Adding a blend to a Promethium (HM-2)

[delackattack]

I would work one problem at a time. You need to find out why you got smoke and, more importantly, did the smoke damage something. I'd isolate it on a breadboard and check voltages first. When you are sure in your mind that the Jfet is ok, then I'd start worrying about how to fly it into the existing circuit.

(So does that mean you were running 9v to one of the pot lugs?)

PS: Your particular blend circuit is explained in Post #21. Here's a schematic to it:
View attachment 20152

And here's some discussion about it where the author says use only Linear pots:

Mini JFET Blend

Collection of vero (stripboard) & tagboard layouts for 100s of popular guitar effects, with over 1000 verified designs. DIY your own boutique effects!

tagboardeffects.blogspot.com

Can I check the voltages with a Multimeter?

 

[delackattack]

I do have a couple Buff N Blend pcbs laying around that I could try as well but I feel like a fella earlier had mentioned oscillation issues with it?
Probably worth a shot

 

[Mike52]

I am pretty sure I was running it to a pot lug.
I wired a new one into the pedal but now I’m coming across the same issue.
Volume all the way with the blend is at zero, kind of a mix at 50 percent and dead silent when I max the blend.
I’ve also wired it both to the C23 cap and the DC jack.
I’m having my guitarist bring my first one back so I can re examine what I did or didn’t do to get to work on my first try.

This is a perfect opportunity to use a breadboard. If you ran 9 volts to one of the pot lugs then the schematic is a clue to what you are possibly facing. The schematic tells the story.

I barely know what I'm talking about, (and I keep hoping that more experienced builders will weigh in!!! Hello, [tap, tap] is this thing on?) but my suspicion is that the negative side of that 10uf capacitor saw +9v DC, or some part of it. If that is the case then it is possible that that is the source of the savory aroma of "Oops" that you experienced. You can Google "what happens when electrolytic capacitors are installed backwards" to learn a little about the issue. But I'm not positive, just suspicious.

In any case, this is a simple circuit to run and test. You plug the 9v node into the breadboard power rail and you plug the ground into the negative rail and then hook those rails to the respective ends of a fresh 9v battery. Then you just read voltages at the drain and the gate (referenced to ground, of course). If you don't understand why that is important, definitely read up on Jfet transistors and how they work.

As you can see by the schematic, the drain should get 9 volts and the entrance to the gate is a voltage divider so that should get exactly half of the voltage that the drain is receiving. Make sense?

Now I just want to play guitar and build pedals that I want to play and then play them. Debugging circuits sucks and I don't want to waste time doing all that crap. But the fact is, when I spend the time studying circuits and debugging problems my knowledge base grows and at the same time it expands the opportunities for me to modify pedals to my particular liking, or construct something exactly the way I want it constructed. I'd strongly encourage you to chase down this problem and fix it now, rather than shift gears and go on to something that looks easier (especially when two other people in this thread have mentioned known problems with it). You will be happy that you did. Just sayin ....

 

[delackattack]

This is a perfect opportunity to use a breadboard. If you ran 9 volts to one of the pot lugs then the schematic is a clue to what you are possibly facing. The schematic tells the story.
View attachment 20181
I barely know what I'm talking about, (and I keep hoping that more experienced builders will weigh in!!! Hello, [tap, tap] is this thing on?) but my suspicion is that the negative side of that 10uf capacitor saw +9v DC, or some part of it. If that is the case then it is possible that that is the source of the savory aroma of "Oops" that you experienced. You can Google "what happens when electrolytic capacitors are installed backwards" to learn a little about the issue. But I'm not positive, just suspicious.

In any case, this is a simple circuit to run and test. You plug the 9v node into the breadboard power rail and you plug the ground into the negative rail and then hook those rails to the respective ends of a fresh 9v battery. Then you just read voltages at the drain and the gate (referenced to ground, of course). If you don't understand why that is important, definitely read up on Jfet transistors and how they work.

As you can see by the schematic, the drain should get 9 volts and the entrance to the gate is a voltage divider so that should get exactly half of the voltage that the drain is receiving. Make sense?

