Aion Refractor V2 Issues: Gain pot does nothing

[bretvh]

Hello everyone. First post. I'm seeking help with tracking down an issue with the Aion Refractor V2 build I recently did as a surprise gift for a friend. I've done some searches and found some potential causes, but nothing has panned out, so I'm asking here to see if anyone has any suggestions as to what may cause the issue I'm seeing. For example, one thread with a similar issue turned out to be a bad cap, but the OP never followed up to specify which cap. Ironically, I did this same build a few years ago (different PCB from a different company) and had the same issue and never fixed it. Note that I did not recycle any parts from that build, fwiw.

Anyhow, details are below. If there is anything I have neglected to include or that would make it easier to help troubleshoot, let me know and i will get it here as fast as I can.

Thanks in advance!


The issue:

Basically, the tone and volume pots work fine. I can hear the tone sweep and the volume has enough output to push my preamp tubes a little. Unfortunately, the gain pot does nothing. Well, it sounds slightly louder when the pot is all the way "down." Otherwise, no change in gain.

Here is the build doc for the version with which I am working:

https://www.dropbox.com/s/4xhwadzx2rxbm8z/aion-refractor-centaur-documentation-v2.pdf?dl=1

Mods/differences from recommended circuit, potential issues:

• I do not have the 3PDT daughterboard, so I hand wired the switch. That said, it works, save for the gain pot.
• I didn't have a Tantalum cap for C16, so I used an electrolytic
• As noted below, I only had a MAX1044, so as per the docs, I used a 1N4739 for Z1
• I had an issue early on with a solder bridge between R10 and GAINA pin 3 and in the process of cleaning it up, some of the mask was removed. The two points have continuity on the board, and in the schematic it appears as though they are connected directly anyhow, but I thought I would mention this since I am having issues with that pot.

Here is what I have tried so far:

• Checked voltages in, which are 9.74
• Checked for solder bridges and poor joints
• Reflowed most of the solder connections on the board, as well as the pots
• Started tracing circuit with my probe: So far I have audio signal on all of the ICs, but I note that I only have signal on one pin each section of the dual gang gain pot. I have not had time to do more in-depth testing or probing beyond that.
• IC voltages seem inline with what I have read they should be around the interwebs (see below)
• I have yet to verify resistor values, although I do generally measure and test each resistor in my component tester before mounting—that said, I also tend to make dumb mistakes like that, so I am not ruling that out
• Verified electrolytic caps are mounted in the proper orientation, and ICs as well

IC1 (TL072):

1 4.88
2 4.88
3 3.75
4 .8
5 4.86
6 4.88
7 4.88
8 9.73

IC2 (TL072):

1 4.94
2 4.88
3 4.87
4 -9.4
5 4.87
6 4.88
7 4.82
8 17.64

IC3 (MAX1044 - I did do the recommended 1N4739 swap in the docs for Z1):

1 9.74
2 4.83
3 .2
4 -4.7
5 -9.4
6 5.04
7 7.14
8 9.73

Note that the diode closest to IC3 is registering 17.64 on one end.


Photos (these turned out blurrier than I thought and I won't be able to re-take them until later tonight):

A closeup of the mask blunder I made at R10 and pin 3 of GAINA (this is before the PCB was totally populated):



Partial view of PCB bottom BEFORE reflowing joints (has been cleaned with alcohol):



PCB Top BEFORE reflowing joints, completed (and yes, my wiring is messy with this build... but the switching appears to be working):

[antonis]

Hi & Welcome..

Try to desolder Gain pot & wire both wipers  to any convenient GND instead of VB..
(lug 3 & 1 wired to their initial pads..)

edit: GainA lug 3 & R10 must be connected (otherwise IC1B works as unity gain buffer..)  so mask blunder doesn't mind as long as there is solder joint between them..

[bretvh]

Thanks @atonis! I will give this a try later. And GAINA pin 3 and R10 do show continuity, so I suspect that they are ok as-is, but I will confirm.

Also wanted to add that I do have the pots shielded on the back—dust caps on tone and vol, electrical tape on gain.

[antonis]

It's just a suggestion find out what's happening with Gain pot..
(you will upset IC1B bias and might crackle but it should work for testing purpose..)

[bretvh]

Ok it's fixed! You were right—the trace between GAINA pin 3 and R10 was broken. I replaced the pot, nothing changed, so I soldered a bridge in there and it works now. Thanks for the help!

[duck_arse]

awww, too soon! I wanted to ask why/how you have .8 volts on pin 4 of IC1 when it should be connecting to ground, but now I can only say also welcome.

oh, also and, no ground wire on that right side jack?

[bretvh]

Ha! Good question on the .8 volts. I can only imagine it had to do with the broken trace, but now I kind of want to measure it again to see if it's zero!

No ground on the output jack because that's how Aion has indicated it should be wired due to the buffered bypass. It's star grounded, but the output jack connects to ground through the 3PDT, I believe.

Thanks for the welcome!

[antonis]

I don't presume your belief can be stronger than virtually loose nut so, if you plz, wire right jack as  "politely" indicated by Stephen..