dc thump? click/thump has me stumped

[pucho812]

so I'm looking at an a-range channel from a desk and which I engage the low pass filters, the first and third switches 9K and 15K work perfectly. When I kick in the second switch, 12K position I get a nice loud click with midrange thump sound while engaging and disengaging that position. It works when engaged, just the thump when going in. Since all 3 position feed the same low pass amplifier stage I would have thought the issue to be in that section except for the fact only the middle position seems to do it. So far all the wiring checks out and parts seem to test out. Any ideas?

 

[Bo Deadly]

Could be capacitor leakage but probably not since the caps are all certainly smallish film. If the electrolytic coupling caps were leaking I would think all of the switches would thump. But if you put your finger on the pins of that switch as you toggle and it goes away, that would indicate a charge is building up and something is leaking.

But my money would be on bad contacts of that switch. Squirt some deoxit in there and toggle it a bunch of times.

 

[JohnRoberts]

If the switch is changing components in an op amp NF a dirty switch might momentarily go open circuit. If the op amp loses NF it could peg to a rail or make rude noises. or not...

JR

 

[megasat16]

I suggest OP clean the sockets first and retest the op amp again in 5-10 mins. Sometimes, let the op-amp and socket pins cleaning is it's all needed and thermal stress on the pins can be lousy.

Good Luck. Hopefully, it's the only thing that needed.

 

[pucho812]

Could be capacitor leakage but probably not since the caps are all certainly smallish film. If the electrolytic coupling caps were leaking I would think all of the switches would thump. But if you put your finger on the pins of that switch as you toggle and it goes away, that would indicate a charge is building up and something is leaking.

But my money would be on bad contacts of that switch. Squirt some deoxit in there and toggle it a bunch of times.

I thought about that especially since it is the only switch that does it.

for reference here is the low pass filters section in question..
left to right 15k switch, 12k switch and 8k switch

 

[abbey road d enfer]

so I'm looking at an a-range channel from a desk and which I engage the low pass filters, the first and third switches 9K and 15K work perfectly. When I kick in the second switch, 12K position I get a nice loud click with midrange thump sound while engaging and disengaging that position. It works when engaged, just the thump when going in. Since all 3 position feed the same low pass amplifier stage I would have thought the issue to be in that section except for the fact only the middle position seems to do it. So far all the wiring checks out and parts seem to test out. Any ideas?

+1 for JR's suggestion. Seems like the bas of Q18 is left without capacitive path to ground for a split second. You can try to ascertain this diagnosis by adding a low value cap (about 100pF) between the base of Q18 and ground. Since replacing the switch is probably a major endeavour, this temporary fix may become permanent...if it works, of course.

EDIT. I see your schemo is somewhat different than the one I had. My suggested fix may not work.

 

[megasat16]

damn ugly switches is all i see. shocking.

 

[pucho812]

+1 for JR's suggestion. Seems like the bas of Q18 is left without capacitive path to ground for a split second. You can try to ascertain this diagnosis by adding a low value cap (about 100pF) between the base of Q18 and ground. Since replacing the switch is probably a major endeavour, this temporary fix may become permanent...if it works, of course.

EDIT. I see your schemo is somewhat different than the one I had. My suggested fix may not work.

yes a slightly different schemo. I assure you it works as in the schemo as there is a desk full that all work as that except for the one switch. will squirt it today and see what changes.

 

[abbey road d enfer]

yes a slightly different schemo. I assure you it works as in the schemo as there is a desk full that all work as that except for the one switch. will squirt it today and see what changes.

Oh, I have absolutely no doubt it works. In fact better than the topology in the schemo I had. Yours is a true Sallen & Key arrangement, mine was two passive filters cascaded + a follower.

 

[pucho812]

Well there was no change when I touched the 12K pushbutton switch, so not a grounding issue. I also cleaned the switches and the 12K switch still clicks. even more bizarre is if I have the 15K switch in first, the 12K switch does not click at all. But if I do the 12K switch first I get the click and I can get residual click if I hit the other switches(15K and 8K) at the same time where it will get click and lower in volume until no click. I can do all 3 switches... Aside form the click it functions so hmmmm.

 

[Bo Deadly]

Well there was no change when I touched the 12K pushbutton switch, so not a grounding issue.

That's not conclusive. If discharging it with your finger worked, that would be conclusive. But just because it didn't work is not. It could be that an electrolytic is leaking super bad and your finger just isn't enough to dissipate the charge.

even more bizarre is if I have the 15K switch in first, the 12K switch does not click at all. But if I do the 12K switch first I get the click and I can get residual click if I hit the other switches(15K and 8K) at the same time where it will get click and lower in volume until no click.

So that is new. You said before the other switches had no influence. So now I'm starting to think one of the electrolytics is leaking badly.

If the leakage is really bad, you might be able to measure it with your DMM. Especially if it's got a higher Z input (some meters are 10M). If the meter Z isn't that high, you could use a scope with a 10M probe.

Specifically, measure the voltage relative to 0V on the two signal common pins of the 12K. Then toggle the switch and look for a change in voltage. If you see more than 100mV or so change, that's enough to create a pop. If it's one pole and not the other, that will also tell you which of the two electrolytics it is.

 

[JohnRoberts]

If you put a scope probe on the audio what does the click/thump look like..?

Perhaps probe around the different circuit nodes to see if it reveals itself.

JR

note: for troubleshooting a DC coupled scope probe provides a DC path and small capacitance to ground.

 

[pucho812]

That's not conclusive. If discharging it with your finger worked, that would be conclusive. But just because it didn't work is not. It could be that an electrolytic is leaking super bad and your finger just isn't enough to dissipate the charge.



So that is new. You said before the other switches had no influence. So now I'm starting to think one of the electrolytics is leaking badly.

If the leakage is really bad, you might be able to measure it with your DMM. Especially if it's got a higher Z input (some meters are 10M). If the meter Z isn't that high, you could use a scope with a 10M probe.

Specifically, measure the voltage relative to 0V on the two signal common pins of the 12K. Then toggle the switch and look for a change in voltage. If you see more than 100mV or so change, that's enough to create a pop. If it's one pole and not the other, that will also tell you which of the two electrolytics it is.

the electros were tested, they are not leaking.
Will Scope it out today and see