Now I just want to play guitar and build pedals that I want to play and then play them. Debugging circuits sucks and I don't want to waste time doing all that crap. But the fact is, when I spend the time studying circuits and debugging problems my knowledge base grows and at the same time it expands the opportunities for me to modify pedals to my particular liking, or construct something exactly the way I want it constructed. I'd strongly encourage you to chase down this problem and fix it now, rather than shift gears and go on to something that looks easier (especially when two other people in this thread have mentioned known problems with it). You will be happy that you did. Just sayin ....

Okay, im gonna put it on a breadboard. But, would a breakoutboard make a difference as to how i should be wiring this up? I only ask cause the first one I did used a 3pdt and wired it "the ole fashion way" and its the one that worked on the first try

 

[Mike52]

Okay, im gonna put it on a breadboard. But, would a breakoutboard make a difference as to how i should be wiring this up? I only ask cause the first one I did used a 3pdt and wired it "the ole fashion way" and its the one that worked on the first try

The point of breadboarding is to see if your blender still works as it should. Does it have 9v where it's supposed to, does it have half voltage where its supposed to, is the electrolytic still functioning. That is a separate issue from how to fly this circuit into the host pedal. And I think how you could most effectively wire it is covered in this thread by someone else who is using the exact same blender circuit that you are using (Post #21 and on).

You could actually use the breadboard to test this as well. Just clip in wires to the suggested locations in the host pedal and see what happens. You can fly +9 volts and ground from the host pedal to the breadboard power rail. And then start the experiment.

Your 3pdt is just a switch that drops the blender circuit in and out of the host pedal on command, right? The 3pdt had to be connected to the host circuit somehow. But it sounds like what you are trying to do is make the blender circuit always on while the pedal is on, so adding an additional 3pdt may not be the right solution for you.

But now that I think about it, it seems like you could get filtered 9v and ground from a 3pdt PCB pad if you are using them, since they are getting their 9v from the circuit's power rail. This guy, as just one handy example, has additional grounding points and 9v power takeoff. (I'd be curious as to whether you could grab 9v from the LED circuit somehow so that when the host pedal is turned off, the blender circuit is also turned off. That circuit takes a signal from the "LED_BLK across a dropping resistor to the anode and cathode pads. Wouldn't that mean that there's 9 volts on the LED_BLK solder pad when the pedal is on? Hmm.)

Anyone else want to weigh in here?

I know we are late in the game here, but this also might be a good project to take the the Troubleshooting forum. Might get more experienced eyes on the problem.

(also, the thing that's been nagging at me is your wires. You have too many of them. That's why it looked so odd.


Your input and send are the same thing. You are scavenging a guitar signal from the host pedal so that the blender can process it. Only one wire comes into the circuit, not two. Then when you've got the signal and the circuit has done its magic, you send it back. Wiring pots always confuses me, but this thread will explain all that.)

 

[delackattack]

The point of breadboarding is to see if your blender still works as it should. Does it have 9v where it's supposed to, does it have half voltage where its supposed to, is the electrolytic still functioning. That is a separate issue from how to fly this circuit into the host pedal. And I think how you could most effectively wire it is covered in this thread by someone else who is using the exact same blender circuit that you are using (Post #21 and on).

You could actually use the breadboard to test this as well. Just clip in wires to the suggested locations in the host pedal and see what happens. You can fly +9 volts and ground from the host pedal to the breadboard power rail. And then start the experiment.

Your 3pdt is just a switch that drops the blender circuit in and out of the host pedal on command, right? The 3pdt had to be connected to the host circuit somehow. But it sounds like what you are trying to do is make the blender circuit always on while the pedal is on, so adding an additional 3pdt may not be the right solution for you.

But now that I think about it, it seems like you could get filtered 9v and ground from a 3pdt PCB pad if you are using them, since they are getting their 9v from the circuit's power rail. This guy, as just one handy example, has additional grounding points and 9v power takeoff. (I'd be curious as to whether you could grab 9v from the LED circuit somehow so that when the host pedal is turned off, the blender circuit is also turned off. That circuit takes a signal from the "LED_BLK across a dropping resistor to the anode and cathode pads. Wouldn't that mean that there's 9 volts on the LED_BLK solder pad when the pedal is on? Hmm.)
View attachment 20185
Anyone else want to weigh in here?

