SOLVED XC Phase. sounds like a toy fire engine?

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motive

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Got my matched FETs in from pedal parts and kits and put this guy together, set up on an auditorium testing platform. When its set to block setting I'm getting this high pitched squeal toy fire engine/air being slowly released from a balloon sound that persists on both the 45 and 90 settings, the signal modulates like you would expect when the pot is turned. when I switch to script, 90 is a really heavily distorted signal that basically sounds like an overdrive with the gain and volume all the way up I can hear some modulation when I turn the pot here too, and the 45 stage setting I just get a REALLY quiet kind of clean signal.

Thought it may be a wonky op amp, I replaced all three with completely new chips, reflowed all the sockets and joints etc.

I tested through some voltages
zener is right at 4.8
and the gate pins on the fets are all tuned to about 2.4v

This is the first phase circuit I'm building with FETs so I'm sure there is a step I'm missing, but any insight would be really helpful!

Thanks!

20211012_113552.jpg 20211012_085352.jpg
 

motive

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unfortunately. its the rev 1 board so the orientation silkscreened is backwards

from the build doc.
"Due to a silkscreen error on Revision 1 PCBs, Q1 - Q4 should be installed
backwards compared to the silkscreen on the PCB when using 2N5952 transistors."

tried flipping them, no joy.
 
unfortunately. its the rev 1 board so the orientation silkscreened is backwards

from the build doc.
"Due to a silkscreen error on Revision 1 PCBs, Q1 - Q4 should be installed
backwards compared to the silkscreen on the PCB when using 2N5952 transistors."

tried flipping them, no joy.

10-4. Didn't know that.
 
Only thing I can say more is that some of the solder joints look a little blobby, especially the one north of the yellow 'IN' wire.
 

motive

New member
thank you, great fb, the switches were not fully soldered in. reflowed the joints on the back and properly soldered in the toggle switches.

unfortunately no real change. drat. feels likes something is off with the op amps, but I'm not sure how to diagnose properly
 

blackhatboojum

Well-known member
Did you bias your jfets by ear first or did you just automatically bias the gates to around 2.5v? 2.5v isn’t automatically the best voltage. I biased my xc phase by ear. It sounds it’s best when it’s biased to around 1v max at the gates.

What are your opamp voltages?
 

finebyfine

Well-known member
Getting oscillation + the rate knob having an impact + general signal in/out makes me think the op-amps are probably fine, or at least wouldn't be the first place I checked. Did the matched jfets come with Vgs specified? If you didn't bias it I would bias it to 3.8v and then go down by ear
 

motive

New member
I haven't really dialed that in yet, but I can basically vary the pitch of the tone with the trim pot. Was more concerned with the loud screeching before I got too into fine tuning haha.

IC voltages

IC1
1 4.8
2 4.8
3 4.6
4 0
5 5.6
6 6.3
7 3.2
8 9.1

IC2
1 4.8
2 4.8
3 4.8
4 0
5 5.8
6 5.9
7 6.1
8 9.1

IC3
1 5.5
2 4.8
3 4.8
4 0
5 fluctuating 4 - 6v
6 fluctuating 4.8-5.6
7 fluctuating 1 - 8v
8 9.1
 

blackhatboojum

Well-known member
I don’t see anything too out of the ordinary with your voltages. Another question… are you getting good connections from your phaser pcb to the auditorium test platform? Mainly the input and output jack grounds?
 

motive

New member
yeah I even disconnected it and wired it up to a footswitch in case there was something going on with the platform. same issue. the only other thing I can think is that the zener is wrong or something? just not sure what the heck would cause it to sound like a toy laser gun with 0 input coming in.
 

blackhatboojum

Well-known member
I’m pretty sure it’s not your zener. The first stages of your op amps are the right reference voltage. Your second stage voltages seem a little weird to me though. Going by memory, I think mine were all around 4.5v. I dunno, I’m going to have to pull out the schematic and poke around my own phaser pedal later today before I can be of anymore help.
 

blackhatboojum

Well-known member
Emitter: 4.8
Base: 4.5
Collector: 3.0
Those aren’t out of the ordinary either. I dunno what to say on this one. The way you described the issue seems like some kind of oscillation or feedback but I have no idea why. My only advice would be to go over all of your solder joints and make sure they have good solid connections. Check the test platform too. Maybe someone with more experience could chime in and help resolve this one. Sorry I couldn’t be more help😕.
 

motive

New member
Update.
I think I found the issue. seems like there was a bad trace between r13 and r14. slapped a jumper between them and its working beautifully.

fun fact. if you are using the auditorium platform, the output clamp is already grounded, so you can make a really simple and effective audio probe with a 10n cap and some jumper wire.
 
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