Lab Rat - slow signal loss

[sclemmer]

I finished my Lab Rat build this evening and I am having an issue where the signal is weak when I turn on the pedal and slowly fades to nothing after 3-4 minutes. Here’s how I got here:
1. Boxed up the pedal and plugged it in, low volume even with the volume maxed out. Overall signal slowly faded to nothing, I used an LM741 instead of the LM308 so I thought maybe that was the problem.
2. Swapped out the LM741 for an OP07, went to plug the pedal back in and realized it was on a 100ma port on my Cioks. Moved it over to a 400ma port and volume was where it should be. (Or so I thought)
3. Played for a bit and started to adjust the gain. Half way through the rotation of the gain pot, the signal completely cut out. The 9mm A100k pot I got from Tayda was not the normal Alpha but a darker green cheaper looking and feeling pot. I somehow had another “real” 9mm Alpha 100k so I pulled the cheap one and put this “real” one in.
4. Plugged the pedal back in, gain now works as it should but the low volume that slowly fades to no signal is back.

Any ideas? How much power does this pedal need and what would be causing the low volume and gradual complete loss of signal?

 

[vigilante398]

A picture is worth a thousand words, potentially more when debugging. Can we see the build?

 

[sclemmer]

 

[vigilante398]

Weird question: for the clipping switch, did you use On/On or On/Off/On? And does it have the same behavior in all positions of the clipping switch?

 

[sclemmer]

Clipping switch is On/On purchased from Tayda via the link in the BOM, pedal behaves the same on both switch positions. Also, I thought the issue with the gain pot was resolved but, when the pedal starts dying, the output gets extremely gated once you get past noon on the gain pot. One more thing, the pedal is no longer completely losing output, it just drops to about 10% of what it was when first turned on.

 

[vigilante398]

Clipping switch is On/On purchased from Tayda via the link in the BOM, pedal behaves the same on both switch positions. Also, I thought the issue with the gain pot was resolved but, when the pedal starts dying, the output gets extremely gated once you get past noon on the gain pot. One more thing, the pedal is no longer completely losing output, it just drops to about 10% of what it was when first turned on.

Still a weird symptom. Have you (carefully) measured your high voltage rail? I would be interested to see what it's at when it starts up and what it's at when the volume drops.

 

[sclemmer]

Still a weird symptom. Have you (carefully) measured your high voltage rail? I would be interested to see what it's at when it starts up and what it's at when the volume drops.

I have not, can you tell me where to measure that voltage from?

 

[vigilante398]

I have not, can you tell me where to measure that voltage from?

My recommendation is to stick the negative probe of the multimeter in a screw hole of the enclosure and the positive probe on the cathode of the UF4007. I use the enclosure hole because it can hold the probe without needing your hand in it, and when sticking your hands in and around high voltage circuits, you should only use one hand at a time so you don't risk passing high voltage across your heart/organs.

 

[sclemmer]

Understood. Will get that measured and report back.
And thank you, I appreciate the help so far.

 

[sclemmer]

When I powered it on, the voltage was 254.
After 15 minutes of playing, voltage was 240.
I played through both clipping options and that did not seem to affect the voltage. However, the gain control still has the issue where past noon, the sound starts cutting out and at full clockwise position there is no sound.

 

[vigilante398]

Can you verify that you used the correct value for the gain pot? I've had a couple times where I inadvertently used a A1M for the gain and A100k for the level, and it had similar symptoms. The gain control is in the feedback loop of the LM308, so if it's significantly larger than expected, you could cause issues with the opamp.

Are you still using the 741 in it, or did you swap it for the OP07?

 

[sclemmer]

For sure A100k on the gain. I also have the OP07 still in there. More pics.

 

[sclemmer]

Another strange behavior, the cut off point of the gain pot changes depending on the IC.
Op07 - cuts at 50%
TL071 - cuts at 75%
I don’t have any LM308 to try unfortunately.

 

[sclemmer]

I took a break from this to build a Nobelium, which worked and sounds phenomenal. Is there anyone who can share IC voltages or something so I can figure out what is wrong with this thing?

 

[sclemmer]

I found a MadBean Slow Loris I built probably 10 years ago that had an LM308 in it. The pedal never worked and I didn’t troubleshoot back then so I robbed the chip and threw it in the Lab Rat. Fired it up and no sound. Guess I now know why the Slow Loris never worked…
Anyhow, I decided to put the LM741 back in the Lab Rat and give it another shot. To my surprise everything works, gain pot turns all the way up without cutting out now!
Thanks for the help!