MXR Phase 90 - Intermittent staticky crackle & popping noises

[jojokeo]

Hi all. I have a 70's block letter unit that I wanted to utilize once again on my board. But it's making ugly static noises while the effect is engaged only. Bypassed all is well (as it should be). The static little crackle and pops are worse when the pedal is first turned on and becomes less as it stays on. It never actually goes away entirely. I don't recall this being an issue before with it but I modernized it by making it true bypass w/ a DC power jack and LED. With a battery installed it behaves the exact same way. I've cleaned the in/out jacks, replaced most wires, and touched up many solder joints that looked iffy. I replaced one of the tantalum caps too. I've taken a wooden probe stick to the board to move things around but can't seem to pinpoint anything.
I'M STUMPED! PLEASE HELP!
Thanks in advance,
Joe

[GibsonGM]

 Any original electrolytic caps in there that might need to be replaced?   Any sockets for IC's that might be dirty/corroded?   Usually, it's things like that.   Play enough, and you'll find out what's doing it!

Always the chance your cable could be screwed up, too (?)    IME, often this crud is caused by failing jacks, believe it or not!    You could try audio probing if nothing else works....

[R.G.]

In old tube amps, this can also be caused by loose/dirty connections of the bushings of jacks and pots to the chassis. Hum too.

[zombiwoof]

It could be something as simple as a pot that needs cleaning with DeOxit D5 or a similar cleaner.  I once bought a vintage pedal on Ebay that barely worked, and had the crackle you describe.  Just cleaning the pots made the problem go away completely.  Just another idea.
Al

[jojokeo]

First thank you for your replies! Second, I'm a seasoned amp and pedal builder and am familiar with the myriad of standard issues to problems.

It's not a guitar cable, jacks, switch, pot, dirty connection, etc. - please re-read my initial comment on stuff I've ruled out, checked, and verified. The signal is absolutely fine in bypass mode. It's only present while circuit is engaged.
It only has two electrolytic caps on the board, both are Tantalum types. I thought these to be much more stable and reliable, and not prone to drying out like standard electrolytics??? I did replace one of the two (nearest the first op amp stage and effect input wire) with no change in crackley behavior & noise. I put the Tantalum back on the board.

The circuit board was a little affected by the original insulating pad material they used to put inside those boxes. It deteriorated onto the board and I cleaned it all away several years ago and the pedal sat since. It never had the issues I'm reporting currently. I wish I could post a picture so words/descriptions will have to do. I re-soldered all borderline components but some aren't as good as I'd like due to age and condition of wire. I've hit them several times w/ the iron a little hotter than normal to get as good of contact as it's going to get. I've cross checked continuity on traces & solder joints best as could be determined. The board contains 6 - 741 op-amps, of course none of them are socketed as it's a bit of a tight fit in there. There's 4 - 2n5952 Jfets & a 2n4126 tranny. All else is just ceramic caps & resistors. The phasing part of the effect works perfectly as it should. If it wasn't for the crackley pops going intermittently, it would be 100% working and good for shows and recording. But due to this, even though it's not super horrible, just can't be used for either right now.

I was hoping someone may have had a similar experience & pinned it down to a particular op-amp or transistor or location on the board? (but maybe not?) Since the effect actually works fine my guess is that it likely would be something early in the circuit of the first one or two stages??? (why I replaced that first tantalum cap and wires to board)

That's a lot of crap I've just typed to read I know, not to mention time already spent on this thing. I could've already built a new one likely  but for vintage value, mojo, technical challenge of finding what it is - I will give it a last honest effort as I've owned it for almost 40 years. Damn, that really makes me feel old right now...  

Thanks again you guys, you are my last resort and hope.
Joe

[R.G.]

Given that, clean it with rubbing alcohol and a toothbrush, let it dry, then re-melt *all* the solder joints with a touch of fresh rosin core.

If that's no help, you probably have an internally intermittent component or a cracked trace.  Components do break. Carbon comp resistors were made from phenolic tubes with terminals on each end and carbon pudding inside. The shells crack, the terminals crack away from the ends. Capacitors can develop internal breaks of the connection of the leads to the plates.

You might try poking every component several different directions with the eraser end of a pencil as well.

