Magnetron Delay single repeat

[DrippyEgg]

Hello all,

I’m embarking on my third build after a dazzling success followed by an abysmal failure. I think I got too big for my britches after my first build worked first try and bit off more than I could chew for the second.

Anyway, I’ve been working on the magnetron delay and have populated the board. When I hooked it up to my test pedal, I was ecstatic to hear the warbly delay but unfortunately I can only generate a single repeat. I read through some other forum posts and reflowed the solder to the feedback pot but no success. Any tips?

P. S. I got the PT2399 from Small Bear so should be legitimate. Thanks!

 

[JamieJ]

Are your lower row of pots shorting out against the board? Put a piece of plastic between and retest to see if that changes it.!

 

[DrippyEgg]

Are your lower row of pots shorting out against the board? Put a piece of plastic between and retest to see if that changes it.!

I included the second photo to show the clearance between the board and the pots and I’m testing the circuit outside the box so that clearance remains constant. That being said, I wasn’t certain that pot contact could short the board so I tried with some plastic wedged in there but no dice.
Also, it seems when the feedback pot is turned fully clockwise the repeat is slightly quieter.

 

[JamieJ]

Even though you have left clearance from the board, the top and bottom row of pots are touching. Bend the two bottom B100Ks up to make sure they are not touching and see if you get the normal amount of repeats.

“Pot condoms” are alway helpful!

 

[DrippyEgg]

Even though you have left clearance from the board, the top and bottom row of pots are touching. Bend the two bottom B100Ks up to make sure they are not touching and see if you get the normal amount of repeats.

“Pot condoms” are alway helpful!

Sorry for the delay (lol), there is actually some clearance there but just to be safe I did as suggested but no effect. But thanks again, like I said before I suspected that this could be an issue but wasn't certain so this'll definitely help for future builds.

 

[phi1]

The photo that shows clearance with the pcb avtuallylooks like there’s something out of focus bridging (could be the angle of the phot though), and I know you already tested with plastic barrier.

It seems like somewhere in the feedback path is shorting to gnd. (That would also help explain the quieter repeat at feedback pot max). I would use a multimeter to check resistance to ground at certain points along that path (from feedback pot pin 2 to C3). While you’re at it, check series rectangle from feedback pot pin2 to C3(-) for 11kohm.

 

[Betty Wont]

Are the pins of your upper pots touching the bodies of your lower pots?

 

[Cybercow]

On the component-side photo, it's clear that a number of the solder points did NOT flow thru the PCB mounting holes completely. I'd definitely conduct a reflow of solder on those starved solder points. And as others have pointed out, the pot bodies should NOT be touching the legs of the others, and the three pots that hang over the solder-side of the board could use some insulation between the pot housings and the PCB.

 

[DrippyEgg]

Thanks for the feedback everyone! I really appreciate the support. To reiterate, there is no contact between any of the potentiometers. When doing future builds, I'll definitely order some plastic covers for the pots and keep my eye on the clearance.

It seems like somewhere in the feedback path is shorting to gnd. (That would also help explain the quieter repeat at feedback pot max). I would use a multimeter to check resistance to ground at certain points along that path (from feedback pot pin 2 to C3). While you’re at it, check series rectangle from feedback pot pin2 to C3(-) for 11kohm.

Awesome! Thanks for the detailed suggestion. Resistance to ground between feedback pot pin 2, R9, and C3 to ground are all around 38 megaohms. RE: the series rectangle I'm not sure what that is.

 

[phi1]

Ah series resistance, not rectangle! Just measure resistance from pin2 to C3(-), should be around 11k.

The resistance to ground at those points you measure would depend on feedback pot position (the resistance between pin2 and gnd). You measured 38M? It should never be more than 100k (pot all the way up). So that’s strange. Or did you mean 38k?

 

[DrippyEgg]

Ah series resistance, not rectangle! Just measure resistance from pin2 to C3(-), should be around 11k.

The resistance to ground at those points you measure would depend on feedback pot position (the resistance between pin2 and gnd). You measured 38M? It should never be more than 100k (pot all the way up). So that’s strange. Or did you mean 38k?

Resistance from pin 2 to C3 was 11k.

Oddly, I’m only getting resistance to ground if it’s hooked up to power but when I do I can confirm that it’s 38M.

 

[phi1]

All those points have a resistance path to ground that should read when the pedal is un-powered. (0~100kohm, depending on pot position). Maybe you have a bad pot? Or damaged solder pads? I'd use the continuity (beep) setting on the multi-meter to make sure pot pins are connecting to the components they are supposed to.

 

[nate433]

Have you double checked the values of your capacitors? When i had a similar issues (different delay pedal, but only got a single delay) it was that one of my caps was the wrong value.

 

[DrippyEgg]

Sorry all, fell off the earth there for a while.

