Having trouble debugging Phase 45... any advice?

[deadlyshart]

Hi guys, I built the Phase 45 from the stripboard layout from this tagboardeffects page. From what I can tell, this is the schematic for it with the same values and everything. I'm using MPF102 JFETs because someone suggested it as an alternative to the 2N5952, which I was having trouble sourcing. I used the JFET matching technique from this page and found two that had pretty close gate-source cutoff voltages (-3.1V and -3.2V).

So I assembled it, but only pretty much the dry signal is getting through (which I'll explain more about below). I carefully went through the whole trimmer range but I don't think that's the problem (yet).

Both the op amps have the correct voltages on pins 4 and 8. I checked the voltages on the pins of the JFETs and compared them to the ones suggested on this page (though they're using a different JFET). They say (with a 9.4V power supply) there should be 4.0V on the S and D pins, and ~2.2V on the G pins. I'm getting 3.6V on my S and D pins (same voltages for both JFETs), and the voltage of the G pin can be adjusted with the trimmer (I can make it 2.2V easily). Also, the voltage on the G pin varies a bunch with time.

So I input a 1kHz sin wave on the input and followed the signal with my scope. It gets to the first op amp stage and the signal can be seen on pins 1,2,3 of it, meaning that op amp is probably working. However, when it goes past any of the components after pin 1 (the output), the signal is gone, except for the one that goes straight to the output. It's a little hard to explain in text, so I've drawn this pic that shows where the signal goes and doesn't. There might be a small point of confusion because between the schematic and stripboard layout, they switched a couple of the op amp stages:



Where "yes" in blue means that I can see the input signal" and "no" in red means that the signal isn't there when I'm pretty sure it should be.

Like I said above, it may be a bit confusing because in the schematic the order of op amp stages the signal goes through is IC1b, IC1a, but in the actual layout it's IC1a, IC2a (where IC1 would be the bottom IC and IC2 is the top one). Sorry, hopefully that's clear... Anyway, so from what the signal's doing, it might make sense that that 2nd op amp stage is messed up, so I tried switching it out for a fresh TL072, and it gave me the same result.

What can I try? My usual bag of tricks is exhausted... thank you for any advice!

[GibsonGM]

Oh, the pain stripboard has caused..but ok, that's for another post.

Offhand, I'd be looking for a short somewhere in your "NO" area...from the output of that first opamp (.05u cap...) up to the next opamp.
Good chance there's a bridge to ground, or components shorting together....why not take a look?

Hope that's helpful...

[deadlyshart]

Oh, the pain stripboard has caused..but ok, that's for another post.

Offhand, I'd be looking for a short somewhere in your "NO" area...from the output of that first opamp (.05u cap...) up to the next opamp.
Good chance there's a bridge to ground, or components shorting together....why not take a look?

Hope that's helpful...

Haha, I'll keep that in mind... I've actually had fairly good luck with stripboard so far! I drill out the holes and then check the continuity across each hole to make sure it maxes out my ohmmeter.

The thing is, I checked those pins 1,2,3 of the 2nd op amp (the "no" area), and they're biased at 3.6V (as you'd expect because it's connected to the JFET directly above), so that suggests they're not shorted to ground right?

thanks!

[Kipper4]

Yer I'd double check the connections from the output of IC1b to the inputs of IC1a.
Just because the dc is satisfied it doesnt guarantee the ac is getting through.
Also stick the meter on the output of IC2a (pin1) and see if the lfo is wobbling.

[deadlyshart]

Yer I'd double check the connections from the output of IC1b to the inputs of IC1a.
Just because the dc is satisfied it doesnt guarantee the ac is getting through.
Also stick the meter on the output of IC2a (pin1) and see if the lfo is wobbling.

Thanks for the reply. So the thing is, where I'm checking for the connections, I'm clipping on the components themselves. So for example, looking at the output of the first op amp (that *does* have a signal), I can check it on one leg of that .05uF cap (and there's a signal), and then clip it directly to the other leg of that cap, so there isn't really any way there can be a broken connection, though I guess that doesn't eliminate shorts.

