BMP OpAmp v4 – squealing (solved)

[Anonymous]

As the title says, my BMP Opamp v4 build always squeals when the effect is engaged, whether there is an input connected or or not.

I changed pretty much every important part at least once but the issue still persists. Voltages look fine (8v on pin 8 of each op-amp) and no magic smoke has been released. I changed the sockets, the opamps, and one potentiometer for known good parts, but the issue is still there. Instrument input is clean, and this does not happens when bypassed. If I play with the tone and sustain pots, I can get it to be really low, but never go away completely.

It looks like some grounding issue since when I touch certain exposed terminals with tweezers/pliers/screwdriver (like pin 3 of the sustain pot or pin one of IC2) it stops… I adapted the BOM and replaced some parts with close enough values according to what I could find or what I had on hand. Attached are a few pictures of the assembled board and the parts I replaced.

Thanks!

[Anonymous]

Quick clarification: I said “squealing”, but it’s not really squealing. It’s more like a high-pitched sine wave. I have no clue where it comes from yet. I need to double-check a few things with my oscilloscope.

[Barry]

I would agree that it is a grounding issue.

I do not think any of the changes matter much.

I would start by re-flowing everything (all pads) which would include bending the pots up and getting under there also. Re-flow the off-board wiring as well and tidy up wherever you see an opportunity.

[Anonymous]

I started investigating again this morning and retried the “tweezer test”, and I think it’s coming from the input jacks, so I’ll change those. The ones I have are stereo, so I’ll do quick run to the electronics store to get some mono jacks.

I also recorded two small videos that show the problem (yes, I need to clean my workspace…). I’m not sure it’s a gound issue, or something vibrating that shouldn’t… I’ll still reflow everything and perhaps change one or two film box caps.

• https://youtu.be/PI_tzTa5zaw
• https://youtu.be/NKi0pW58t-M

[Anonymous]

I finally found out what the issue was. It was indeed a grounding issue, but not the kind I expected.

First, I reflowed all the joints and changed the jacks for mono ones (just in case), then, once my oscilloscope was charged enough, I traced the signal at every single exposed terminal to see where the noise was injected. The culprit happened to be pin 5 of IC1. On the oscilloscope, pin 7 gave this gnarly sine which doesn’t belong here:

Pin 6 was odd, since the pitch would change when I touched it with the probe. Slightly less gnarly sine (that still doesn’t belong here) that looked like that:

Pin 5, however, showed absolutely nothing – BUT! The noise stopped when in contact with the grounded probe.

I checked a few more places in the circuit, and pin 7 of IC2 looked square-ish:

The board output looked like ungrounded noise from parasitic AC current:

Knowing that grounding pin 5 of IC1 makes the noise stop I checked the diagram to see where it goes. It goes to ground (when all is fine) through C2. I reflowed C2 again with no change. I replaced C2 and suddenly it’s working. I’m not sure what was wrong, because the “dead” part tests fine…

Now I’ll salvage the Buff n’ Blend that was originally in front of it and put it back in there.

Thanks!

[Barry]

I am glad you solved it.

That was a proper troubleshoot.

Interestingly I have also had people test parts with that tester that said a part was bad when it was not.

I see it goes both ways in the wrong direction.

[Anonymous]

It’s an OK little device to help identifying parts and values. It gives generally accurate results most of the times, but some other times it has issues. It seems this happens when the battery is low, so I guess it’s not that unexpected. I mainly use it to get ballpark hFE values of transistors and testing the odd unidentified parts that sat in the bottom of my parts bins for who knows how long.

Last update on that project: I put the Buff N’ Blend back, but the “squealing” returned 🙁 I think the buffer is most likely what’s causing it. It does that with the two JFET I tried (J113 and NTE469), and it seems to be sending 5V-ish to the board input. Is that normal? Perhaps I’ll create another thread just for that issue, but right now it works perfectly with the standard Vari-Brite 3PDT board.

Thanks!

[Big O]

I use one of these little handy dandy testers as well for testing components to confirm approximate values and identification.  I think it cost me $20.

I don’t use it for more critical measurements of transistors, diodes, etc.  Instead I use a DCA-55 to get more reliable measurements.  But for quick identification purposes, it is a great little tool.

[Author]

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