Vintage MXR Phase 100 Troubleshooting - 1981 No Effect

[ammalato]

Hi All:

I have what appears to be a 1981 (Pre LED / 9v Jack) MXR Phase 100 that isn't working.  The pedal had previously been modified at some point to accompany an old 9v jack (which has since been removed and restored back to battery only wiring).  My guess is that at some point bad power lead to a failed component somewhere.  The issue is that there is no effect in the "on" position.  There is a slightly, more nasally tone when the pedal is on, but there isn't any phasing effect.  I've read about the common issues (dead Vactrol, IC, Transistors, etc) but since I'm less familiar with this circuit I'm hoping for some guidance on how best to isolate the issue.

There doesn't appear to be any variation in resistance when measuring the vactrols (resistance from in lead to outer lead(s) sits around 89ohm without variation.  Perhaps I'm measuring wrong.  Also, I tried to touch an led to various leads on the vactrols (I wasn't sure which was which) but no combination yielded a light, it seems.  I'm curious if anyone has suggestions on various testing points / voltages to compare against.  Perhaps an IC is dead?

I checked all of the wire terminations and they all look solid and correct.  I don't see any broken/lifted traces on the PCB either.

Here are some pics.  Thanks for the help.

[Mark Hammer]

Good preliminary description and troubleshooting, and the pics will likely prove helpful.

For a modulation pedal like this, the strategy is to check the various "bottlenecks" of the audio signal, and check the modulation sources controlling the audio signal.

You've done some of that.  From your description, audio IS getting through, so that would suggest that all chips are receiving power and working fine, and the stompswitch is fine.  The issue would seem to be with the modulation.  There is a 20k trimpot that sets the "center" of the LFO sweep.  Consider tweaking it.  Alternatively, there are two 15uf caps in the LFO circuit.  If they are electrolytic, they might have dried out over time and warrant replacement.  If they are tantalum type, then they may be fine.

[Electron Tornado]

An analog voltmeter would be very helpful in testing the LFO as well as checking that the LFO signal is reaching each of the vactrols. The LFO signal will make the needle swing.

[antonis]

An analog voltmeter would be very helpful in testing the LFO as well as checking that the LFO signal is reaching each of the vactrols.

No argue but a "Welcome" should be useful, as well..

[CheapPedalCollector]

Dead TL022 is very common as well. Ccheck the opamps inverting/noninverting/output pins on each side, if you find you don't have around 1/2 supply voltage on them you found a dead one, replace it. Shorted ones will read full supply voltage and open ones will read very low like 0-1v. Tantalums also go bad.

[ammalato]

Ok, help a simple man out here... based on the pin outs show below, what exactly am I testing? Pins 2,3,5,6 from ground?

Dead TL022 is very common as well. Ccheck the opamps inverting/noninverting/output pins on each side, if you find you don't have around 1/2 supply voltage on them you found a dead one, replace it. Shorted ones will read full supply voltage and open ones will read very low like 0-1v. Tantalums also go bad.

[duck_arse]

Ok, help a simple man out here... based on the pin outs show below, what exactly am I testing? Pins 2,3,5,6 from ground?

yes, and pins 1 and 7 as well.

[ammalato]

Ok, so I tested pins 1,2,3 and 5,6,7 on all the IC's.  Every IC except the first one on the left (which I detail below) reads right around 3.9 volts across all 6 of those pins (My battery is weak and measure 8.16vdc so, to your point those are all right around the half way mark of the supply voltage.  The first IC all the way on the left reads as follows...

Pin 1: 3.86vdc
Pin 2: 3.86vdc
Pin 3: 3.86vdc
Pin 5: 2.745vdc - 4.85vdc (square wave)
Pin 6: 2.95vdc - 4.7vdc (smooth wave)
Pin 7: .6vdc - 6.98 vdc (square wave)

Based on this, and other posts I've read it seems this is consistent with other users' readings, so I suppose they're working and the vactrols are next to be questioned?

Ok, help a simple man out here... based on the pin outs show below, what exactly am I testing? Pins 2,3,5,6 from ground?

yes, and pins 1 and 7 as well.

[ammalato]

Providing some additional measurements, this time from the vactrols. Measuring just the LED sides (both cathode and anode) from ground. See measurements in the pic below. The Voltages are all static, no deviation.

[CheapPedalCollector]

OP AMPs are good then, and vactrols are probably fine.

Seems likely that the tantalums (blue caps) are failing or failed, as it seems the LFO is not working.

[ammalato]

Interesting. Any recommendation on how best to verify or what to test next?

I touched an LED to the +/- ends of the vactrols thinking that they should illuminate in some fashion, and they didn't so not sure if that indicates something wrong with the Vactrols, or something earlier on in the path (i.e., preventing the Vactrols from getting proper voltage)? 

OP AMPs are good then, and vactrols are probably fine.

Seems likely that the tantalums (blue caps) are failing or failed, as it seems the LFO is not working.

[CheapPedalCollector]

Oh, wait I read your other post wrong, the LFO seems working.

