Issue with Thermionic Deluxe (PCB getting 300 degrees hot)

[joe99812]

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!

 

[giovanni]

What a majestic oscillator!
What Chuck said.

 

[joe99812]

Cool, I'll clean it up.

The IC's are plugged in and I was testing it with all pots installed originally. I removed the low-gain channel pots so that I could see the PCB.

Thanks for the help!

 

[Chuck D. Bones]

Make sure the socket contact are all clean too, flux likes to get in there and make a sticky mess.
We should focus on getting Channel A working, so pull the ICs from Channel B to ensure that Channel B cannot interfere with Channel A testing.

 

[joe99812]

Chuck, to answer another one of your questions:
1. I unplugged the IC's from the low gain channel and same thing is happening
2. I plugged the pedal in to my guitar and get the same squealing noise if I'm not strumming. If I send signal through the pedal by playing the squealing goes away and I hear the pedal doing stuff..

Still need to inspect the solder joints and make sure there are no bridges. I scrubbed the board a lot.

 

[Chuck D. Bones]

Can you show us some better pix, post cleaning?

 

[joe99812]

Updated pictures (I have been unable to remove 100% of the whiteish residue, I've scrubbed this thing a LOT with a toothbrush and wiped it down as best I can with a cloth to pick up residue):

https://imgur.com/zNBc5yb

https://imgur.com/Ej1VPv2

https://imgur.com/cd2Sk4h

I did find one solder joint on the low-gain channel that was bad so I fixed it (but I'm not testing the low-gain channel right now). I also put my iron on every other joint to heat it up again. None of them needed more solder, I was just trying to reheat all the joints in case one was bad.

One thing I did notice was that on my IC's some of the pins have about 4.5v, one was 9v, and one other one was at 3.x volts (forget exactly). I thought that was a bit odd. This was on both the high and low gain channels first IC (IC2A and IC2B).

Not sure what else to test at this point. I do have a cable rigged up to do a signal check throughout the circuit. I noticed that the noise is introduced after the first resistor in the "IN A" circuit.

Given that both channels have similar noise (just at different levels), I seems like whatever is causing it should be in circuitry that both channels share. Another alternative would be that I screwed up the exact same thing in both circuits, or I have damaged pots/capacitors in both circuits. Not sure how to test any further though so advice would be appreciated.

 

[joe99812]

Did more testing:
1. If I plug this pedal in to my amplifier and this pedal is unpowered AND the switch is set to bypass, I still get a hum coming through the amplifier that is not present if I just plug my guitar indirectly in to my amplifier
2. If I plug this pedal in to my amplifier, switch the pedal on, and switch to the low-gain channel that currently has no pots or IC's, I get the same baseline level of noise that I get from point #1

 

[joe99812]

Alright, I think I'm just being an idiot and the pedal is working fine..

I plugged my guitar in to other pedals that have a true bypass and that are off and they have the same feedback coming through as what I described in #1 and #2 above. I think that's just normal.

I dialed the volume knob WAY back. The volume knob is freaking crazy on this thing and boosts the volume WAY WAY louder than the clean signal. When I dialed the volume knob so that my overdriven tone (gain to full) was a bit louder than my clean tone, I don't have any of the squealing. I do have some humming, but as you noted, this is probably just because I have the gain CRANKED. I also get similar humming through my amplifier if I crank the gain (although the total volume of the amplifier would be a lot louder than what's going through the pedal, but still, lots of gain).

The pedal also sounds pretty good to my ear with the volume dialed back to a more reasonable level.

Maybe if someone else has this pedal they can confirm that cranking the gain + volume knob leads to some crazy feedback coming through? In any case, I'm thinking that the pedal is actually working.. Sorry for all the questions, and some of this seems to have been nonsense on my end but it did at least lead to my PCB getting cleaned up a lot so thank you for the tips.

 

[mjh36]

Maybe if someone else has this pedal they can confirm that cranking the gain + volume knob leads to some crazy feedback coming through?

Yeah the level controls on some of the pedals these days are powerful it surprised me too. I made a vero board Runoffgroove Tonemender that barely sees above zero. Plus, it depends on your source volume.

I probably run my Thermionic Deluxe at about 10-25% on the volume knob (have not ran it maxed out so can't confirm but I'm sure there'd be squealing). What I like to do as I make all these new pedals, or in general, is turn my amp/speaker level to where I want it first, then turn on the pedal with the volume at 0% and turn it up slowly to until unity volume with my amp.

