SOLVED Oxide Distortion is Too Quiet

[apierz]

Solution: Using an audio probe and tracing the path of the signal, I found that the second op-amp stage was not actually sending out an amplified signal on its output, pin 7. I reflowed the solder on the parts in that stage and replaced the spec'ed IC, a CA3260, with a JRC4850D.

------------------------------ Original Post ---------------------------------------------------------
I decided to take another crack at the Oxide build. I bought a new board and repopulated it from scratch with the exception of one component (C13 a 100nF cap) which I tested and it came in right at 100nF after installation on the new board. Everything is to spec except C4 (I have a 47pF cap instead of a 51pF cap). Unlike last time I got a signal right away and all knobs were responsive. However, when I tested it in my signal chain it was noticeably quieter then the bypass level, even with the pedal's volume and drive knobs turned all the way up. I did some testing with the audio probe and the signal seems fine until there is a large drop in volume across R10 (a 47K resistor). I don't know if this is intended or not but the signal volume never recovers after this resistor. I checked the resistor value with my DMM and it came in right at 47K. I checked some other schematics for the Mostortion and they all have a 47K resistor in that spot of the circuit.

Any help is greatly appreciated.

 

[swyse]

Is your second op amp stage not boosting the signal by about 5.7 times to make up for the signal level lost to the tone stack?

 

[apierz]

Is your second op amp stage not boosting the signal by about 5.7 times to make up for the signal level lost to the tone stack?

No, the volume coming out of pin 7 on the IC is about the same as it is coming out of R10. Does that indicate a problem in the second op amp stage? I've tried two op-amps (one 3260 and one JRC4588D) and neither made an appreciable difference in the volume.

 

[jesuscrisp]

Maybe try reflowing everything, sounds like the second op amp stage isn't working

 

[swyse]

My guess is the issue is one of the following
Pin 6 on the IC
R13
R20
C15
I'd reflow those or check them for continuity because it sounds like your IC is operating like a buffer and not giving you a gain boost.

 

[Bricksnbeatles]

Is your second op amp stage not boosting the signal by about 5.7 times to make up for the signal level lost to the tone stack?

Where are you getting 5.7x from? With R20 at 10k and R13 at 47k, I’m calculating a gain of 1.2 [A=1+(Rf/Rin)]

You get a gain of around 5.6 with R13 at 2.2k, but yeah… I’ll admit I just woke up so I’m taking a pretty rough glance at the schematic, but I’m not seeing where you’re getting 5.7, and the meager increase in volume seems to make sense to me unless I overlooked a key element

 

[swyse]

1+(47000/10000)
1+4.7=5.7

 

[Bricksnbeatles]

1+(47000/10000)
1+4.7=5.7

Well I did say “unless I overlooked a key element”, right?

I’d say overlooking the fact that I accidentally was dividing 10k by 47k and not the other way around is pretty key. I’m not a coffee drinker, but I’m pretty sure I need my morning coffee after that blunder

 

[swyse]

Well I did say “unless I overlooked a key element”, right?

I’d say overlooking the fact that I accidentally was dividing 10k by 47k and not the other way around is pretty key. I’m not a coffee drinker, but I’m pretty sure I need my morning coffee after that blunder

An easy mistake to make! I always thought it was weird that it was called Rin when I’m running it to ground so that helps me remember somehow.

 

[Bricksnbeatles]

An easy mistake to make! I always thought it was weird that it was called Rin when I’m running it to ground so that helps me remember somehow.

Yeah, I just took note of the fact that it was r20 and r13 setting the gain of the second op amp stage, but didn’t actually take note of which was in which place before I closed out of the schematic to check in the pictures that the values were both correct. Once I saw that they were, I double checked the calculation but didn’t return back to the schematic so I thought r20 was the feedback resistor rather than r13

 

[apierz]

Thank you to everyone for your help, espcially @swyse! There was definitely a bad looking solder connection in the second op-amp stage which resolved that issue. After more testing it seems like all 3 of the CA3260 were bad or went bad during testing. I can see why they discontinued them! I replaced it with a JRC4850D which was giving me good volume out of pin 7. Then I was getting a dying tube sound out of Q2 so I replaced it. After fixing a few wire solders I think it is now working as intended. I'll do a little more testing but hopefully I can finally put this build to bed.

Thanks again!

 

[Chuck D. Bones]

There is nothing wrong with CA3260s. ICs get discontinued for all sorts of reasons. CMOS opamps like the CA3260 are very susceptible to static damage. If in fact that part was bad, and I see no conclusive data here, it was most likely damaged by mishandling, either by you or the guy who sold it to you. Where did you get them and how were they packaged? ESD tube? Metallized mylar baggie?

 

[apierz]

I got them on Ebay. They came set in foam in a bag. One worked briefly then stopped producing anything bit a high pitched whine from the output of the second op-amp. The other 2 didn’t have any output at the first op-amp stage. It could definitely be down to mishandling on my end but I’ve never had an IC go bad when installing/testing let alone three in an hour.

 

[Chuck D. Bones]

Most of the people selling semiconductors on eBay
a) have no way of knowing if the parts they're selling are legit
b) have no way of knowing if the parts they're selling are functional
c) have no clue how to handle the parts to avoid ESD damage
And some of them are deliberately selling fake parts

I hope you didn't pay too much for those. My advice for folks who want to buy semiconductors on eBay: have the tools and skills to test them. Demand a refund for anything that is out of spec. I've sent parts back that weren't packaged properly.

If you don't already know, learn how to handle CMOS and MOSFETs safely. Static electricity will damage or kill them.