Obsidius preamp problems

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Vildhjuggah

Member
When I turn on my obsidius preamp the distorted signal doesn't come through (only raw sound and some sizzle). If I flip the toggle switches a million times it will come through, but then I try to flip it to the ones I like (or again at all) it cuts out again. To get it to the setting I like and work at the same time it takes awhile. Also, when I switch the pedal off and back on it normally goes back to no distortion (only raw sound and some sizzle) coming through.
I honestly don't know where to start diagnosing this problem. I'd guess check the switches but this happens no matter which switch I flip so I honestly don't know.
 
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When you're ready, get your delay going with a more-or-less constant tone. Connect that to the input of you Obsidius. Connect the Audio Probe's ground clip to the ground lug on the IN or OUT jack. Power up the Obsidius. Set the BLEND to 0 (dry), DRIVE to 0 and LEVEL to noon. Set your amp's volume low. Connect the Obsidius output to you amp. Engage the stomp switch (not bypass). Once you have verified that signal is getting thru the Obsidius, unplug the amp from the Obsidius and plug the audio probe into the amp. You'll want to keep the amp volume low while you're moving the probe around because there will be hum and pops and all sorts of mean nasty ugly things.
Start with the probe on IC1-1. You should hear the same sound you heard when the amp was plugged into the Obsidius.
Check the following with the Audio Probe in the order given and report back when you find a dead spot. Wherever you find a dead spot, the problem will be between there and the previous spot where you had sound. The volume and eq will change as you move through the circuit, that is normal.
1) Q1-1
2) IC3-3 try ATTACK switch in both positions
3) IC3-1
4) IC2-3 try GRUNT switch in both positions
5) IC2-2
6) IC3-7
7) IC4-7
8) IC4-1

If the signal is dead at step 5, measure the DC voltage on IC2-1 w.r.t. GND.
 
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So because on the circuit board picture it doesn't label which ic (or transistor) is which number I'm gonna clarify as I go through

1) Transistors (not sure which is q1 but guessing the left one, putting both just in case)
If looking at the circuit on component side and in reference to the flat side of the transistor;
Q1 Left transistor:
Left pin no signal, right pin loud signal.
Q2 Right transistor:
Left pin loud, right pin quiet signal.



Ic1 bottom left (seems to be the signal unaffected by switches so assuming that's one)
Ic 2 is the cd4039 according to parts list
Ic 3 middle (I'm guessing cuz that seems to respond to the attack switch only which is what you said test it with)
Ic 4 bottom right (seems to be the last one in signal since it's affected by both eq switches so guessing that's 4)
That correct?

2)ic3-3 working with attack switch
3)ic3-1 slightly louder than original signal
4)ic2-3 quiet but sometimes loud (seems to be the signal popping in and out thing maybe?)
5)ic2-2 louder than 3
Bonus question) 7.6 volts on ic2-1 to ground (running on battery at 8.5v)
6)ic3-7 sometimes quiet sometimes loud
7)ic4-7 quiet
8)ic4-1 quiet

Testing while ic3-7 is loud
7/8) when tested while ic3-7 is loud it's about the same as original signal
4/5) probing these again seemed to make ic3-7 go quieter again
 
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You had most of it right. Q1 is on the right, Q2 is on the left. If you look on the PCB, you can see a trace on the top layer connecting Q2's gate (pin 3) to two 1M resistors and a 22nF capacitor. That's how I know which is which. Or you can measure the voltage on pin 1. Q2-1 will be at +9V. IC1 is lower left. IC4 is lower right. IC2 is the big guy at the top. IC3 is just below IC2.

1) 1st stage (Q1 & Q2) is working correctly.
2) Signal is getting to 2nd stage.
3) 2nd stage is working correctly. IC3-1 should get louder if you turn DRIVE up.
4) Sound like there is an intermittent connection at the input to the 3rd stage. Check both sides of R17 with the probe. There are two 6.8K resistors above IC1. R17 is the one on the right. The signal on the bottom side of R17 should be quieter than the top side. We're looking for intermittent signal.
4a) +7.6V on IC2-1 sounds about right. Normally, CMOS gates draw hardly any current. But when we bias one into a linear region so we can use it as an amplifier, it starts drawing current. In this case, about 9mA. It will vary from one chip to the next.

The problem is on the input side of IC2. Does switching the GRUNT switch make the problem come and go? If so, then the switch or its solder joints are suspect. If you used too much heat on the switch terminals, it may be damaged.

Measure the DC voltage on IC2-3 and IC2-2 with no input signal. They should be equal (assuming you are using a good DMM) and somewhere between 2.5 and 5V.

IC sockets are always suspect and I don't like to use them myself. However, a socket is definitely a good idea when using 4049s in a pedal because some are noisy. Inspect all of the solder joints around R17, IC2, D1 & D2. Pull IC2 out of the socket and clean the socket out with IPA.

A word about DMMs. I have two on my bench. A good one that has 10M input impedance and a shitty one that has 1M input impedance. I use them both, but I won't try to measure high-impedance circuits like the input side of IC2 with the shitty one. If you use a cheap HF DMM that has 1M input impedance, the voltage on IC2-3 will read 2/3 of the voltage that is on IC2-2.

We're getting closer to fixing this beast!
 