I know we are late in the game here, but this also might be a good project to take the the Troubleshooting forum. Might get more experienced eyes on the problem.

(also, the thing that's been nagging at me is your wires. You have too many of them. That's why it looked so odd.

View attachment 20187
Your input and send are the same thing. You are scavenging a guitar signal from the host pedal so that the blender can process it. Only one wire comes into the circuit, not two. Then when you've got the signal and the circuit has done its magic, you send it back. Wiring pots always confuses me, but this thread will explain all that.)

So I wouldn’t need the IN and OUT on the SFB or vice versa?

 

[Mike52]

So I wouldn’t need the IN and OUT on the SFB or vice versa?

I don't understand the question. Have you read this post and his followup posts? He used your circuit and made it work on the real pedal.

Adding a blend to a Promethium (HM-2)

I modded my real HM-2. The schematic at the Peperspedals link shows it wired right in between the flip flop switching (Q4 & Q10). The Promethium also has the in and out buffers, so it should work the same. You can see where "send", "return" and "output" are wired, when using the Tagboard...

forum.pedalpcb.com

He may be wrong, or he may be right. But you can figure it out by linking the two circuits to your breadboard as I described above.

As to your wires. Your picture shows that you have 9 wires coming off the circuit. As I look at the schematic, I can only see seven wires at most (and one is redundant in this specific case). There should be a (1) +9v, (2) a ground, an (3) input, a (4) send connected to the input, and then three wires to connect the SFB to the three lugs of a pot. What do the other two wires on your board do?


And a followup question: Have you checked the continuity between all the wires that are supposed to be connected by the circuit and all the traces. I just see a lot of solder globs that could be touching other circuits, and your pot wires look like they aren't going to the right places on the output side of things. It almost looks like white #2 is connected to 9v and like white #3 is connected to both of the other blue wires. This is where color coding wires is really helpful in preventing mistakes.

 

[delackattack]

I don't understand the question. Have you read this post and his followup posts? He used your circuit and made it work on the real pedal.

Adding a blend to a Promethium (HM-2)

I modded my real HM-2. The schematic at the Peperspedals link shows it wired right in between the flip flop switching (Q4 & Q10). The Promethium also has the in and out buffers, so it should work the same. You can see where "send", "return" and "output" are wired, when using the Tagboard...

forum.pedalpcb.com

He may be wrong, or he may be right. But you can figure it out by linking the two circuits to your breadboard as I described above.

As to your wires. Your picture shows that you have 9 wires coming off the circuit. As I look at the schematic, I can only see seven wires at most (and one is redundant in this specific case). There should be a (1) +9v, (2) a ground, an (3) input, a (4) send connected to the input, and then three wires to connect the SFB to the three lugs of a pot. What do the other two wires on your board do?
View attachment 20188

And a followup question: Have you checked the continuity between all the wires that are supposed to be connected by the circuit and all the traces. I just see a lot of solder globs that could be touching other circuits, and your pot wires look like they aren't going to the right places on the output side of things. It almost looks like white #2 is connected to 9v and like white #3 is connected to both of the other blue wires. This is where color coding wires is really helpful in preventing mistakes.

Sorry, juust got back to this. The extra two wires are send and return. This is also the past circuit ive made that i had mentioned placing wires in the wrong postions

 

[Fuzzonaut]

Resurrecting a hella dead thread here but does anyone have a copy of this Dunwich schematic that they could share with me? The Oshpark link doesn't exist anymore.

It's here now.

 

[jesuscrisp]

It's here now.

Don't want to be a party popper (or maybe I do), but this looks like a pretty poor way to add a clean blend. Could have at least amplified the clean path signal a bit or made the mixing stage more than a buffer instead of that 1M blend pot that really only just puts resistance in either path.

Might as well just use the signal straight off the input buffer, or, you know, put a proper mixing stage in there if you already add an extra dual op amp...