SOLVED VFE Mini Mu Troubleshooting

[rhaas]

Hi all!

I posted about this on the Madbean forum but I'm still trying to resolve a couple issues. In summary:

1. My initial build was completely silent when engaging the effect. I tracked this down to a solder joint that I had missed.
2. After resolving Issue 1, I was still seeing screwy voltage readings on IC1 (the OPA134 output stage amp). At this point, I was getting sound through the pedal but the envelope effect was not working. I ended up swapping in another OPA134 from the same Mouser order and it exhibited the same symptoms. Only after I swapped in a CA3130EZ (the only other single op-amp I had on hand) did the voltage readings mostly resolve (with one exception described below), and the effect mostly began working.
3. Even with the CA3130EZ, I'm seeing ~4V DC on pin 6 (output). I think this is the cause of some nasty scratching noise when I turn the level knob.

Looking at the schematic, does anyone have any suggestions for identifying/remediating the source of the DC voltage on pin 6? Also... I'd love to know what's going on with the two OPA134s that I tried. I'm kind of baffled as to how differently they worked in the circuit compared to the CA3130EZ (see voltage readings below).

IC1 with OPA134 installed
1 8 (should be -8.4)
2 0 (should be 0)
3 0 (should be 0)
4 -8 (should be -8.4)
5 0 (should be -8.4)
6 0 (should be 0)
7 8.3 (should be 8.8)
8 8 (ignore)

Thanks!

 

[Cybercow]

What are the voltage measurements at the outputs of that VFE switching board? And where they respectively meet the mainboard?

If the voltages there are off, with all the chips in their sockets, try removing all the chips form their sockets and measure the VFE switching board voltages again.

 

[rhaas]

What are the voltage measurements at the outputs of that VFE switching board? And where they respectively meet the mainboard?

I don't see expected test voltages in the VFESB_v3 build doc but here's what I measured. They seem fine? (I checked both the switching board output and the main board input — thankfully they match. )

With effect on:
IN: 0
OUT: 0
LED: 2.3V
GND: 0
V-: -7.9V
V+: 8.3V

With effect off:
IN: 0
OUT: 0
LED: 0
GND: 0
V-: -7.9V
V+: 8.3V

 

[Cybercow]

Where those measurement taken with the ICs in the sockets or out? (Just trying to be thorough here.)

IC1 should have about -7.9V on pin pin #4 and about +8.2V on pin #7. (Pins #1 & #8 are offset trims and not used, so their respective voltage measurements should have no bearing.)

And how about IC2 & IC3? IC2 should have about -7.9V on pin #4 and about +8.2V on pin #8. And IC3 should have about -7.9V on pin #11 and about +8.2V on pin #4.

 

[rhaas]

Where those measurement taken with the ICs in the sockets or out? (Just trying to be thorough here.)

I appreciate the thoroughness! The measurements were with the ICs in.

IC1 should have about -7.9V on pin pin #4 and about +8.2V on pin #7.

Great, that is in fact what I'm seeing with the CA3130EZ installed.

IC2 should have about -7.9V on pin #4 and about +8.2V on pin #8.

Good there as well.

And IC3 should have about -7.9V on pin #11 and about +8.2V on pin #4.

And this aligns with what I'm seeing.

I went back and double-checked all the pins against the build doc, and the only significant discrepancy I'm seeing is that ~4V on pin 6 of IC1.

 

[benny_profane]

CA3130 is not an appropriate part for a split rail +/-9V design. Its max DC supply is 16V.

Are your voltage readings in the list correct? There you list 0V on pin 6. How is it possible that you're getting different readings on pin 2 and 6?

 

[rhaas]

CA3130 is not an appropriate part for a split rail +/-9V design. Its max DC supply is 16V.

Good callout — I only subbed that in temporarily because it was the only other single op-amp I had. I have some TL071s on the way.