[jojokeo]

Thanks RG! I have already taken an orangewood stick w/ a point tried moving many of the components while on - and with my fingernails too. Anything to get the popping to increase or show itself. There are many areas on traces and components w/ slight greenish corrosion because of that broken down insulating pad material. It had a slight corrosive effect on the component side. There is one particular resistor where the ends look kinda blackish and that stuck out to me. I'll give your suggestion a go and see where I get.
In my amps this kind of characteristic would be definitely likely in a bad solder joint or old carbon comp resistor. But this is not like how old resistor's get that background noise hiss going due to the high voltages present. It's not a continual hiss but rather an intermittent crackle or pop with moments of SWEET SILENCE in between. It seems worst upon initial turning pedal "on" then lessens as it stabilizes but just doesn't stop. To me this is a characteristic of a cap? The frustration continues...(but this is how we learn!)

[zombiwoof]

I didn't see anything in your original post about cleaning the pots, that's why I suggested it could be that simple.  It was just another suggestion that I thought you might have overlooked, as I have experienced that same problem in vintage pedals and just cleaning the pots fixed it.  Just throwing out ideas.

Al

First thank you for your replies! Second, I'm a seasoned amp and pedal builder and am familiar with the myriad of standard issues to problems.

It's not a guitar cable, jacks, switch, pot, dirty connection, etc. - please re-read my initial comment on stuff I've ruled out, checked, and verified. The signal is absolutely fine in bypass mode. It's only present while circuit is engaged.
It only has two electrolytic caps on the board, both are Tantalum types. I thought these to be much more stable and reliable, and not prone to drying out like standard electrolytics??? I did replace one of the two (nearest the first op amp stage and effect input wire) with no change in crackley behavior & noise. I put the Tantalum back on the board.

The circuit board was a little affected by the original insulating pad material they used to put inside those boxes. It deteriorated onto the board and I cleaned it all away several years ago and the pedal sat since. It never had the issues I'm reporting currently. I wish I could post a picture so words/descriptions will have to do. I re-soldered all borderline components but some aren't as good as I'd like due to age and condition of wire. I've hit them several times w/ the iron a little hotter than normal to get as good of contact as it's going to get. I've cross checked continuity on traces & solder joints best as could be determined. The board contains 6 - 741 op-amps, of course none of them are socketed as it's a bit of a tight fit in there. There's 4 - 2n5952 Jfets & a 2n4126 tranny. All else is just ceramic caps & resistors. The phasing part of the effect works perfectly as it should. If it wasn't for the crackley pops going intermittently, it would be 100% working and good for shows and recording. But due to this, even though it's not super horrible, just can't be used for either right now.

I was hoping someone may have had a similar experience & pinned it down to a particular op-amp or transistor or location on the board? (but maybe not?) Since the effect actually works fine my guess is that it likely would be something early in the circuit of the first one or two stages??? (why I replaced that first tantalum cap and wires to board)

That's a lot of crap I've just typed to read I know, not to mention time already spent on this thing. I could've already built a new one likely  but for vintage value, mojo, technical challenge of finding what it is - I will give it a last honest effort as I've owned it for almost 40 years. Damn, that really makes me feel old right now...  

Thanks again you guys, you are my last resort and hope.
Joe

[R.G.]

It's not a continual hiss but rather an intermittent crackle or pop with moments of SWEET SILENCE in between. It seems worst upon initial turning pedal "on" then lessens as it stabilizes but just doesn't stop.

Hmmm.

There is a mechanism for degrading semiconductor junctions that happens when the base-emitter junction gets reverse-broken without apparent damage. It still works, but is noisier and lower gain. This can be caused by the arrangement of filter and bypassing caps inside the circuits at power on and off.

See "When Good Opamps Go Bad" and "What are all those parts for?" at geofex.

It may be that you have a damaged input on one of the opamps and/or the PNP in the mixer. Getting better as it warms up is what led me to this. Could be other stuff, but it sounds like you have covered most of them. As Sherlock said, "Eliminate the impossible, dear Watson, and whatever remains must be - the truth."

[jojokeo]

UPDATE: noise problem fixed!!!  Turns out it was the first op-amp misbehaving! I retouched up all solder joints but still same issue, then went in with prime suspect #1 & replaced the first LM741 and it's all good...finally!
Thanks everyone!
Joe