Vox channel switch pop

[lowell]

It's a Cambridge 30 Reverb. The pop is quite loud...loud enough to be audible while playing. I went in and noticed someone has repaired this thing before so I guess I should double check their work. I did find that they replaced C28 with a jumper so I put that coupling cap back in. Pop is still the same. If I ground the negative side of that cap the sound AND the pop go away. So I think the issue is IC3 or before that. I tried adding a 1M resistor from the top of relay SW3 to ground....didnt help. Both +/-15v rails are good. Finally the pop remains with ALL controls at zero and pops when switching to and from both channels.

Schematic:
http://music-electronics-forum.com/a...mbridge30r.pdf

[Enzo]

Never a good idea to just replace bypassed parts unless you know why. For all you know that could have been the result of a service bulletin.

What usually causes pops is uneven DC voltages getting connected. IC3 is the switch. Is there DC on its output pin in either state? How about its input pins, is ther DC on any of them in either state? Does it have good 15v rails hitting its power pins?

[MarkusBass]

As Enzo has written, check DC voltages on the inputs of IC3. Check voltages on R14 and R36 (or directly on inputs of IC3). You may also clarify what you mean by relay SW3. Looking at the schematic it seems that this is just a simple switch that connects an inductor L4 to the circuit and it has nothing to do with channel switching. Is it a relay? Can you post a photo?
You can also see that C28 is in series with C29 and it was most probably decided at the factory that is is not needed.
Grounding C28 is a very bad idea since in this way you short output of IC3 to the ground (and opamps do not like such conditions). I hope that IC3 is still alive.

Mark

[Dave H]

The switching pop may be a design "feature" of IC3. The data sheet says there is 200mV of "shock noise" when switching channels. The noise can be reduced by adding a capacitor (Csw) to the switching transistor but it will still be 150mV.

[lowell]

Enzo,

IC3 output pin
0v Channel 2
22mv Channel 1

IC3 inputs
Pin2 4mv
Pin7 .8mv

15v rails are both good.

My bad regarding the relay. It looks like one on the schematic! Was a glancing over it.

I added Csw 100uf per the data sheet. It definitely helped... I think it's enough to where it may not be noticeable when performing. Of note is that it no longer pops at all when switched from Ch2 to Ch1. If there's any suggestions regarding the voltages I've posted let me know as I'd be interested in lessening the pop more.

Thanks for Csw suggestion Dave

[lowell]

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.

[MarkusBass]

Lowell,

Please used basic troubleshooting method: divide and conquer. For example, disconnect the preamp from the power amp. Is the problem still there? Disconnect part of the preamp, and so on. This will allow you to determine which part of the amp is critical for the problem. If you find it, focus on this part of the amp. You may also try to connect a different preamp to this power amp. There are many methods to narrow the problem.

Mark

[lowell]

I believe I've done just that Markus. As details stated in my first posts. Its not before the switch IC, and doesnt poo if the IC output is AC shorted to ground.

[MarkusBass]

Its not before the switch IC

I think that previously you said that "the issue is IC3, or before that". You haven't mentioned any test that excludes "before that" part. To check it, I would short to ground wipers of both VR2 and VR7 pots (turn them CCW before that). Still the same? In such a case I would assemble the switch on a breadboard and see it the circuit has the same problem. If yes, this is "by design" and you can call it "feature".

Mark

[lowell]

No worries Markus, I'll check it out thanks! Haha "and doesn't poo if AC output..." whoops

[Enzo]

Lowell, the thing to do is use your scope or a signal tracer to explore the noisy channel step by step to see/here where the noise enters the system. Is it at pin 7 of IC1 only or pin 1 as well? It is noisy with all controls at zero, but does advancing the gain control VR1 make it even noisier or not? Is pin 7 of IC1 clean but the noise is at the treble control wiper? etc. localize the problem.

By the way, are D1,D2 really parallel the same way like that? Or is one of them really facing the other way. Either one could be noisy, and a quick lift would tell you that.

I wouldn't sweat the few millivolts negative on that dirt channel input pin. They are on the dirt channel side, and that isn't the noisy channel, is it?

I doubt the noise is involved with the pop.
Of course I could be wrong.

[MarkusBass]

Haha "and doesn't poo if AC output..." whoops

This sentence wasn't clear enough for me .
I've just realized that our advise to measure the voltages directly on inputs of the switching IC wasn't perfect. It would be much better to measure the voltages "before inputs" - and by this I mean directly on sliders of VR2 and VR7. You remember that once you were checking signal on input of a inverting opamp and there was almost no signal (despite the fact that the opamp worked correctly). This was due to so called "virtual earth" on the inverting input. That's why it is much better to measure the voltage "before" the input. For example, if C9 or C22 are leaky, they could cause symptoms like the one that you have. So, please measure the voltages on sliders of VR2 and VR7 (in reference to the ground, scale range 200mV, or less).

Mark

[lowell]

Ok finally measured this. There is 0v on VR2/7 sliders in both switch positions.

[lowell]

Well I'd like to just go ahead and replace this switching device in here. All other options have been exhausted and it just must be the switch itself. If this doesn't fix it then it is what it is. It's still popping pretty loud and I can't imagine they would've let this pass at the factory. Does anyone have any good sources for the NJM2120? I see some on Ebay but not sure if I should trust them or not.

[g1]

2121 will also work. Mouser may have one or the other. D suffix for DIP package.

http://www.mouser.com/ProductDetail/...I2KOllXwyxw%3d