Regarding the voltage readings, I might have caused some confusion: the full list in my first message was with the OPA134, and the "4V on pin 6" reading was with the CA3130EZ. Maybe the "4V on pin 6" issue will clear up with a more appropriate part (TL071)? But if so, it begs the question of why the OPA134s aren't working.

 

[benny_profane]

The envelope has nothing to do with the output stage. All that IC is doing is providing gain recovery and serving as an output buffer.

 

[rhaas]

The envelope has nothing to do with the output stage. All that IC is doing is providing gain recovery and serving as an output buffer.

Yes, and that tracks with the behavior I noticed when I briefly tested with the CA3130EZ installed (the envelope effect worked but the volume knob was very scratchy).

So maybe that scratchy volume knob issue (which seems like it could be caused by the DC voltage on pin 6, right?) will be resolved with the TL071. I'm still confused as to how the voltage readings were so off with the OPA134s, though.

 

[benny_profane]

I'm still confused as to how the voltage readings were so off with the OPA134s, though.

Do you mean 1, 5, and 8? Pins 1 and 8 are for offset nulling and not used here (i.e., ignore) and pin 5 is not internally connected to anything (i.e., ignore). Your values are correct in the places that matter.

Use an audio probe to isolate the problem, and follow the advice from the MBP thread.

 

[Cybercow]

And +1 for the CA3130EZ being an incorrect part for IC1. A TL071 is a better replacement for the OPA134. But the OPA134 is the best chip for IC1.

 

[coltonius]

If the effect engages without an envelope, I would double check the optocouplers. My money is they are somehow installed incorrectly.

 

[rhaas]

Update: mystery solved! @benny_profane - thanks for your suggestion in the MBP thread about the voltages in the build doc being incorrect. It dawned on me that after swapping the optocouplers out of desperation (with the OPA134 installed), I didn't bother to actually plug the thing in and play it because I thought the IC1 voltage readings were still wrong. The readings were closer to matching the build docs with the CA3130EZ, so that's when I gave it another spin and first noticed that the envelope effect was working.

Well... I just put an OPA134 back in and despite the voltages being "wrong" relative to the build doc, the thing works! No scratchy noises on the volume knob either. So a summary of the actual issues I encountered:

1. I missed a solder joint on initial assembly.
2. I either mis-oriented a NSL-32R3 on initial assembly (or installed a defective one... but probably the former).
3. I got kind of thrown by the IC1 voltages in the build doc, which don't seem to be correct.

Thanks all for your help. I'll also follow up in the MBP thread for closure.

 

[coltonius]

Update: mystery solved! @benny_profane - thanks for your suggestion in the MBP thread about the voltages in the build doc being incorrect. It dawned on me that after swapping the optocouplers out of desperation (with the OPA134 installed), I didn't bother to actually plug the thing in and play it because I thought the IC1 voltage readings were still wrong. The readings were closer to matching the build docs with the CA3130EZ, so that's when I gave it another spin and first noticed that the envelope effect was working.

Well... I just put an OPA134 back in and despite the voltages being "wrong" relative to the build doc, the thing works! No scratchy noises on the volume knob either. So a summary of the actual issues I encountered:

1. I missed a solder joint on initial assembly.
2. I either mis-oriented a NSL-32R3 on initial assembly (or installed a defective one... but probably the former).
3. I got kind of thrown by the IC1 voltages in the build doc, which don't seem to be correct.

Thanks all for your help. I'll also follow up in the MBP thread for closure.

The voltages on my Tractor Beam are way off too, but it works perfectly so I did an and buttoned it up. (One IC pin was supposed to be +0.2mV but read as -9v.)

Bless Peter for his designs; I'm quite obsessed with a few of them! It's clear from the schematics, docs, etc... they were never meant to be mass produced by third parties though.

 

[rhaas]

Bless Peter for his designs; I'm quite obsessed with a few of them!

For sure! I only had a short time to play it this evening (and envelope filters are tricky beasts to dial in) but so far it compares really favorably to my Naughty Fish. I'll do an actual build report sometime next week.