Very, Very Quiet Promethium Distortion/HM-2

[EmptyBuckets]

Hey guys, not sure what to make of this. I tossed together a Promethium Distortion and it's SUPER quiet. If I crank the volume pot all the way I'm at maybe 20% of line level. Everything appears to be working as expected, it's just too quiet!

Any ideas on possible culprit? Here's a couple gut shots. The ICs aren't socketed but I did them last and went quickly. The two diodes (D4-D5) are socketed Soviet Germanium D9Ks which I had read can be used instead of 1N34As. Could that have an effect on volume, though?

 

[zgrav]

I suspect the issue has more to do with a bad solder connection somewhere than a bad part.

 

[EmptyBuckets]

I suspect the issue has more to do with a bad solder connection somewhere than a bad part.

But if it's just a soldering issue wouldn't that mean it has to be D4 since that's where things die when testing with the audio probe? If that's the case I'm not sure that my soldering would be the problem - I've now soldered the diodes 4 times now (first sockets, then straight to the board, then swapped in the opposite direction, and finally now with the new 1N34As, not to mention reflowing things a few times at various points throughout.

I'm a bit stumped as to what my next move should be outside of just re-soldering everything. I did give it a good scrub with some iso so I'll try to get a new photo up of my soldering work to see if anything sticks out to someone but I think it's looking pretty decent!

 

[zgrav]

do you get sound on both sides of D4? How about D5? So you hear any difference in each of the diodes with the audio probe?

If you check the schematic, the only things that D4 should connect to are D5, R19, and R20. Confirm those connections with your meter and then make sure that D4 is not accidentally contacting C7 or any other nearby part of the PCB. Also check to see if either side of D4 has connection to ground. You can also temporarily jump each side of D4 together to see if it is really the diode that is blocking the sound. Does any of that make a difference in what you hear through the pedal?

 

[EmptyBuckets]

do you get sound on both sides of D4? How about D5? So you hear any difference in each of the diodes with the audio probe?

If you check the schematic, the only things that D4 should connect to are D5, R19, and R20. Confirm those connections with your meter and then make sure that D4 is not accidentally contacting C7 or any other nearby part of the PCB. Also check to see if either side of D4 has connection to ground. You can also temporarily jump each side of D4 together to see if it is really the diode that is blocking the sound. Does any of that make a difference in what you hear through the pedal?

Nice loud, nicely distorted audio on top/K of D4 but the bottom/A has barely perceptible audio when at all the way at max volume. D5 is even more quiet / essentially silent. Same results as when I first probed back when I was still using the Soviet diodes. I reflowed everything adding a little more solder here and there. A few of these look a bit flat because a I had snipped a few leads too short after I first soldered them in.

Checked continuity and D4 is indeed only connected to D5, R19, and R20. No connection anywhere else around. Not shorting to ground either.

How exactly would I go about jumping D4 together? That's not something I've done before.

Here's some newer photos, post-iso scrub but re-dirtied a little by the reflowing. (the diodes are left long & tall because ideally once I get things figured out here I'll pull them and put back in the Soviet diodes)

 

[zgrav]

by "jumping D4" you would temporarily touch a wire to each side of it so the signal bypasses the diode. then see if you hear a difference. either the diodes block the sound or there is something on the other side of the diode that is draining the signal (short) or blocking it (bad connection). Look at the schematic in the build document and note the each side of D4 and D5 should connect to each other -- use you meter and confirm that is OK. Then use your meter to make sure one side of D4 and D5 connect to R19 and the other sides connect to R20. Also double check to make sure you have the right values for R19 and R20 (10K each).

 

[EGRENIER]

Nice loud, nicely distorted audio on top/K of D4 but the bottom/A has barely perceptible audio when at all the way at max volume. D5 is even more quiet / essentially silent. Same results as when I first probed back when I was still using the Soviet diodes. I reflowed everything adding a little more solder here and there. A few of these look a bit flat because a I had snipped a few leads too short after I first soldered them in.

Checked continuity and D4 is indeed only connected to D5, R19, and R20. No connection anywhere else around. Not shorting to ground either.

How exactly would I go about jumping D4 together? That's not something I've done before.

Here's some newer photos, post-iso scrub but re-dirtied a little by the reflowing. (the diodes are left long & tall because ideally once I get things figured out here I'll pull them and put back in the Soviet diodes)

View attachment 25136
View attachment 25137
View attachment 25138

From this last picture it looks like you have a 1u at C11 instead of a 1n.... So that can't help...

I just went through a long troubleshooting session with a Frost Bite and after 2 weeks of searching for issues, turns out I had a 33n instead of a 33p in my drive stage. Looks like that can suck a lot of signal out

I would also double check your C19 and C20 as you had mentioned volume lost in that part of the circuit also...