The board usually will pick up a little more noise anyways while outside the enclosure. So if the knobs are working better and it's sounding normal, I'd say box it up again and see how the hum is once enclosed and with lower pedal volume but higher amp volume to get it back to where you need it.

 

[Chuck D. Bones]

I noticed in the first video clip that two of the internal LEDs are glowing brightly, indicating a large signal at IC3A-1. This is not normal when there is no input.

 

[joe99812]

Chuck, how would I troubleshoot why that is?

I manually checked the resistance of all the high gain channel resistors and they all look correct (minus the couple that cannot be checked on board). I noticed that when I switch to low gain channel, the 2 high gain channel LED's you are talking about do not dim at all. When I switch from low-gain channel to high-gain channel, the low-gain channel LED's seem to turn off. The low-gain channel LED's do turn on when switching to the low-gain channel, even without any input.

Voltage readings for the high gain channel (I am running on a 9v battery that doesn't have full charge):
IC2:
Pin1: 4v
Pin2: 4v
Pin3: 3v
Pin4: 0v
Pin5: 3.5v
Pin6: 4v
Pin7: 4v
Pin8: 7.5v

IC3:
Pin1: 4v
Pin2: 4v
Pin3: 4v
Pin4: 0v
Pin5: 4v
Pin6: 4v
Pin7: 4v
Pin8: 7.5v

IC4:
Pin1: 4v
Pin2: 4v
Pin3: 4v
Pin4: 0v
Pin5: 4v
Pin6: 4v
Pin7: 4v
Pin8: 7.5v

 

[Chuck D. Bones]

Your IC voltages are all good. You're using a cheap DMM that's loading down the circuit for some of the measurements.
1) Do you have a DC power source? We're gonna run down that battery troubleshooting this and we don't want to have to stop in the middle because you're out of batteries.
2) Do you have an audio probe and know how to use it?

 

[joe99812]

Yeah I have a DC power source. Yes I have an audio probe. I have watched videos on using so, so I think I know how to use it?

 

[Chuck D. Bones]

Put the board back in the box. We want the benefits of the box's grounding and shielding. You can leave out the Channel B pots and opamps for now.
Set all of the controls to zero except set TREBLE & GAIN to max.
Apply power.
Start at the input jack, probe there. You should hear the same sound engaged or bypassed.
From here on, leave the pedal engaged.
Probe these points:
IC2A-1
IC2A-7
IC3A-7
IC3A-1
IC4A-7
IC4A-1
We have just walked down the entire Channel A signal chain from input to output. At some point, the sound will be abnormal (noisy, oscillating, whatever). The problem will be between there and the last place it sounded good.

 

[joe99812]

Thanks for the tips!

I just did the probe walk. IC2A-1 sounds clean. IC2A-7 has a little bit of gain sound, if I turn the volume of my looper pedal off (i.e. not feeding any signal) there is a very very soft hum.

IC3A-1 is a LOT louder than the IC2 signals. With volume of my looper off, it has a fairly loud hum. With the looper sending a signal you can hear the gain (more than IC2A) plus you can hear the hum if the signal getting sent is too quiet.

IC3A-7 is similar to IC3A-1, except a bit quieter.

IC4A-1 has no signal.
IC4A-7 is similar to IC3A-7.

I can start probing the individual components between IC2 and IC3 now.

Note that I did this with the PCB outside of the case. I can install it in the case if it's essential but it's sort of a PITA to keep installing and uninstalling.

 

[joe99812]

Also I just wanted to clarify something: If I have this pedal plugged in to my looper pedal and my looper pedal is supplying no signal, those LED's are off. If I supply input (even a little input) the LED's will turn on. At super low input they will flicker a bit and then they'll fully turn on as I increase volume.

If I unplug the input from this pedal, the lights are always on and there is a hum. Note of course that this is outside of a case.

 

[joe99812]

I've been using my audio probe and here's what I've found so far.

I am testing both the signal from the looper pedal and with the looper volume turned all the way down so no signal is provided.

Here's what I'm seeing with my audio probe:

R11A Right side: Humming not loud, guitar signal sounds fine (little gain) with input
R11A Left side: No humming with zero signal. Can only hear anything if the signal is set really loud and what I hear sounds faint and a bit distorted, like the signal isn't coming through all the way.

