Thermionic Deluxe Oscillates on tight knob

D

DiamondFlesh

New member
Hi There!
I’ve just build the Thermionic deluxe and everything is working as it should be. However, when I set the tight knob below half, the pedal is going to oscillate. Setting the trimpots on zero won’t solve the problem. It happens on both channels. Ive measured all resistors and readings are fine. Also when I disengage the pedal, the on/off led is still bleading so I assume it’s a voltage problem? Any help is much appreciated!
 
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DiamondFlesh said:
Hi There!
I’ve just build the Thermionic deluxe and everything is working as it should be. However, when I set the tight knob below half, the pedal is going to oscillate. Setting the trimpots on zero won’t solve the problem. It happens on both channels. Ive measured all resistors and readings are fine. Also when I disengage the pedal, the on/off led is still bleading so I assume it’s a voltage problem? Any help is much appreciated!
Click to expand...
Did you fix your issue with the pedal?
 
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No, I didn’t find the cause. It’s manageable and the pedal sounds good, plus I don’t think I will use the pedal with the tightknob all the way down. I think I leave it as it is unless it’s something simple. Thanks
 
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These are the only good pics I have atm. I’ve double checked everything and I can’t find any bridges. Everything works fine and the pedal sounds great. Just the oscillation part is strange, and I have to keep the trimpots on zero to keep it manageable, so it’s more a overdrive than a full on distortion. C12C7958-C922-4540-B927-6701F6901662.jpeg 11095D3E-1F52-4ADC-938F-28DE4CCD6B2E.jpeg
 
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DiamondFlesh said:
These are the only good pics I have atm. I’ve double checked everything and I can’t find any bridges. Everything works fine and the pedal sounds great. Just the oscillation part is strange, and I have to keep the trimpots on zero to keep it manageable, so it’s more a overdrive than a full on distortion. View attachment 9250View attachment 9251
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Can you post a better picture of the components, I can't make out the Resistors colour bands.
Can you confirm what is written on the side of the Trimmer's, It should say 104.
Resistors R14A & R14B are 10K, Bands should be Brown, Black, Black, Red , Brown, Can you Confirm?
 
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I dont have the pedal within reach atm but
when I zoom in on the original photo, the resistors color rings are both brown black black red brown on r14a and r14b. Also I measured and checked all resistors a few weeks ago and everything seems fine. I can’t give you a answer about the trimpots right now. As soon as I have the pedal in hands, I will check the trimpots
 
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Chuck D. Bones said:
Does turning any of the knobs affect the pitch of the oscillation?

Have you tried using a different power supply or a battery?

Any parts substitutions?
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Only the tight knob affect the pitch (until center and then it’s gone). It seems like the tight knob is acting as sort of a threshold. If I set the gainknob higher than the tightknob, it’s going to oscillate (also until center, from there it’s not oscilating anymore).

I have tried it with multiple power supplies, no notable differences.

What do you mean with parts substitutions? I just followed the pcb values, so I didn’t change any of the values mentioned on the pcb.

English is not my first language, so I hope it will be clear.
 
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The characteristic of oscillators is they need a certain amount of gain to oscillate, so it's not surprising that some of the knobs, TIGHT & GAIN in this case, act like a threshold control. The fact the the pitch changes indicates that the unintended feedback path that's causing the oscillations is wrapped around the GAIN and TIGHT controls. Do any of the other knobs affect the pitch or threshold of the oscillation?

Your wiring looks very clean and the chassis is bare metal on the inside, so that's all good.

I see insulators on the back of the top row of pots. You have insulators on the other row of pots too, correct?

Your answers are clear.
 
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Chuck D. Bones said:
The characteristic of oscillators is they need a certain amount of gain to oscillate, so it's not surprising that some of the knobs, TIGHT & GAIN in this case, act like a threshold control. The fact the the pitch changes indicates that the unintended feedback path that's causing the oscillations is wrapped around the GAIN and TIGHT controls. Do any of the other knobs affect the pitch or threshold of the oscillation?

Your wiring looks very clean and the chassis is bare metal on the inside, so that's all good.

I see insulators on the back of the top row of pots. You have insulators on the other row of pots too, correct?

Your answers are clear.
Click to expand...
No other knobs affects the pitch or threshold of the oscillation, just the tight knob. Gain does nothing with the pitch. All 14 pots has insulators and I’ve put some extra isolation tape on top of the lower pots to avoid contact with the upper pots.
 
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