Help troubleshooting issue with AION FX Cerulean Kit

[anotherjim]

Sorry, I must have read crosseyed. Thought there was 3v or so on the gate! But then I didn't know what 1,2,3 referred to and made an assumption!
I'm not sure I like the example voltage of near 7.8v on the drain unless it's meant to distort, but JFET performance varies all over the place from part to part and your example might actually be better biased than Aions was.

[jrbilodeau]

Hmm, yeah I'm not sure. The pedal definitely isn't sounding right, even at low volume and low drive there's a bunch of distortion and it sound kind of crackly. It definitely isn't sounding much like a blues breaker. Maybe I should unsolder the component and test them individually to make sure all the values are correct?

I've gone over the schematic and tested all the point listed and ground and am getting 0V and all the points that read VR are coming up with the same voltage...same with points listed as VA.

My current multimeter doesn't have a setting to test capacitors, so I'm thinking of picking up a new one. I read that the ANENG 8008 and 8009 are pretty good for the price. As far as I can tell, the only difference seems to be 8008 has a square wave generator where as 8009 has non contact voltage detection. Would either of these features actually prove useful in pedal building? If you guys have any better recommendations I'm up for suggestions.

Thanks

[jrbilodeau]

I was just doing a search in the forum and can across this post that sounds really similar to mine. https://www.diystompboxes.com/smfforum/index.php?topic=125642.msg1197707#msg1197707. The voltages that he listed are quite close to what I'm getting and I did some testing and when with the hard and soft clipping switches and I also get very little difference regardless of their position. He mentioned having a bad transistor, and I know that I did the test meantioned in the earlier post and it seemed to come back as expected, but is it possible that it's still bad? before going further, would it be wise to order a new transistor and try that to see if it fixes thing?

[idy]

Buying another transistor from reputable source is not a bad idea. Maybe you are thinking of an order for your next project?

If you are so suspicious of the Q you could carefully remove it and try to breadboard the booster stage to see if it works. All those components between CX2 and CX4. Nice simple project, FET booster.

I don't see that you have tried a signal probe yet.(?) Necessary and simple implement! Always good to try that before you get all medieval with the soldering iron. I would not dream of blaming a component until I have signal traced a circuit. But FETs are dodgy.

[jrbilodeau]

Looked up a few guides on building a audio probe, and I found a .1 uF cap in some old electronics stuff I had from back in my college days. I'll try building an audio probe and see what results I get, and then see about getting another JFET if need be. I found a MPF102 N channel JFET in my old parts kit. Although it's not an equivalent replacement for the 2N4303 or 2N5457 which the kit lists as parts for Q1, would swapping this be a possible test until I can get the right part shipped....depending on how the audio probe goes first that is.

Also if I was to get a better multimeter what do you guy's think of the ANENG 8008 or 8009? My current Mastercraft one doesn't have a capacitor reading setting, and the diode test, doesn't have a audible beep, it seems to be more just on the 2K resistor setting.

[Kevin Mitchell]

Late to the party (because that's what cool kids do, right?)

Go with AN8008. I bought one to replace my big MM since it didn't have a frequency setting. Been going strong for years. 4-digits and inexpensive. Can't go wrong.
However no transistor test sockets if that concerns you.

For your jfets, just be mindful of the pinouts. When used as a gate most jfets usually work however if used as a source of variable current you may have to adjust the bias to accommodate. Spectrum behavior may also differ in that case.
Just something to chew on, likely not going to be a problem for you 

[jrbilodeau]

So I made an audio probe tonight and did my best to trace it (I'm still new at this). Things sound pretty good until I got to the the drain or source on the JFET, or at RX4 or RX5. Then everything got all crackly. I had a bit of a hard time figuring out how to properly trace the hard clipping diode section, as I got audio on both sides all of the diodes in the hard clipping section that ranged in volume and amount of distortion. This test was done with the switch in the middle position. Also with the switch in that position I got audio reading on all 3 lugs. The top one was the loudest, the middle one was really faint, but could still be heard and the bottom had slightly less volume that the top one.

With the hard clipping switch set to the top position, I got audio at the 1st lug, and very faint audio at lugs 2 & 3
With the hard clipping switch set to the bottom position, I got audio at the 3rd lug, and very faint audio at lugs 1 & 3

The voltage that i'm getting at VA is 9.14V in front of RX4 (22K resistor), but after crossing the resistor it drops to 3.52V

Btw, do you guys have any good ressources that you'd recommend to learn how to understand how these type of audio circuits work? I took a few courses back in college 15+ years ago as part of my computer programm, but went more into the computer side of things, and don't remember alot of this. I have some basic understanding of things like resistors, capacitors, diodes, opamps, but when you put it all together and how the various parts all interact with one another and how they affect the tone, thats a bit over my head. I'd like to be able to better understand how the circuits work and understand how to troubleshoot, and tinker with mods and stuff. I realize that it'll be a learning process, so just curious on good ressources to help get the learning started.

