Issue with Thermionic Deluxe (PCB getting 300 degrees hot)

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joe99812

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Hello,
I am building a thermionic deluxe and it does not work. I've used my kitchen infrared thermometer and found that the upper right portion of the PCB (where the power rail runs) is getting around 300 degrees. This is only happening along approximately a centimeter or so of the rail, so I don't believe it is a short. The power wire itself remains cool, and the rest of the power rail (that leads from the power wire to this spot on the PCB) is a bit over 100 degrees, obviously substantially cooler.

My theory is that the rail wire is damaged and causing high resistance. I have measured the resistance from the power +9v power wire connecting to the board to the D5 diode and it looks normal, but I'm assuming that under load there is more resistance on this part of the rail and it cannot handle the amount of current. Does that theory make sense?

If so, can I bypass this rail? The only thing I see the rail connected to looking at the PCB diagram (https://docs.pedalpcb.com/project/PedalPCB-ThermionicDeluxe.pdf) is the D5 diode. So can I just run the power wire directly to the diode and bypass the rail?

Any other idea as to what is going on?

hot spot:
https://imgur.com/a/bOSxxbI

Thanks in advance!
 
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High resistance wouldn't get hot, the voltage would drop across the trace but it wouldn't create significant heat.

It has to be a short of some sort.

Check the values of R32, R35, R36. Those look like they've been hot (and possibly the incorrect values).

It could just be the pic, but it looks like all of your 22K resistors are 22 ohm, 33K resistors are 33 ohm, etc.

I would go back and double check all of the resistor values.
 
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Oh jeez, every resistor I installed is off by an order of 1k.. Can I salvage this if I remove all the resistors and put the right ones in, or is everything toast? It looks like the IC's are all getting proper voltage and the LED's work (except the LED's for the switch status, those ones immediately burned out). I guess mostly concerned about the caps and board.

Are my resistors all being wrong likely to have caused this insane heat? Just wondering if I need to continue diagnosing that, or if the resistors are the likely culprit.
 
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The heat is from the resistors so very likely the cause they don't look like they've gone up in smoke just heat damaged rather than burnt out

You may be lucky if your capacitors are of a high enough voltage rating they will be fine and some ICs can take up to 30v TL072 is around 18v max

With electrolytics you'll see a bulge in the top or the innards coming out of the bottom if they're fried

Even if a few components are trashed the pcb will be ok
 
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giovanni said:
Out of curiosity, what happened with the incorrect values? Did you misread the BOM?
Click to expand...

When I first started building I had a little trouble with resistor values (R, K, and M) since some build docs don't use the "R" code and instead just use "ohm". I confused myself a few times using something like 10r where a 10k ought to have gone. Could be something like that.
 
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I honestly have no idea what happened with the resistors. I remember reading the list of stuff I needed, seeing the "K", and seeing otherwise identical resistors on Tayda without the "K" (like 2.2 ohm instead of 2.2kohm) and I guess I figured they were the same.. The clearly aren't, so just a massive brain fart on my part. Extremely stupid.

I had measured voltage on all the IC's and saw incoming voltage of 4.5v on some of the pins and 9v on pin 8, which I believe is expected.

The board itself is not burned and seems fine. Should I just re-order all the parts (capacitors, IC's, pots, foot switches)? FWIW the electrolytic capacitors appear undamaged. All the capacitors are at least 24v with some of them being 50v or 100v. I do not have a capacitor tester. I can look up how to test pots and IC's, not sure if that's possible with just a standard multimeter.
 
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I see two straightforward options.
1) Replace all resistors that are not the correct value. Everything else is probably fine.
2) Scrap that board and start over from scratch. You can re-use the socketed ICs.

I don't know what you know or don't know about building electronics. There are methods of removing parts with minimum stress to the board. If you need to know the specifics, just ask.
 
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Alright, I'll order resistors and give it a shot.

I have experience soldering but not working with PCB's, mainly just doing other projects like working on cars. I'm happy to have some tips. I had to remove some components and used a solder pump to remove solder and then heated up the remaining solder and inched the component out one side at a time, little by little. If there's a better way to do it I'm all ears.
 
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Cut the leads on any parts you're going to remove, then use a solder pump, solder sucker or solder braid to remove the solder from the hole and gently pull the cut lead out with tweezers. The idea is to put as little heat on the board as possible.
 
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Alright, the new resistors finally arrived and I got them installed. The pedal sort of works now.. When I have my guitar plugged in it does raise the gain.

When the guitar is not plugged in it's making some crazy sounds. I've attached a video. When there is no input cable connected to the pedal at all, it makes a high pitched squealing noise. When there is a cable connected (other end is attached to my unpowered looper pedal) I get a loud low hum.

The recording is on the high gain channel with every setting turned to maximum aside from "Tight" which is set to the minimum.

Reproduces the same if I plug in a 9v battery instead of the power supply.

I'm going to take everything apart and look for solder bridges. Any other tips? I have ensured that it is not a ground issue on the 1/4 jacks themselves by measuring resistance to ground (0) and also verifying that grounding them to the chasis with the multimeter does not affect this sound.


https://imgur.com/a/82iZeyl


Thanks for the help so far, I'm glad it at least sort of works now :D.
 
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When the pedal is switched off there is no hum.

The hum is present when the cable is connected to my powered off looper pedal (input) and amp (output). The squealing happens when I unplug the input cable from the pedal. In both situations, the pedal is powered on and in the high gain channel.

This happens both with a wall wart and a 9v battery.

Joe
 
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PCB picture:
https://imgur.com/p2XEl1d

Video of me adjusting the knobs individually to see the effect it has on the sound:
https://imgur.com/SALeL17

All of the knobs are set to maximum by default EXCEPT tight which is set to minimum. You can see that setting tight to maximum makes the squealing even louder. Volume also effects it a lot, if I turn the volume down a lot of the squealing goes away but there is still a hum.
 
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Try shorting the input and see what happens. Running a high-gain pedal with the input open invites hum and noise and is not representative of how the pedal will be connected in normal use.
Are the channel B IC sockets populated? With the pots out, it's anyone's guess what that channel is doing.
The board is filthy. I recommend that you scrub off all of the flux residue with a toothbrush and liberal amounts of IPA (isopropyl alcohol). Don't let any dirty IPA get into the pots. Then inspect all of the solder joints under magnification.
 
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