loud pop when turning pedals on/off - AKA - replacing capacitor on an active bass?

[polvo]

Sorry for a kind of cryptic subject line. I've been down a bit of a chatgpt rabbit hole trying to figure out what's going on here, but I don't know if I can trust what it's telling me. Think I need some human input on this one!

 

Problem: I get a loud pop when turning my pedals on or off. This is only on one particular bass. 

 

Likely cause (according to chatgpt): Output coupling capacitor is likely leaky or failing, causing DC leakage and popping. BUT My bass has nothing that looks like the replacement capacitor I found online (e.g. https://ebay.us/m/62aZgK). It's a Cort B4 Plus AS (wiring schematic here: https://www.cortguitars.com/wp-content/uploads/2024/07/cort-b4-plus-as-basses-schematic.pdf). 

 

My questions for the wise heads of basschat:

• Based on the tests I've done (listed below), is the capacitor likely the problem or is chatgpt hallucinating? If not, what else could I try?
• If I can't see a capacitor, does that mean it's likely build into the circuit board of the active electronics module? And if so, is that something I could realistically fix myself with basic soldering skills?

 

Tests I've done to identify the cause of the problem (summary by chatgpt):

• Measured DC voltage at output jack: ~1.75-2.0 V DC present with battery, 0 V without battery (confirms DC leakage).
• Battery voltage verified healthy.
• Output jack switching continuity: Functioning as expected, switches battery ground on connection.
• Push-pull active/passive switch continuity tested: Switch behaves correctly on both positions.
• Pickup resistance measured at circuit board inputs: Neck pickup ~3.25kΩ, bridge pickup ~9.2kΩ (normal ranges).
• Output coupling capacitor tested for DC leakage with multimeter: Resistance readings 0.3 MΩ and 0.046 MΩ, lower than ideal for a blocking capacitor, suggesting leakage.
• Battery connected vs disconnected test: DC leakage present only with battery connected, consistent with capacitor leakage causing DC to output.

Edited Sunday at 18:45 by polvo

[itu]

Please include the whole system: bass, fx, amp. 

[polvo]

34 minutes ago, itu said:

Please include the whole system: bass, fx, amp. 

 

It's a Cort B4 Plus AS RM bass.

 

Tested with multiple different pedals and amps, it repeats the same issue with all of them.

[itu]

One more: which power are you using with pedals? Have you tried batteries? 

 

I think you are close, as this sounds like DC somewhere. 

[polvo]

Also getting the same issue across different power supplies.

I was previously using one that gave me a lot of buzz when gigging so I recently switched to a Cioks PSU in the hope that would solve all my noise problem. It completely cleared up the buzz, but I'm still getting the pops.

 

Haven't tried batteries on the pedals... that's not gonna be a sustainable solution but worth testing out just to see if it makes any difference. I'll test that later today.

[Jerry C]

The capacitor that's leaking would be on your active preamp board, ahead of the putput volume pot. Can you attach a pic of the preamp?

[tauzero]

Is the preamp potted (encapsulated), or is it a naked circuit board?

 

PS: you could try connecting a high value capacitor between the preamp output and the jack socket - as there's a DC bias you could use an electrolytic. High value as it'll be in series with the output capacitor so it'll minimise the capacitance reduction.

Edited 13 hours ago by tauzero

[chyc]

7 hours ago, tauzero said:

PS: you could try connecting a high value capacitor between the preamp output and the jack socket - as there's a DC bias you could use an electrolytic. High value as it'll be in series with the output capacitor so it'll minimise the capacitance reduction.

 

You can purchase such a thing in pedal form. There may be extra sorcery inside this one, but I'd guess it is just expensive packaging around a capacitor, albeit in a high quality metal box.

 

https://www.lehle.com/Lehle-DC-Filter

[Uncle Rodney]

I've had a look at that schematic, what about turning the BG's Volume knob down to 0% as in fully off, does the problem still appear then? This assumes the BG is causing the problem and could be acceptable procedure, turn BG to zero then switch on/off. I notice the drawing suggests the EQ board is a Mark Bass product, I may have misunderstood that.

[polvo]

Thanks for all the replies. Here's a bit more info and some photos....

 

The problem does not occur when volume knob is turned to 0%, or when the battery is removed.

It does still occur when the battery is plugged in but the active/passive switch is set to passive.

 

(This means at least I have a workaround as I can just roll of the volume between songs to activate switches.... but that's not really helpful in the middle of a song!)

 

 

 

 

 

 

 

 

[polvo]

1 hour ago, Uncle Rodney said:

I've had a look at that schematic, what about turning the BG's Volume knob down to 0% as in fully off, does the problem still appear then? This assumes the BG is causing the problem and could be acceptable procedure, turn BG to zero then switch on/off. I notice the drawing suggests the EQ board is a Mark Bass product, I may have misunderstood that.

 

Replied above - yes, reducing volume to 0% is a workaround for now. I've been putting up with it for a while but with some big gigs coming up I've now decided it's no longer "acceptable procedure"!

 

13 hours ago, Jerry C said:

The capacitor that's leaking would be on your active preamp board, ahead of the putput volume pot. Can you attach a pic of the preamp?