So here are a couple pics and stuff. I actually renamed the ICs in the schematic to match the stripboard layout, because I realized it was going to get confusing because of the previously mentioned switching of op amps between the two diagrams. So these two are consistent:





Yeah so I realized the same dry signal is also on the output pin of the other op amp of IC2 (the way it was for pin 7 as well. In addition, there's what a *tiny* but definitely still there signal on the *inverting* pins of IC2 (for both op amps of IC2). I guess this makes sense because it is "amplifying" from that input to the output, so if the signal is there on the output a smaller one must be on the input right?

Anyway, so pin 7 of IC1 (LFO output) is definitely oscillating. About every 2 seconds it oscillates between 0V and 8.6V. I'm sure it's definitely not the IC's now, I switched them, and whichever is in the IC1 position works.

So I'm pretty confused. The LFO output and the gate pins of each JFET are oscillating with correct seeming voltages (right?).

Just to be sure (though it doesn't matter because I can't hear any difference in the output): I *should* be able to see a different waveform in the output if it's working correctly, right? I'm inputting a sin wave so I assume it would be a weird looking one that's wobbling or something due to the phasing.

Here are front/back pics:

[Kipper4]

Check your fet pinout?

[deadlyshart]

Check your fet pinout?

Yeah, every pic I can find of the MPF102 pinout shows the DSG order like so: http://www.learningaboutelectronics.com/images/MPF102-pinout.png

so I think they're facing the right way. Maybe I'll try another relatively close pair of JFETs tomorrow, but that seems like it's probably not the problem...

ah so this is a good resource. I found this image:



from this page: http://mklec.com/project-kits/guitar-effects-kits/mxr-phase-45-clone-kit

that's clearly using the exact same stripboard layout as I am, so it's useful to compare. You may notice that their JFETs face the other way, but that's because the order of the 2N5952 pins are backwards with respect to the MPF102 pins, so I think I'm safe there.

[Kipper4]

Yep my mistake. I was thinking j201 pinout.
What the heck since the fets are socketed I'd try them in reverse pinout and see.
Worse case scenario it cost the price of 2x mpf102's

I can't believe you've boxed it up before testing it out.
I always test before boxing. It's a lot easier to debug out of the box.
I sometimes test while boxing too. So the last boxed build i did I left the circuit input output disconnected until the last minute so I could test the circuit worked before soldering to the switch.

I hope you can get it sorted.

[duck_arse]

can you give us your pin voltages on the IC's please? also, try pulling the fets out of the sockets (don't try them backwards) and see if you get audio thru to the output.

[deadlyshart]

can you give us your pin voltages on the IC's please? also, try pulling the fets out of the sockets (don't try them backwards) and see if you get audio thru to the output.

Yeah, so I actually already tried them backwards (which in retrospect doesn't even make sense because the gate definitely has to be in the same place, but the order is DSG... so flipping them would just make it GSD...ugh), before reading this... I hope I didn't fry them!

Anyway, with the FETs in, these are the voltages (using the new IC1 and IC2 naming in my later post):

IC1:

1: 3.6
2: seems to vary between 4 and 4.4
3: 3.7
4: 0
5: oscillates between 3.3-6.1
6: oscillates between 3.5-6
7: oscillates between 0-8.5
8: 9.6


IC2:

1: 3.6
2: 3.9
3: 3.6
4: 0
5: 3.6
6: 3.8
7: 3.7
8: 9.6

Removing the FETs... well the thing is, the audio signal was always getting through via the path in the first post, but it's a totally dry signal, same as the input. So taking out the FETs did the same thing.

Thank you for the advice though... do those IC voltages seem right?

[Kipper4]

Well that's good news. Your volts all seem ok.
Your getting a clean dry signal at the output too
And you say your getting oscillation from the lfo. So that narrows it down to the variable resistors (in this case the mpf102)
And probably the biasing network of the fets.

[deadlyshart]

Well that's good news. Your volts all seem ok.
Your getting a clean dry signal at the output too
And you say your getting oscillation from the lfo. So that narrows it down to the variable resistors (in this case the mpf102)
And probably the biasing network of the fets.

Yeah I'm betting it's them too to be honest. They were just some ebay ones. Maybe I'll just buy the 2N5952's or something, but I can't find any great domestic sources...

So the gate-source cutoff voltage I measured for these ones were -3.1V. Is that an okay voltage? There was another pair that was a little lower, in the -2V range.

