Pythagoras L78L33 Temperature

slubtle

New member
Hello folks, hopefully a simple situation for you to help with. I was debugging my Pythagoras build and noticed all the spots where there ought to be +3.3v were only showing +0.3v.

So I start poking around the power regulation section and find the L78L33 is too hot to touch, which...seems not good for a bunch of reasons.

Am I right in thinking that there ought to be ~9v DC at the input, a transistor which *isn't* too hot to touch, and a lovely regulated +3.3v on the output? Given the finger-searing hotness, have I got a dud regulator or is something upstream pushing it too hard?
 

slubtle

New member
I think I found it, or at least something which doesn't seem right.

I did some probing for components with shorts to ground, and right off the bat R1 shows no resistance. Pulled that component, figuring it was a bad/cooked resistor, and there's a dead short between the pads underneath.

If the rest of the board is populated, but R1 is removed, is it correct to say there ought to be no connection between the pads of R1?
 

PJS

Active member
You are correct - there should b no connection between those points. Somewhere you have introduced a short. Most likely poor soldering. Show us some photos maybe, and visually check everything.
 

Cucurbitam0schata

Well-known member
Did you end up cooking the L78L33? Turns out I just did the same thing with my Leprechaun / Rainbow Machine. Thought I'd poke around the forum before I start poking around on my build. Thanks!
 

PedalPCB

Administrator
Staff member
If the rest of the board is populated, but R1 is removed, is it correct to say there ought to be no connection between the pads of R1?

Just now seeing this, my apologies.

If you used the wiring diagram in the build docs (or one of the 3PDT breakout boards) the pads of R1 will measure as shorted if the pedal is in bypass because the PCB input is grounded, this is normal.

In any case, shorting R1 wouldn't cause the L78L33 to overheat.


As for the Leprechaun, if you wired up an expression jack I would start there.
 

slubtle

New member
I've hit a wee bit of a wall, but suspect I've ruined the FV1 unfortunately.

Goodness only knows how, but an LT1054CP managed to get into my TL072 bin at some point in the past. When gathering the components for this build, I blindly grabbed a chip from the bin and didn't double-check it. Lesson learnt! Always check your components before installing!

Once I resolved that, my +3.3v came right. I also redid all the offboard wiring and replaced the L78L33, just in case too.

Unfortunately it hasn't resulted in a working build, but it feels a step closer. The output is still a horrendous clicking, buzzing noise, no trace of the input signal blended in and only the mix/volume pots have any effect on the sound.

I'm waiting on a hot air rework station to arrive so I can pull and redo the FV1, hopefully that helps. But as I say, I'm fairly sure I've borked it at this point. And of course sourcing another FV1 during this ridiculous chip shortage is *delightful* too 😬
 

daeg

Active member
Unfortunately it hasn't resulted in a working build, but it feels a step closer. The output is still a horrendous clicking, buzzing noise, no trace of the input signal blended in and only the mix/volume pots have any effect on the sound.
If I'm understanding right, when you turn the Mix pot to zero, and play the guitar, you hear clicks and pops?

That sounds like a misbiased opamp to me. Check the voltages of the TL072.
 

slubtle

New member
I'll definitely give that a look, but I didn't explain clearly. The mix and volume pots both act as volumes. At min value for either pot, zero sound is produced (including the dry signal). When both pots have any value above min, the output is the clicky/buzzing nightmare.

The more time goes on, the more I think the FV1 is either fried or was a fake to begin with.
 
  • Like
Reactions: fig

PJS

Active member
So, let me understand. You are getting clicks and buzzes even with the mix control set all the way dry? Or does that give you the normal dry guitar sound? Also, does bypass work?
 
Top