 

[Coda]

A quiet probe around D4 and 5 doesn’t surprise me…there is still a lot of opamp to go through in the circuit. C12 also definitely appears to be wrong. You have an electrolytic in that spot. According to Coda’s Rough Guide to Capacitors, nF = box, uF = electrolytic. That cap is pulling way more out of the signal than it should be…

 

[EmptyBuckets]

From this last picture it looks like you have a 1u at C11 instead of a 1n.... So that can't help...

I just went through a long troubleshooting session with a Frost Bite and after 2 weeks of searching for issues, turns out I had a 33n instead of a 33p in my drive stage. Looks like that can suck a lot of signal out

I would also double check your C19 and C20 as you had mentioned volume lost in that part of the circuit also...

Oh wow, can't believe I missed that! When I went and swapped all 1u electrolytics for 1u film caps I just went on ahead and swapped that for a 1u, too. Going to swap that as soon as I have time to get to the bench again. Thanks for spotting! I would love for this to solve the problem hah!

A quiet probe around D4 and 5 doesn’t surprise me…there is still a lot of opamp to go through in the circuit. C12 also definitely appears to be wrong. You have an electrolytic in that spot. According to Coda’s Rough Guide to Capacitors, nF = box, uF = electrolytic. That cap is pulling way more out of the signal than it should be…

Thank you but I think you must be looking at a photo on page 1 of the thread. I had originally populated with 1u electrolytics but I've since swapped C12 and the others for proper 1u film caps.

 

[EmptyBuckets]

From this last picture it looks like you have a 1u at C11 instead of a 1n.... So that can't help...

I just went through a long troubleshooting session with a Frost Bite and after 2 weeks of searching for issues, turns out I had a 33n instead of a 33p in my drive stage. Looks like that can suck a lot of signal out

I would also double check your C19 and C20 as you had mentioned volume lost in that part of the circuit also...

Just swapped the 1n in and no noticeable change... Argh!!

Going to try @zgrav 's jump suggestion, hopefully this evening, and will report back

 

[EGRENIER]

Just swapped the 1n in and no noticeable change... Argh!!

Going to try @zgrav 's jump suggestion, hopefully this evening, and will report back

Did you double check on C19 and C20 ?

 

[Chuck D. Bones]

All of your voltages look good except D4 cathode. You can re-check that one because it should be exactly the same as D5 anode. What model DMM are you using?

Audio Probe IC2 pin 1 & IC1 pin 1. Set all knobs to 11 when probing.

Until you have a better idea where the problem is, swapping parts is a bad idea.

 

[EmptyBuckets]

Did you double check on C19 and C20 ?

Double checked C19 and C20 and R19 and R20 but they're all as they should be.

All of your voltages look good except D4 cathode. You can re-check that one because it should be exactly the same as D5 anode. What model DMM are you using?

Audio Probe IC2 pin 1 & IC1 pin 1. Set all knobs to 11 when probing.

Until you have a better idea where the problem is, swapping parts is a bad idea.

Re-checked D4 & D5 and I'm still getting the same results as before: .3-.6 on D4 cathode, zeros across the other three. I'm using an AstroAI AM33D multimeter.

Pin 1 on IC1 & IC2 were both totally silent when probed.

by "jumping D4" you would temporarily touch a wire to each side of it so the signal bypasses the diode. then see if you hear a difference. either the diodes block the sound or there is something on the other side of the diode that is draining the signal (short) or blocking it (bad connection). Look at the schematic in the build document and note the each side of D4 and D5 should connect to each other -- use you meter and confirm that is OK. Then use your meter to make sure one side of D4 and D5 connect to R19 and the other sides connect to R20. Also double check to make sure you have the right values for R19 and R20 (10K each).

Jumping gave me very similar results to when I probe the anode of D4, distorted but very, very quiet.

 

[zgrav]

Double checked C19 and C20 and R19 and R20 but they're all as they should be.



Re-checked D4 & D5 and I'm still getting the same results as before: .3-.6 on D4 cathode, zeros across the other three. I'm using an AstroAI AM33D multimeter.

Pin 1 on IC1 & IC2 were both totally silent when probed.



Jumping gave me very similar results to when I probe the anode of D4, distorted but very, very quiet.

so something is draining most of your audio signal past that point. an extra short to another part, a bad connection, or wrong part value. what is your measured resistance to ground on each side of the diode where the sound level drops?

 

[Chuck D. Bones]

Re-checked D4 & D5 and I'm still getting the same results as before: .3-.6 on D4 cathode, zeros across the other three. I'm using an AstroAI AM33D multimeter.

What do you mean "across?" Make measurements from each circuit point to gnd unless otherwise specified.

Pin 1 on IC1 & IC2 were both totally silent when probed.

Since you hear signal at IC2-7 and nothing at IC2-1, the problem is between those two points. My guess is that in all of this parts swapping, you broke a pad or trace.