[Kipper4]

So we need to try and get the transistors working as a variable resistor with the help of the trim pot right?
When you turn the pot does the voltages go up and down?
If you have 0-8v5 it may be that if that sort of voltage getting to the drain it might be off.
It's been a while since I did this maybe I should dig my notes out. If I made some hmm.

[DrAlx]

With the FETs taken out you should see decent 1 kHz signal at both IC2 pin 2 and 3. If not then I would guess that the 47nF cap on pin 3 of that amp has a bad connection on one or both ends. Either that or the cap is wrong value (too small). Check that cap and its connections.

EDIT: With all FETs and ICs taken out of the board, and with power off, use a multimeter to check continuity between leg of the 47nF cap and the socket for IC2 pin 3. Similarly check continuity from other leg to IC1 pin1 socket.

EDIT2: The reason I think the cap could be the problem is that the opamp that follows it seems to be acting as an inverter (i.e. a unity gain inverting amplifier) rather than as an all-pass filter (or "phase shifter"). An opamp acting as an inverter will register hardly any signal at its two input pins. If that cap were not there, then the op amp would definitely act as an inverter instead of a phase shifter. Hence cap is the main suspect.

If the cap is OK, then with all ICs and FETs out of the board, and with power off, measure resistance between socket for IC2 pin 3 and D1. It should be about 20k according to schematic. If it is very low or zero, then that is another thing that would cause the phase-shifter to act like an inverter.

[pinkjimiphoton]

not positive that reading values in circuit is plausible. i was always taught to unsolder one side of passives to prevent reading the peripherals attached to them.

in my experience, broken phasers often seem to have a bad pot or trimmer somewhere that's gone open. admittedly, i have minimal experience with them, but...

[deadlyshart]

With the FETs taken out you should see decent 1 kHz signal at both IC2 pin 2 and 3. If not then I would guess that the 47nF cap on pin 3 of that amp has a bad connection on one or both ends. Either that or the cap is wrong value (too small). Check that cap and its connections.

EDIT: With all FETs and ICs taken out of the board, and with power off, use a multimeter to check continuity between leg of the 47nF cap and the socket for IC2 pin 3. Similarly check continuity from other leg to IC1 pin1 socket.

EDIT2: The reason I think the cap could be the problem is that the opamp that follows it seems to be acting as an inverter (i.e. a unity gain inverting amplifier) rather than as an all-pass filter (or "phase shifter"). An opamp acting as an inverter will register hardly any signal at its two input pins. If that cap were not there, then the op amp would definitely act as an inverter instead of a phase shifter. Hence cap is the main suspect.

If the cap is OK, then with all ICs and FETs out of the board, and with power off, measure resistance between socket for IC2 pin 3 and D1. It should be about 20k according to schematic. If it is very low or zero, then that is another thing that would cause the phase-shifter to act like an inverter.

Hey, thanks for the advice.

With no FETs or ICs: high continuity between leg of 47nF cap and pin 3 of IC2. Same for the other leg and pin 1 of IC1. 19.7kOhm between D1 and pin 3 of IC2.

Hmm. So I have another pair of MPF's that had a similar cutoff voltage (-2.45 and -2.62V, close enough?), and I actually just checked my components stash and found a bunch of J105's, which are apparently an N-channel JFET, so maybe I'll try them too?

anyway, I'll report back, thank you!

[deadlyshart]

Welp guys, I'm an idiot!

I tried the other pair of MPF245's with close cutoff voltages (-2.45V and 2.62V) and I could immediately see a difference in the output waveform! I was curious what the phaser would actually turn a sin wave into. It turns out it basically makes the sin wave modulate the amplitude! (might be obvious to others but it wasn't to me  )

All I had to do was adjust the trimmer, which changes the depth of the modulation. The max amplitude of the sin wave seems to stay the same, but it changes how far down it drops when it modulates. I adjusted it so it went as low as I could make out, and it works! I'll say for posterity that I read online that the trimmer has a *really* tight range where it works, but it wasn't *that* tight when I did it, just pretty tight.

Anyway it sounds great now and thank you all! I'll upload pics soon.

[Kipper4]

Cracking job