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the decay knob on my swah doesn’t seem to do anything. I’ve taken it apart and checked I used the right value pot. I checked R25 is 4k7. where do I go from here?
I would check to see if you have about 5V coming out of IC3. Then check that the voltage at the collector of Q2 is about 5V. Check solder connections around all components associated with the DECAY portion of the circuit.
I’m getting 5V out of IC3. And I’m getting 5V at the collector of Q2. I triple checked my soldering and used a multimeter to check as well. I put an audio probe at the base of Q2 and I get a faint farty signal. I don’t know if that’s helpful for diagnosing this.
I just tried some different BC550s but that didn’t make any difference
I would use the audio probe to trace the signal through the circuit starting at pin 1 of IC2a. Follow the schematic and check at pin 14 of IC2d, emitter of Q3, etc.
I’m getting a strong signal at pin 1 of IC2. I’m getting a strong signal at pin 8. I’m getting strong signal at R21. I’m getting SOME signal at pin 14 but it’s quiet. Q3 is getting a quiet signal.
Everything around this section, the signal gets quiet:
Check for VB voltage (4.5V) at pin 12. Check Connections for all components in that area. If no luck, I wonder if the IC is bad at ICD?
well I’m only getting about 3.8V at pin 12. My power supply is about 9.5V.
Here’s what I’ve checked so far:
I’m also getting 3.8V at pin 5 and 6 of IC2B.
7.5V going into R29.
7.5V going into IC3.
8.75V going into R28.
9.5V going into D4, 8.75V coming out.
Is that too much voltage drop for a diode? I used 1N4001s.
3.8V is correct. 1/2 of 7.5V. Voltages seem to be within tolerance. How about the components? Reflow solder joints? Remove the ICs and reflow the sockets.
It was IC2 (TL074). I replaced it with another TL074 that I happen to have from tayda and the decay knob worked! The original one I ordered from Barry. Just bad luck for me I guess. I suspect it was the D section that was bad since that’s where I was getting weirdness with the audio probe.
Thank you so much wilkie1 for your patience and help with this.
You are most welcome. Here to help.
i spoke too soon. i boxed it back up and the decay knob wasn’t doing anything. i took it back apart and still wasn’t doing anything. i think i messed with the bias knob instead of the decay knob after replacing the TL074. dang i thought this was over. the other things i tried was reflowing all the IC solder joints and anything else that looked even slightly questionable. i replaced the LM13700. and i replaced both BC550s. testing after each thing i did.
i tried reflowing again but decay still isn’t working
i desoldered the C1M decay pot to make sure it was good
OK. Go back to signal tracing. Pin1 IC2, pin 14 IC2, collector of Q3, pin16 of IC1. You need to have some guitar playing during this test.
IC2:
pin 1 – i hear my dry signal
pin 8 – i hear my signal but it’s louder and clipping a bit
pin 14 – i hear a quiet fuzzy signal. while at pin 14, adjusting the decay knob does nothing.pin 3 3.8 V
pin 10 3.8 V
pin 12 3.8 V
pin 13 3.7 VQ2:
base – i hear faint choppy signal
emitter – i hear what sounds like a heartbeatQ3:
base – i hear the same heartbeat
emitter – i hear a heart beat but there’s more beats
collector – i hear nothingIC1:
pin 16 – i hear my signal with auto wah going on
by the way, i’m now testing with a looper pedal with guitar playing. i was previously using a function generator.
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