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Alright so after I get off work I need to test
(Making a new number list)
1) test if drive is making ic3 louder
2) see what happens on each side of r17
3) measure ic2-2 and 2-3 (my multimeter wasn't very expensive but it wasn't a free one from harbor freight haha but we'll see how it measures these)

Normally it was while switching the switch, but I had reflowed the connections just in case. But also, since I originally installed the potentiometers and switches on the front side before I looked at a few finished builds on the forum, it did take a while to get enough solder off the switch connections to pull it out, it's possible that the switch was damaged then, but it's so inconsistent on when the distorted side of the signal finally pops in.
So to be absolutely sure whether it's the switch or not.
4) check joints on switch, maybe take out switch and try using jumpers instead of the switch to see if that works

But the things that were making me think it wasn't the switch was that sometimes it'd pop in while flipping the attack switch previously, but that was working perfect in yesterday's testing. When I turn the pedal off while it's working properly, then back on, it wouldn't be working right anymore either. Also there was a few random times where ic3 got loud while poking around and not flipping the switch.

5) clean ic3 socket (also just in case check the electrical tape on back of pot that's above it)
 
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Correction: the IC2 socket is the one that's suspect, clean that one.
I would not trust electrical tape, it is too easily cut by sharp leads. I use either dust caps or a thin cardboard, like a business card or piece of a cereal box.
If you had to remove and replace the toggle switches, chances are that they were damaged. in the unsoldering process. Been there, done that. I won't reuse toggle switches that have been removed. A failure in the ATTACK switch would be evident when probing IC3 pins 1 or 3. A failure in the GRUNT switch would not affect IC3 pins 1 or 3, but would affect IC2.
Since you mentioned R & R'ing the pots and switches, you need to inspect carefully for collateral damage. Look for lifted pads, cracked traces and parts that were nicked by the soldering iron. Even a small nick or crack on a film resistor can break the resistive element. The pix look fine, but you can see better than we can. The signal popping in and out when you probe makes me thing there is a loose solder joint, intermittent short or a cracked trace.
 
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Fair enough.
None of the pads lifted. (props to the PCB maker) I hope there's no track cracks. I'll check for any resistors that were touched by the soldering iron cuz I think there was one.

Once I get home I'll do all that, probably reflow the ic 2 socket as well.
 
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Yes, thru-plated holes are pretty robust, but enough heat will mess them up too.
My advice is to not reflow a solder joint unless it looks bad or tests bad. You can do more harm than good by reflowing solder joints that don't need it.
Try poking the audio probe around the area where the sound would come and go, you should be able to zero in on the intermittent connector. Also try tapping various places and parts with a soft insulated tool. A small chopstick works well for this (no joke!).
 
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Pedal is working for some reason atm, all i did was check the switch with the multimeter before doing these tests

1)Drive is making ic3 louder
2) r17 seems to be same volume on both sides
3)Ic2-2 and 2-3 are 2.79 and 2.75 (still running on battery at 8.5 v

Imma play through it for a bit
Flick some switches see if it's staying on right
 
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messing up again, clean socket and board with ipa
working better, i probably just need to replace the switches, next time i order parts ill grab some so its worth the shipping (though i should probably try hardwiring switch point first)
 
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Yeah, jumpering is a good idea to prove that the switches are the problem. Toggle switches can fail in several ways:
1) a contact pair closing that should be open
2) a contact pair opening that should be closed
3) a contact shorting to the case
First try it with the switches installed. At some point you'll need to remove the switches because you might be experiencing failure mode 1. As long as the switches are not in contact with the box, failure mode 3 won't be an issue.
 
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The switch failures I was describing above would be inside the switch where we can't see them, but we can observe them with an audio probe or DMM. If you're going to jumper the switches, it would be easier to do with the board outside the case, yes? I'm suggesting that we test the board out of the case as well.
Make sense?
 
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I replaced the switches and they should definitely be working properly.
The pedal however still isn't.
I ran through some of the tests you've had me do before.
Currently the issues I'm having in the test (with blend 0 raw, volume noon, drive 0, switches in middle position)
Ic 2-3 nothing (voltage 2.86)
ic2-2 (voltage 2.89) hiss and some sound
R17 you said the 6.8k one on the right (I'm assuming while looking at the components) getting nothing either way
The one to the left I get good signal on top nothing on bottom

So
"Inspect all of the solder joints around R17, IC2, D1 & D2. Pull IC2 out of the socket and clean the socket out with IPA"
Joints seem fine
I cleaned the socket
 
Last edited:
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First, let's be clear about board orientation. When we say left, right, up, down, top or bottom, we are referring the the view of the board as it appears on page 1 of the build docs.

You said you have good signal at R10 "The one to the left I get good signal on top nothing on bottom."
You said you have no signal at R17 "R17 you said the 6.8k one on the right (I'm assuming while looking at the components) getting nothing either way."

That narrows the search for the short, open, bad part, or whatever is causing your pedal to malfunction. It's somewhere between the output of the 1st stage (good signal at the top of R10) and the input to the 3rd stage (no signal on R17).

Verify you have good signal at IC3-1 (output of 2nd stage). If you have signal there, but no signal on either side of R17, then the problem is right around the GRUNT sw, C11-C13 and R17.

If you don't have signal at IC3-1, then check for signal at the bottom of R13 (470K to the left of the CD4049).

I do not trust the electrical tape you have under the BLEND pot. Sharp trimmed leads can poke holes in it. IC2 & IC3 are right under the BLEND pot and that's where we've been losing signal.
 
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