 

Pics above - but I don't really know what I'm looking at and can't see anything recognisable to me as a capacitor.

 

9 hours ago, tauzero said:

Is the preamp potted (encapsulated), or is it a naked circuit board?

 

PS: you could try connecting a high value capacitor between the preamp output and the jack socket - as there's a DC bias you could use an electrolytic. High value as it'll be in series with the output capacitor so it'll minimise the capacitance reduction.

 

Sounds like that (or the pedal version posted by @chyc) might be less fiddly than replacing the capacitor inside the preamp circuitry, and a cheaper solution that replacing the entire preamp. Will check it out, thanks.

 

 

 

[Stub Mandrel]

From what's above, the problem is likely to be a failed capacitor underneath what looks to be nasty stiky-backed foam.

 

The undulating foam suggests surface mount components, which can be tricky to work with

 

The good news is tracing back from the output jack via the volume pot should lead you, eventually, to the dodgy capacitor (if that is the cause).

[polvo]

10 hours ago, tauzero said:

Is the preamp potted (encapsulated), or is it a naked circuit board?

 

PS: you could try connecting a high value capacitor between the preamp output and the jack socket - as there's a DC bias you could use an electrolytic. High value as it'll be in series with the output capacitor so it'll minimise the capacitance reduction.

 

Just checking I've understood this...

Would something like this be what I need? https://ebay.us/m/EnJIZt

 

And where in the circuit would it go? Between volume and output jack? Or immediately after preamp? 

 

 

[chyc]

8 minutes ago, polvo said:

 

Would something like this be what I need? https://ebay.us/m/EnJIZt

Those are for speaker crossovers, so no they're not the right ones. If you rip off the charcoal padding on the blue PCB in the circuit diagram you should see a physically much smaller capacitor, and with luck there will be the rated capacitance written on it. On the circuit diagram above, it is not shown, but would be on the blue thing in the centre, on the other side of the board.

[tauzero]

I'd suggest a bipolar electrolytic capacitor, 100uF, between the volume control wiper (centre terminal) and the jack socket. https://www.ebay.co.uk/itm/135605376775?_trksid=p2332490.c101875.m1851&itmprp=cksum%3A135605376775cef0a91da54e454782348804eec8ffed|enc%3AAQAKAAABAGqm9FAFUYsD%2Bg2P7amRF5PbrMKBN936TfFmdyEufDvNNKlouoyOXUVGTPycutqBFaPaOz%2BQqnhN8qhZ%2BG5l%2BL7DDJt8BGa%2FL%2FwS2wEfKZPNxF6YW3%2BOx%2F7ihlnjbvlvBdhOpkkf4UKXyGM%2BbVkoLJB52mgrxZb48efu8b%2FhufVM8MgbVsMtNLBULIZNQOKjZc5QWzm1qqCJ6%2FxIL4Bf7IPNWqZCHUiTIXx8mqkMApNuDghAJGD4Cl0x4rpQFzfYOmJWkS7uv9s19vuRnPSWKX6%2F7kol5CacR7y0rSDod%2F1DYURL1WsxQVrGELLor6eq4lvgL5Ifzlg%2Bthp6HhqDbZA%3D|ampid%3APL_CLK|clp%3A2332490&itmmeta=01K5V1RHF038YS2JAVXMHRP6AW&var=434962815180 - the 100uF and axial (one lead each end) would probably be easier to fit than radial (both leads at one end).

 

Depends how urgent this is. If you've got a little time, it's going to be worth getting the sticky foam off the component side of the preamp circuit board. There's going to be a capacitor connected to the orange wire just by the red wire in the upper connection block on that diagram, and that's most likely the guilty party. It may well be a case of replacing the entire board though.

 

I think there's another work round, which is to briefly switch to passive when switching a pedal, although that's not ideal either.

[polvo]

29 minutes ago, tauzero said:

I think there's another work round, which is to briefly switch to passive when switching a pedal, although that's not ideal either.

 

No, I still get the pop even when switched to passive.

It only stops with volume at 0% or with the battery removed.

 

[Uncle Rodney]

Doing this from a distance rather than being hands on the thing. I think that PCB is the culprit. It is probably going to be what's called "surface mount" electronic components which are mega fiddly to do anything with unless fully equipped with big eyes (magnifier eyepiece) and a suitable soldering set. So not really fixable for most people.

The "problem" is a well known effect in circuit design, so I would be tempted to investigate the options, see what Cort have to say, because... if this is a one off fault, they should offer a replacement, it looks to be an easy plug connector. But if it's an oversight in design, custard pie in face is called for, because... if this is a Mark Bass EQ or any well known designer, this is such a basic "design out" problem. One of the first tests any designer will do is to check for this power on/off glitch.

Ask Cort for their opinion. The big clue is, BG volume set to 0% and there's no problems.

Edited 1 hour ago by Uncle Rodney

[Stub Mandrel]

If if happens with the preamp switched out it is NOT  a problem with the preamp as the circuit as drawn abovd totally isolates it.

 

If there's a dc voltage on the output jack with the preamp switched out, then possibly it's an earth fault or maybe a partial short from the battery connection to the signal on the jack socket.