
circuit that is generating the VX and VREF voltage.(circuit diagram 1
of 3) :
When I disconnect L3 and D3 from the TL074, the voltage output on pin 1
of the TL074 is still fluctuating.
I have checked the voltage supply to the TL074 and the ones to resistor
R17 and R16 and they appear to be stable.
When I remove the feedback resistor R18, the fluctuation stops on the
output of the TL074.
So I believe that the TL074 could be the troublemaker, but how could I
easily check this without having to replace this part first ?
Could a failed TL072 cause such behaviour ?

Thanks in advance for any help !
steven