All those points have a resistance path to ground that should read when the pedal is un-powered. (0~100kohm, depending on pot position). Maybe you have a bad pot? Or damaged solder pads? I'd use the continuity (beep) setting on the multi-meter to make sure pot pins are connecting to the components they are supposed to.

Feedback pot to R9, C11, C12, and level pot all clear for continuity. I also checked polarity of all relevant components. I did have to remove and replace the first L78L05 (IC4) that I installed because it was cocked once I soldered it in. Could damaged solder pads in that spot cause what what we're seeing here?

Have you double checked the values of your capacitors? When i had a similar issues (different delay pedal, but only got a single delay) it was that one of my caps was the wrong value.

I just checked all cap values as well, everything seems clear.

From what I understand about PT2399 delay circuits, the repeats are generated between pins 11 and 12 of the chip. Should I pay attention to components in that path?

 

[DrippyEgg]

Resistance from pin 2 to C3 was 11k.

Oddly, I’m only getting resistance to ground if it’s hooked up to power but when I do I can confirm that it’s 38M.

Also to clarify, when the pedal is unpowered the multimeter's saying OL when one lead touches a piece of metal and the other touches the corresponding component. When I used the Fluke I was getting 38M but OL with the Harbor Freight multimeter. Just want to make sure I'm measuring resistance to ground correctly.

 

[phi1]

Sorry all, fell off the earth there for a while.


Feedback pot to R9, C11, C12, and level pot all clear for continuity. I also checked polarity of all relevant components. I did have to remove and replace the first L78L05 (IC4) that I installed because it was cocked once I soldered it in. Could damaged solder pads in that spot cause what what we're seeing here?


I just checked all cap values as well, everything seems clear.

From what I understand about PT2399 delay circuits, the repeats are generated between pins 11 and 12 of the chip. Should I pay attention to components in that path?

The pt2399 is a single repeat device. The feedback is achieved by sending some of the PT2399 output through the feedback pot, then ties in with the input. Somehow you’re losing that signal between the output and input, either it’s grounding accidentally or the connection is broken. That’s what I was trying to get at with my other check ideas.

 

[Locrian99]

Also to clarify, when the pedal is unpowered the multimeter's saying OL when one lead touches a piece of metal and the other touches the corresponding component. When I used the Fluke I was getting 38M but OL with the Harbor Freight multimeter. Just want to make sure I'm measuring resistance to ground correctly.

It sounds like you are checking the resistance to the enclosure as ground? Check it vs ground of your circuit it may be the same if the circuit is connected to the metal for the enclosure but is read it as resistance to one of the ground points in the schematic say pin 4 of the 2134 or whatever ic you used for the signal path.

 

[jimilee]

Sorry all, fell off the earth there for a while.


Feedback pot to R9, C11, C12, and level pot all clear for continuity. I also checked polarity of all relevant components. I did have to remove and replace the first L78L05 (IC4) that I installed because it was cocked once I soldered it in. Could damaged solder pads in that spot cause what what we're seeing here?


I just checked all cap values as well, everything seems clear.

From what I understand about PT2399 delay circuits, the repeats are generated between pins 11 and 12 of the chip. Should I pay attention to components in that path?

Yes

 

[DrippyEgg]

The pt2399 is a single repeat device. The feedback is achieved by sending some of the PT2399 output through the feedback pot, then ties in with the input. Somehow you’re losing that signal between the output and input, either it’s grounding accidentally or the connection is broken. That’s what I was trying to get at with my other check ideas.

It sounds like you are checking the resistance to the enclosure as ground? Check it vs ground of your circuit it may be the same if the circuit is connected to the metal for the enclosure but is read it as resistance to one of the ground points in the schematic say pin 4 of the 2134 or whatever ic you used for the signal path.

Okay, checking the resistance to pin 4 of the PT2399 yields more sensible resistance values, though the pot jumps from 0 to 90k in less than a quarter turn and maxes out around 98k for the rest of the sweep. Since it's an A100K I know not to expect a linear progression but is this the expected progression? I anticipated a more gradual increase in the lower portion of the sweep with a sharp increase toward the end. It seems this is the opposite with an early jump followed by a smooth climb.

 

[Cybercow]

Okay, checking the resistance to pin 4 of the PT2399 yields more sensible resistance values, though the pot jumps from 0 to 90k in less than a quarter turn and maxes out around 98k for the rest of the sweep. Since it's an A100K I know not to expect a linear progression but is this the expected progression? I anticipated a more gradual increase in the lower portion of the sweep with a sharp increase toward the end. It seems this is the opposite with an early jump followed by a smooth climb.

Are you measuring the sweep of the 'Feedback' pot while it is in circuit?

If you're measuring the pot and its sweep while it is in the circuit, note that the "Level" pot ( B100K) AND R9 (1K) & D1 & D2 are in parallel with the "Feedback" pot (A100K). That configuration will affect a measured sweep of the 'Feedback' pot while it is in the circuit.