R12A Right Side: Noticeable louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.
R12A Left side: No humming with zero signal. Can only hear anything if the signal is set really loud and what I hear sounds faint and a bit distorted, like the signal isn't coming through all the way.

D1A Right Side: No humming with zero signal. Signal sounds like fine when provided (normal volume)
D1A Left Side: Noticeably louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.

D2A Right Side: No humming with zero signal. Signal sounds like fine when provided (normal volume)
D2A Left Side: No humming with zero signal. Signal sounds like fine when provided (normal volume)

D3A Right Side: Noticeably louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.
D3A Left Side: Noticeably louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.

D4A Right Side: No humming with zero signal. Signal sounds like fine when provided (normal volume)
D4A Left Side: Noticeably louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.

C8A Right Side: No humming with zero signal. Signal sounds like fine when provided (normal volume)
C8A Left Side: Noticeably louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.

R13A Right Side: Quieter than the left side of the same resistor. None of the loud humming with no signal. With signal it has a different distortion sound coming through that sounds sort of strange but that I don't hear in the final signal when I use the pedal.
R13A Left Side: Noticeably louder than base line. I get the humming with no signal and a lot of gain and some of the humming with signal.


Looking at the wiring diagram, the similarities make sense as a bunch of these parts are wired up together.. The fact that the right side of R13A loses all the humming I was hearing makes me think maybe the humming isn't an issue after all. Not really sure if I'm chasing a dead end here.

 

[joe99812]

Also I just wanted to clarify something: If I have this pedal plugged in to my looper pedal and my looper pedal is supplying no signal, those LED's are off. If I supply input (even a little input) the LED's will turn on. At super low input they will flicker a bit and then they'll fully turn on as I increase volume.

If I unplug the input from this pedal, the lights are always on and there is a hum. Note of course that this is outside of a case.

Also after more testing:
If I lower the gain then even unplugging a the input cable will not turn the light on.

Just want to make sure there's actually a problem here.. If I crank the gain to max then I get a decent amount of noise coming through my amplifier when not playing but only hear it coming through the amp if I am sending really low signal to the pedal. Otherwise I don't hear noise coming through the amplifier.

 

[Chuck D. Bones]

Thanks for the tips!

I just did the probe walk. IC2A-1 sounds clean. IC2A-7 has a little bit of gain sound, if I turn the volume of my looper pedal off (i.e. not feeding any signal) there is a very very soft hum.

IC3A-1 is a LOT louder than the IC2 signals. With volume of my looper off, it has a fairly loud hum. With the looper sending a signal you can hear the gain (more than IC2A) plus you can hear the hum if the signal getting sent is too quiet.

IC3A-7 is similar to IC3A-1, except a bit quieter.

IC4A-1 has no signal.
IC4A-7 is similar to IC3A-7.

I can start probing the individual components between IC2 and IC3 now.

Note that I did this with the PCB outside of the case. I can install it in the case if it's essential but it's sort of a PITA to keep installing and uninstalling.

IC2A-1 should be clean and a little louder than the input jack.
IC2A-7 should be considerably louder and distorted with GAIN maxed. Turning GAIN all the way down should clean up the signal and it will be a little louder than IC2A-1.
IC3A-7 should be louder and more distorted than IC2-7 if GAIN is set to zero. With GAIN maxed, IC2-7 and IC3-7 should sound about the same.
IC3A-1 should be a little louder than IC3A-7.
IC4A-7 should sound about the same as IC3A-1, depending on the PRES, TREBLE and BASS settings.
If you turn VOL up, then you'll get signal at IC4A-1. At max VOL, IC4A-1 should be the same loudness as IC4A-7.

Seems like it's all ok, but verify that GAIN and VOL do what I say they should do.

The other measurements you made seem normal. Some of the places you probed and not good places to probe.

I'm starting to think that there's nothing wrong. This is a high-gain pedal and when you crank the GAIN control, it's gonna amplify everything, including any hum and noise that's around. A closed box should help minimize the hum and noise pickup.

 

[joe99812]

Thanks Chuck, that is extremely helpful. I just went through all the different points you mentioned and I believe my pedal passes the test. Gain and volume do exactly what you said they should do. Thank you so much for taking the time to help me with this!

I'm going to get the pedal all assembled and see how it sounds once I have it in the enclosure. I've played with it a bit out of the enclosure and it's pretty awesome. I probably just need some sort of noise gate if I'm going to really crank the gain so I can get rid of the no-signal humming.