Problem with my OCD

[foo]

Hi folks,

I have just finished my first two pedals ...  an OCD V3 and a Jan Ray.

The JR is fine ... Sounds greate.

But I have some problems with the drive pot of the OCD.
It is really high gain and it starts to squeal with the drive pot at the 1oclock position.

Do you have any suggestions what could be wrong?

By the way... I use it in front of a tubemeister 5.

Thanks for your help.

Marco

[Frank_NH]

Hi Foo,

I too had this problem with the OCD (oscillation at high gain).  It was such a problem that I eventually shelved the OCD (banished to "The Island of Lost Circuits").  When I saw your inquiry I decided to retrieve my old OCD build and and check it out.  One thing that I did that was non-standard was to use a different Op Amp, an LF353N.  However, I had recently purchased a TL082 from Radio Shack and decided on a whim to pop that in, since that is the IC called for by the OCD.  Well, what do you know - no squealing!  It seems to work fine now, even at max gain and tone on max treble...

I thought the LF353N ought to work, so I checked the data sheets.  Sure enough, it's pretty close in performance to the TL082.  So I changed the TL082 for a NEW LF353N.  Hey - that works too!  Hmmm.   I guess I'm going to chalk this up to a bad chip.

Anyhow, try changing your IC and see what happens.  You can get the TL082 at Radio Shack for cheap -  perhaps get a couple.  Let me know if this works.  

By the way, I like the OCD with the 500K gain pot versus 1M.  1M is probably too much for what it does, and it gets a bit wooly for me at max gain.  

[foo]

Dear Frank,

thanks for your reply...

I think I will check my circuit and the solder joints next weekend. But I also guess that the IC is the problem.
In this case I have to wait until my next order from an electronic shop because a couple of TL082 is not
worth the shipping costs (I live in a small town in Germany).

But when I have fixed the problem I will inform you.

Marco

[foo]

It seems it was a cold solder joint.
Now it sounds awesome.

Thanks for your help.

Best regards

Marco

[Frank_NH]

Good to hear Marco.  Where was the cold solder joint?  I've been thinking that one way to minimize/avoid squealing is to have proper power supply filtering, especially on the IC bias voltage (4.5 V).  Some circuits seem to exclude some high values capacitors in the power section, though they may not be needed if you're using a battery.

[foo]

To be honest ... I don't know where the cold solder joint was.
I inspected the circuit on the backside and found the cold solder joint and after a re-flow everything was ok...

[induction]

Frank_NH wrote

I've been thinking that one way to minimize/avoid squealing is to have proper power supply filtering, especially on the IC bias voltage (4.5 V).

IvIark's layouts for the OCD (all 4 versions) have 10u caps on both V+ and Vref. That should be good enough, but those caps can be increased if necessary. I never go below 47u for filter caps, and I convert to low-noise biasing if it's not already included in the original schematic (it is included in the OCD). The filtering on Vref helps a lot for reducing hum, but I haven't had problems with oscillation, so I can't say whether it helps for that.

[Frank_NH]

I've been reading various posts on this topic at DIYStompboxes.com, and the recommended approach is a 100 uF cap from +9V to ground, and 47 uF from Vref to ground.  Some also use a current limiting resistor at the +9V input (~100 ohms).  This along with a  parallel polarity protection diode should be enough for most circuits.

For the OCD, the vref filtering is the important bit.  Of course, on mine I substituted a different IC and that fixed my problem with oscillation, so power supply filtering wasn't the issue.