Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: 92Volts on April 20, 2020, 09:14:00 pm
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Hi, I was hoping somebody might have some ideas on a broken TS9 Tubescreamer-- or maybe help me understand more about this circuit.
The problem is the signal is always clean. The LED does switch on/off with the button, but the sound doesn't change.
I'm learning from this site: https://www.electrosmash.com/tube-screamer-analysis
Schematic for this part of the circuit: https://www.electrosmash.com/images/tech/tube-screamer/tube-screamer-jfet-bypass-switch.png
To allow overdrive signal, Q2 gate is biased high through D3, to shut out overdrive signal Q2 gate is biased low through D3. At any point the opposite should be true for Q4 which controls clean signal and is biased high/low through D4.
My measurements show the bottom ends of D3 and D4 (down by C11 and C12) are getting variable voltage of ~30mV up to ~7v, and "flip flop" when the switch is activated. But only one of the FETs gets gate voltage that changes while the other stays down at 30mV to 90mV
I have 2 questions (besides "how do I fix it")--
How the heck does this bias scheme work? These diodes look like they should only pull the gate down. How does the gate get pushed up when the diodes are reverse-biased and should block current?
If only the clean channel switches properly, shouldn't it switch off and kill the sound instead of staying clean when switched? If neither was switching wouldn't it give no sound?
And of course-- how do I fix it? I'm suspecting either dead diodes or transistors but I'm having a hard time understanding how or what went wrong. If one of these diodes was dead could I substitute a different one or is the behavior of it critical to the circuit?
Thanks in advance!
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looks to me to be an "A B" switch, 2 ins 1 out, tells me one in should be clean one in distorted, kick the J/K flipflop change channels. But my brain is on fog sooo :dontknow:
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what happens with only 1 signal at IN1? sound the same in either position?
now take same signal and put at IN 2, same?
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> These diodes look like they should only pull the gate down. How does the gate get pushed up when the diodes are reverse-biased and should block current?
"Block" is relative. I now "block water" in my ditch with a sand dam. Yes water gets through, but very little, and insignificant compared to the water in the other paths.
The back-leakage of a general purpose diode reliably exceeds the leakage of a JFET Gate. I see your objection but it works, in millions of audio systems.
Dead FETs are not unknown. They can be replaced with shorts/opens across D-S for diagnosis.
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Thanks for the help!
The pedal is fixed. Turns out I was overthinking it-- one of lugs on the distortion pot was shorted to the metal chassis.
I tried something similar to shooter's advice and poked a meter probe around the different sides of the switch while messing with the controls :icon_biggrin: screwing with one side shut off the sound in clean mode, and screwing with the other side of the circuit shut off the sound in distortion mode... so it was NOT the switching circuit at fault!
PRR, thanks for the explanation. This helps me understand that while the circuit was working right, I probably couldn't get a sensible voltage reading with a DMM because the input impedance of the meter would load the circuit and throw things off.
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> couldn't get a sensible voltage reading with a DMM because
Right. Too sleepy to do math, but the diode is hundreds of Meg, the JFET Gate is much higher than that, and your DMM is only 10 Meg or so. Yes, the "sensitive" DMM will actually throw the switch in or out of action.
Read what goes TO the diode. If that is right, replace diode (cheap) and any cap on that node. Then replace JFET. Then actually look for an unrelated short.......