Vox channel switch pop

[Jazz P Bass]

Mouser has the 8 pin DIP NJM2120:

NJM2120D NJR | Mouser

[MarkusBass]

It seems to me that in post #6 you wrote that NJM2120 was already replaced.

Here's what I've done, with no positive results.
*replaced IC1 4558
*replaced the NJM2120D switching IC
*replaced C24

[lowell]

Well there ya go haha...this has had me spinning trying to track down the issue so much so that I've lost perspective! Thanks for that Markus! Saves me time and $. Ive also been dealing with some health issues and havent been at my bench very consistently.

[lowell]

Well I'm back in this thing. The reverb wasn't working so had to go in and fix that. Now I'm back trying to track down this static/hiss. It has to be before the switch IC or in the switch itself, which I doubt cause I replaced it. It is after C7 as I disconnected that and it's still there. I lifted the diodes D1/2 and it didn't change. I'm just boggled. I've tried the BFC approach to find where the noise goes away but this doesn't work with opamps does it? It just causes a real loud hum when I do it. Any advice?

[MarkusBass]

What exactly you mean by "BFC approach"?
I thought that you were working on channel switching pop but now it appears that you are trying to solve some other problem (static/hiss, which you hadn't menioned before).
Have you tried simple tests as, for example, shorting sliders of VR2 and VR7 to the ground? This could tell you a little bit more where is the problem. Instead of lifting D1 and D2 you could just short them to the ground. Do you still have hiss in this case?

Mark

[g1]

What exactly you mean by "BFC approach"?

This would be using a large value (big f'n) capacitor as an AC short to ground.

[lowell]

Markus in my post #6 I mentioned this noise issue.

This thing seems real POPpy still. Not sure what happened. The higher pitched pop content is still gone, but it's a big THUD when switched still. Also, Channel 1 clean channel has lots of fried egg noise with all controls at zero. Here's what I've done, with no positive results.

*replaced IC1 4558
*replaced the NJM2120D switching IC
*replaced C24

None of those helped with fried egg noise or switch pop. Here are some more voltage readings.

IC3 Switch

CLEAN CHANNEL ON:
Pin1 +15v
Pin3 +14mv
Pin6 +97mv

DIRTY TUBE CHANNEL ON:
Pin1 +13.8
Pin3 -75mv
Pin6 0mv

Pin3 at a negative voltage seems quite odd to me. Does this clue anyone in on what's goin on here? Pin 1 seems fine as when the switch is on there is ~200ua of current which is well within the data sheet specs.

I just reviewed this thread and had missed Enzos advice on using a scope. I always struggle with the scope but that just means I need more practice. I tend to not be able to see and interpret the readings sometimes. Mainly when dealing issues like this - noise.

Also of note is that the noise tends to be there at turn-on, and SOMETIMES goes away after a little while. Maybe some freeze spray could reveal a hot IC. Thing is, there's no IC between the first opamp and the switching IC so I think it's a resistor, cap, or solder joint.

[MarkusBass]

I see it now. But don't you think that these two problems (pop and noise in first channel) are related to each other? Short the slider of VR2 to the ground and see what happens. There is not much left that could fail. I would look at C24 (1uF) in the negative feedback loop of the first channel of IC3.

Mark

[lowell]

As I guessed would be the case... I'm having a hard time seeing the snap/crackle/pop noise on the scope. When it pops I can see transients on the speaker output on the scope. Anything in the preamp is just a hazy line with no visible transients when it pops. I've swept around with the frequency and voltage.

Of note Markus, when I ground VR2 slider I get a loud hum. I cannot tell if the noise goes away as the hum is too loud.

Enzo, the diodes are NOT as written on the schematic, they are indeed back to back.

[MarkusBass]

Of note Markus, when I ground VR2 slider I get a loud hum. I cannot tell if the noise goes away as the hum is too loud.

In similar, difficult cases I always try to find something that is obviously wrong. And hum when VR2 slider is grounded is obviously wrong. It's because this is typical tone stack just with middle pot replaced with a resistors and in such a tone stack, when you turn all pots counterclockwise, the input of the following stage is grounded. And I haven't heard about increased hum in this case. So I would try to clarify this problem. Are you sure that you connected the slider to the ground (and it was close to R12 ground)? And what happens when you turn treble and bass pots counterclockwise?

Mark

[lowell]

If I ground VR2 slider where R14 is grounded there is no hum. However, the crackling is still present. Turning the tone/volume knobs does nothing to the noise.
It's looking like it's GOTTA be a component on the top side of IC3 on the schematic. C24 has already been replaced.

[MarkusBass]

Previously you wrote that: "when I ground VR2 slider I get a loud hum" . This is not clear for me .
I had similar case recently. Crackling in preamp, which could be removed when various components were shorted to to ground. Finally, I found out that this was caused by failed capacitor in the power supply . Can you supply the preamp with an external power supply?

Mark

[lowell]

Wouldn't channel two have the same problem if it were related to the power supply?

[lowell]

Replaced all components in the feedback loop for IC3. Didn't help. Decided to just resolder IC3's socket on the pcb. Surprisingly, it worked. Noise is gone AND the switch no longer pops. My god. I've spent so many hours on this thing. In an attempt to diagnose the specific problem before trying to blindly fix it by replacing things, I think I spent more time than I needed to. If I'd just resoldered related pcb points I could've fixed it. Ya win some ya lose some. I still won, but geez I was this close to giving up.

[52 Bill]

Decided to just resolder IC3's socket on the pcb. Surprisingly, it worked. Noise is gone AND the switch no longer pops.

Out of curiosity, is the socket from the factory or was it added earlier?