Hrm, maybe some blown vactrols (VTL5C3/2) after all, you can get new ones that are made by XVIVE, they are not cheap though. I do not know if they need to be matched or not. See if you are getting voltage variance out of the LFO into the first vactrol. Maybe a current limiting resistor blew, hard to troubleshoot from afar apologies

[duck_arse]

has someone posted a circuit diagram we can reference?

look at the measured voltages along your vactrol string, each is about 1V7 lower than the next. the forward voltage of a red led fer ex, is about 1V7. yer readings would suggest each of the internal leds is still functional, and probably putting light onto the ldr it's facing. is the oscillator running? are the ldr's functioning?

[ammalato]

look at the measured voltages along your vactrol string, each is about 1V7 lower than the next. the forward voltage of a red led fer ex, is about 1V7. yer readings would suggest each of the internal leds is still functional, and probably putting light onto the ldr it's facing. is the oscillator running? are the ldr's functioning?

So I posted this in image above but I realize it's probably not as clear as simply writing the measurements down here.  So I'll write ALL the recorded measurements below. You're right, the Vactrol String does show a 1.7v variance from LED to LED, and based on what you're saying that sounds consistent with an LED's forward voltage, so I assume that's a good sign...only issue I have is that if/when I touch another LED to those contacts, it doesn't seem to light up for me...should it?   Based on the measurements I'd provided earlier it does appear that the LFO is working, at least as measured off the first IC, since it seems to be cycling voltage in rhythm.  Again, here are all the measurements (taken from ground) I have to date....

IC #1:
Pin 1: 3.86vdc
Pin 2: 3.86vdc
Pin 3: 3.86vdc
Pin 5: 2.745vdc - 4.85vdc (square wave)
Pin 6: 2.95vdc - 4.7vdc (smooth wave)
Pin 7: .6vdc - 6.98 vdc (square wave)

IC #2 - 6:
I tested pins 1,2,3 and 5,6,7 on all the remaining IC's and they all reads right around 3.9 volts across all 6 of those pins (My battery is weak and measure 8.16vdc so, to your point those are all right around the half way mark of the supply voltage. 

Vactrol #1:
LED (-) 2.33v
LED (+): 4v

Vactrol #2:
LED (-): 4v
LED (+) 5.7v

Vactrol #3:
LED (-): 5.7v
LED (+) 7.4v

Additionally, when I measure the resistance between the center lug and either outer lug on the photo-resistor side of the vactrols, I see no variations....Meaning the resistance is static and not increasing / decreasing as I would expect.  They all measure somewhere between 90 - 100 ohms of fixed resistance.  That's what lead me to believe the LED(s) might have been cooked inside the vactrol, but I don't know. 

Transistors:
Q1 Collector: 1.55v to Ground fixed
Q2 Collector: 2.3v to ground fixed

has someone posted a circuit diagram we can reference?

Here is the schematic
https://images.app.goo.gl/geatQHnEYtfh1jf78

and here is a PCB layout from general guitar gadgets, that I THINK mirrors the original MXR layout.
http://www.generalguitargadgets.com/pdf/ggg_p100_lo.pdf

[duck_arse]

Q2 collector is connected to +9V, on the circuit diagram. that would be one place to look for a fault.

[ammalato]

Thanks, I'm realizing now that what's labeled in the schematic I linked as Q1 and Q2 is also labeled as Q3 and Q4 in the PCB layout I shared.  This isn't getting any easier.

Q2 collector is connected to +9V, on the circuit diagram. that would be one place to look for a fault.

[Rob Strand]

There's at least two versions of the circuit with some different part values.

That circuit is quite fussy about part values and adjustments; ggg docs has some info.

If the frequency center trimpot is set incorrectly the LEDs might be stuck on.

With power off, record the current set-up by measuring the resistances across all trimpot pins.  Use both meter polarities.

With the power on,
- set the modulation speed to a high rate
- set the switch to a known position.
- measure the voltage on Q1's collector and record the value
  (this schematic https://postlmg.cc/QFpZDJBd)
  [In the GGG layout that would be Q3, the transistor close to the trimpot.]
- Adjust the trimpot in a direction that reduces the Q1's collector voltage,
  Try steps of 10% reduction.
  (should be when the wiper moved towards the 47k R5)
  Preferably do this with a signal just in case you hit a point where you hear the modulation working.

[duck_arse]

documentation, documentation, documentation. also - documentation. this is why you are asked to provide it, and why we don;t like chasing it when it isn't provided.

also:

Q1 Collector: 1.55v to Ground fixed
Q2 Collector: 2.3v to ground fixed

we want the base and emitter voltages as well, every time, so that we can say you are backwards the transistor, or perhaps looking the wrong transistor, etc. when all three readings don't make sense as presented.

[ammalato]

Hey all, I wanted to follow up here and close out this thread (since I often find it frustrating when troubleshooting a problem online and finding a forum post that describes the same problem, but doesn't provide a solution).  The issue with my Phase 100 was a faulty trimpot, which had apparently become so corroded that the wiper was no longer making contact.  The trimpot was replaced and the pedal phases again.  It does sound weaker than I expect for a 10 stage phaser though, so I'm going to poke around some more...My Script phase 90 has more oomph than any of the settings...could be the battery...which tested a tad weak (8.5v) but the sound definitely feels weaker than a .5v deficit.