[antonis]

So I made an audio probe tonight and did my best to trace it (I'm still new at this). Things sound pretty good until I got to the the drain or source on the JFET, or at RX4 or RX5. Then everything got all crackly.

I don't like that Q1 CS amp fancy configuration with all those NFB items..
Personally, I'd delete CX5, C8 & RX1 and make RX2 an order of magnitude lower..
(but my aspect doesn't help with your issue..)

[jrbilodeau]

CX5 and C8 are used in other optional for this build, but since i'm following the kit documentation for the morning glory style circuit i'm not using them. The non kit documentation for the pcb is found here https://aionfx.com/app/files/docs/cerulean_documentation.pdf

BUILD NOTES
Morning Glory Gain Stage
The JHS Morning Glory adds another gain stage to the end of the circuit for more available output
volume. You can choose to build the pedal with this gain stage or without.
To build with the gain stage, omit C8 and include RX1-6, CX2-4 and Q1.
To build without the gain stage, include C8 and omit RX1-6, CX2-4 and Q1.

King of Tone
The Analogman King of Tone is two Bluesbreakers in one box. All of the modifications from the King of
Tone are available as part of this project (soft clipping, hard clipping, and Presence control).

In the parts list CX5 is listed as being part of the King of Tone variant, but to omit it in the morning glory build...so they show up in the schematic, but aren't used in the build that I built.

[antonis]

Could you plz also omit CX5..??
(Volume pot lug 3 left connected to CX4 negative plate only..)

[jrbilodeau]

That's what I'm trying to explain, C8 and CX5 where both omitted from the build that I did. I've never put any components in that location since I was following the morning glory style build, which didn't use them.

[idy]

Btw, do you guys have any good ressources that you'd recommend to learn how to understand how these type of audio circuits work?

Start with the links at the top of the page. GEOFEX has some great lessons called
"technology of...." Great walk through tubescreamer and a few others. Sadly no Bluesbreaker....

[anotherjim]

When I first saw it, I thought they added a JFET stage to boost the level which with only the single set of hard clipping diodes can seem too quiet for a lot of players, but then they have those caps around it (1uF with 100n in parallel makes no sense). As the JFET stage is inverting, it will cancel out some important frequencies and I'd have to hear it to believe it won't sound horrible! Of course, Antonis pointed this out earlier but he sees it as feedback while I see it as bypass.  Anyway, definitely take CX5 and C8 out of there!
That whole JFET stage appears to be some version of a guitar "Fetzer" preamp circuit (68k input resistor lends credence) stuck under the original straight output with the output caps left misplaced on the schematic.

[jrbilodeau]

I didn't add anything to CX5 or C8, so there's nothing to take out.

[antonis]

OK.. You said that you have "clear" signal till Drain/RX4 and Source/RX5/RX6 joints..
What puzzles me is the later (Source) crackling 'cause at that point you should have the same signal "quality" as with that at Gate (RX2/RX3 joint) - just a bit of lower amplitude..
But with a quiescent V_DS voltage of about 500mV (3.5 -3.01) things get complicated a bit..

Could you plz repeat audio probing with clipping switch set at HARD position (onnly a pair of antiparallel diodes activated)..??
(I'm mainly interested about Source signal, to figure out if your issue relies upon inproper bias or on JFET itself..)

P.S.
For your understanding: Q1 configuration may be considered either Common Source amplifier or Common Drain voltage follower, depending on signal output point of view..
For the later configuration, outcoming signal (from JFET Source) should be almost identical to the Gate incoming one (in phase but of slightly lower volume..)
The above only stand for V_DS higher than incoming signal amplitude..
(not even for equal to, for reasons beyond present scope.. )
For any clipping diode configuration other than D8/D9 pair, Q1 input signal amplitude is higher (considering no losses at Tone and Presence configurations) than allowable for undistorted Source outputV_DS..

[mdermksian]

Any chance you could confirm that swapping the transistor solved your problem? I'm having nearly exactly the same problem as you had and can't quite seem to figure out why. I'm starting to suspect that the transistor might be bad based on what you and the other guy have said.

If swapping Q1 fixed it, could you also let me know what you swapped to?