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Hi all!
Beware: I’m a pedal building newbie.
I have two P.U.R.P. pedals (both PCB version 2.1), one of which is noisy. I found that to be related to the 7660S charge pump. When I moved the charge pump from the noisy unit to the other, the noise moved to the other pedal as well. So something is off with the charge pump I reckon. I measured the voltages against ground on the charge pumps and they were very similar. So no way to single out a faulty charge pump by looking at the voltages.
P.U.R.P. details: https://guitarpcb.com/product/pumpd-up-rangemaster-plus/
I recorded sound samples of the two pedals with my mobile phone. Download: https://www.musikding.rocks/index.php?attachment/427003-audio-zip/
I posted a question in the Musikding forum (I had ordered the kit there). https://www.musikding.rocks/forum/index.php?thread/421187-defekte-ladungspumpe/
I asked if there is another way of identifying a faulty charge pump without swapping it out. I received feedback from other forum members. Forum member cebersp posted a reply (https://www.musikding.rocks/forum/index.php?thread/421187-defekte-ladungspumpe/&postID=461153#post461153) in which he suggested that in his opinion it’s not the charge pump that is the problem, but rather the P.U.R.P. circuit.
Die Ladungspumpe PUMPT halt, sodass an C9 die Spannung schwanken muss. Was über R2 im Signal landet.
Die Schaltung wurde ursprünglich für Batterie gemacht. Hier gehört z. B. ein Rc Filter nach C9 eingefügt.
Translation: “The charge pump _pumps_, hence the voltage at C9 has to fluctuate. This ends up in the signal via R2. The circuit was originally intended for battery use. One should for example add an RC filter behind C9.”
Maybe the GuitarPCB team can take a look at this and possibly add such a filter to a future revision of the PCB if there’s agreement that this would be beneficial.
I’m keeping the noisy unit. So if there’s anything anybody wants me to test I’m happy to do (if it is within my abilities that is).
Thanks for reading!
Kind regards,
Seb
That is quite amazing ! I recall having had problems with the TC1044S charge pump (similar to the TC1660S), some of which staying in the low frequency mode 10 kHz instead of 45 kHz that it should operate at when pin 1 is connected to the + voltage. I had exchanged with Klaus of Musikding about that… and we had concluded the the TC1660S was the way to go as after testing a bunch of them they all correctly switched to 45kHz when asked to do so.
Maybe the problem also occurs now on some batches. Do you have a scope to observe if the output of the pump is modulated at 10 kHz or much higher ?
C9 as you can see is the capacitor that inverts the 7660S output voltage with its + tied to ground so as such not part of the effects circuit but part of the power supply circuit
The 7660S section in this circuit is exactly the same as in the datasheet so I’m not sure a tried and tested method for a simple voltage inverter is the problem it’s more than likely, that particular 7660S or a cold solder joint, bad ground etc, unfortunately not all components of the same type are created equal I’ve built this circuit many times without problems
Did you put the ‘good’ 7660S in the noisy circuit to see if it made a difference
What are your transistor CBE voltages
To me your audio sounds more like DC getting into the audio path
Do you have an audio probe this could help identify where the noise gets into the circuit and eliminate external problems like noise from other nearby electronics
Hi Christian, Thanks for your feedback. No, unfortunately I don’t have a scope.
Hello Billy, thank you for your reply.
The 7660S section in this circuit is exactly the same as in the datasheet so I’m not sure a tried and tested method for a simple voltage inverter is the problem it’s more than likely, that particular 7660S or a cold solder joint, bad ground etc, unfortunately not all components of the same type are created equal I’ve built this circuit many times without problems
I agree the “bad” 7660S must have something going on.
The longer story is that I had PURP #1 and was very happy with it as far as the sound was concerned. But I hadn’t done a very good job on its exterior (the 125b case). It looked a bit shoddy. So I ordered PURP #2 hoping I could do a better job this time (it’s better, but I’m no artist :)).
Once I was done with building #2 I realized there was noise coming out of it that #1 didn’t have. I re-flowed all solder joints in #2 but that didn’t make the noise go away.
Did you put the ‘good’ 7660S in the noisy circuit to see if it made a difference
Yes, next I put the 7660S from #2 into #1 and the one from #1 into #2. Ever since, the noise is in #1 and #2 is noise-free.
So I think bad solder joints can be excluded and the noise must be related to the IC, because the noise moves from one pedal to the other with this IC.
What are your transistor CBE voltages
I measured the voltages on the transistors:
PURP #1 (currently with the “bad” IC)
C -6.91V
E -2.36V
B -2.35VPURP #2 (currently with the “good” IC)
C -6.90V
E -2.03V
B -2.05VDo you have an audio probe this could help identify where the noise gets into the circuit and eliminate external problems like noise from other nearby electronics
No, sadly I don’t.
Kind regards,
Sepp
The power supply I used with both pedals was the same. Same outlet, too. It’s a Truetone 1 Spot Pro CS6. Just in case anybody was wondering.
Sepp,
With the sound traveling from PURP #2 to PURP #1 when the 7066S was changed I believe the 7066S provided with #2 originally (assuming you bought it as a kit) was not good. If it were mine, I would contact the vendor and request a replacement 7066S.
I strongly disagree with the assertion on the other forum that there is a problem with the PURP board – I have an original small version that I have been using for several years and have also built several of it’s PU cousins (PUTP and PUFF) without issues (other than the ones I caused).
Chuck
Thanks for your feedback Chuck! I don’t mind the broken IC. I have a working pedal that looks half-way decent now. I just wanted to post here to provide the feedback I had received on the Musikding forum to the GuitarPCB people.
Kind regards,
Seb
Your results certainly seem to suggest you do indeed have a dodgy 7660S
I’ve had a few but usually one’s with a high pitched oscillation
Definitely make yourself an audio probe you can make a very simple one like this solder a 100n to 1u film or ceramic capacitor to the tip of a mono jack socket and a wire to the other side of the capacitor with something for the probe soldered to the wire end, I’ve used solid core wire here, then solder a wire to the sleeve with a small clip for grounding it
Here’s a PDF that explains how to build Bruce’s more pro looking probe, how to use it and how to trace the audio path
https://drive.google.com/file/d/1T9OsJ0riQhm1QFWPfNQ5q5guikuFKDRN/view?usp=drivesdk
Here’s a PDF that explains how to build Bruce’s more pro looking probe, how to use it and how to trace the audio path
Thank you Billy! Seems like a tool worth having. I’ll likely build one the next time I run into trouble 🙂
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