MXR Phase 100 Repair Help

[pedalbob]

Hi All-

New to this board and my first post but hoping someone can offer some advice.

I bought a used vintage MXR Phase 100 (older but not the oldest ... has all of the white box film caps and not tropical fish not that it should matter). I can post a gut shot at some point if someone wants.

Anyway it wasn't working when I bought it ... just passing clean signal and very mild to no effect.  One of the Vactrols tested bad on the LED side with my diode checker so I replces it with one of the new XVive ones.  Also the trimmer pot was corroded and stuck at a fixed 20Kohms so I replces it.
Still not a lot of effect but better than it was and could get some variation by adjusting trimmer (although seems to be best in null position).

I noticed that the new vactrol had a higher forward voltage than the two originals so I replaced them with 2 new ones.  The diode tester shows me essentially same measurement on all 3 now within a couple 1/100th of a volt.

Now get more effect but nowhere near as much as I remember from my friends old Phase 100 or from videos on youtube.

Speed pot seems to be working correctly and I can sort of hear the LFM trying to do its thing with a pulsing when on the most pronounced settings.

I checked all of the Op Amps and they seem fine as well.

Guessing there is something else in the LFM circuit not fully functioning.

Any thoughts or things I can try or check ?   It's got the tantalum capacitors and not electrolytic so I would not expect them to go bad but I gues they could.

Thanks for any advice,
Bob

[Govmnt_Lacky]

Cannot see a pic but, did you use VTL5C2/2 or VTL5C3/2 vactrols?

[pedalbob]

Hi-

They are the VTL5C3/2  vactrols.  The ones I replaced were marked on the underside (facing board).  I checked LED side to make sure I put the new ones in correctly. (The XVive ones have the anode marked but the old ones didn't).

Thanks

[pedalbob]

Here's a picture

[pedalbob]

Hi All-

I did a little more troubleshooting and raised a couple of questions because I am not an expert at testing transistors but here goes :

1) I was never sure about testing transistors if they are still soldered in the PCB even if the circuit is "unpowered" but I tried anyway
2) The 4 ZTX109 NPN transistors all showed what appears to be a correct forward voltage between the Base/Collector and Base/Emitter (0.65 to 0.68 ish volts)
3) A couple show 0 volts in the reverse direction when I put the negative lead of the diode tester on the Base but in some cases I am getting a significant voltage (about 1.2v volts )
4) I also did a continuity test between the junctions and some are very high (mega ohms) and some are medium resistance (150K ohm)


I am thinking of replacing them but know the ZTX109's are obsolete.  I've read that the BC109C which I can still get is a suitable replecement.

Thoughts ?

[fowl]

You can't really test transistors in circuit, but reading voltages is not a bad idea, if you know what to look for (I don't in this case).

I think silicon transistors rarely go bad in a properly designed circuit.  I would look at "shotgun" replacement of all those tantalum caps before I thought about transistors.

I have no experience with vactrols, but I remember reading somewhere (here?) that the XVive offerings weren't exact matches for the older parts of the same types.  Maybe search the forum for the key terms.

[pedalbob]

Thanks.
Yes I have read similar about the newer Xvive vactrols which was why I replaced all 3 because I figured they would stand a better chance of being "matched".  I'm guessing to actually match a vactrol I would need to measure the resistance variation as the LED is illuminated which is a bit beyond what I have to test with at this point.  I do know that 1 of my original Vactrols was bad.  I guess I could always try to round up a new old stock one and put the other 2 originals back in but not the path I chose for now.

I read a couple of other posts on this forum about Phase 1000 transistor measurements and what I was getting with my measurements was quite different than what the other person had.  That's why I then went for the diode check on each side of the 2 "NP" junctions.

I changed the 1 tantalum capacitor (C3 ?) that looks like it is directly in the LFM circuit this afternoon because I had some 15uF ones and it didn't do much (although I swear it brightened the tone a bit but nothing to modulation).

Think I might carefully pull the transistors one at a time and take some off board measurements.  I can always substitute in some equivalent replacements in the process too.

Thanks for the input. 

[fowl]

Thanks for the input.

Ain't no thang.

I should have included a disclaimer that I am not even remotely close to an expert on this, just throwing my thoughts out there for consideration.

Tantalum caps do go bad, I would think way more frequently than Si transistors.  Those 630V Plesseys will probably see us all buried though.

I think your pedal was made in the later part of 1982.  Not at all certain of that.  But I think the first digit of the 3-digit code on the edge of those TL022s represents the last digit of the year they were made, and maybe the other two are the week.

[PRR]

> the ZTX109's are obsolete

BC109 or probably 2N5089 should replace it.

But I agree: BJT failure is unlikely in low-volt stuff.

I don't see your voltage readings or link to schematic?

[pedalbob]

Thanks for the input folks !

Agree with everything said about the silicon transistors ... I have never had an issue but don't know the history of this pedal since I bought it used and don't know what could have happened to it.  It was evident that a battery had leaked at some point but primarily got on the enclosure bottom side and the footswitch which I have cleaned up.  Also had the normal disintegrated gray foam which I cleaned up.  The Op amp pins showed a little surface corrosion but I measured (voltage in powered state) all of them and they seam fine.

Here is link to schematic : https://www.freestompboxes.org/viewtopic.php?t=2071&start=20

I measured voltages at transistors with Speed control at max and battery (9.18 volts measured)

Q1: C 1.01v, B 0.626v, E 0.000v
Q2: C 8.66v, B 3.74-3.88v(fluctuates), E 3.27 - 3.42v (fluctuates)
Q3: C 3.46v, B 1.18v, E 0.630v
Q4: C 3.46v, B 0.628v, E 0.005v

I'm also starting to question the Xvive vactrols I put in ... I've read a lot of contardictory information online between them being direct drop in clone replacements for th VCT5C3/2 versus they are not.

I measured the resistance (unpowered LED/darkened state) across the photoresistor side of the originals and I get multiple megaOhms.  When I do the same for the Xvive replacements I get a couple of hundred kiloOhms.  Don't have anyway of injecting variable current to LED and creating curves to compare but seams odd.  The megaohms makes sense compared to the datasheet for the original Vactrols.  Looking for XVive datasheet next.

I have 2 good original Vactrols so wondering if I should just find a 3rd and put them back in as a next step.  Looking online though that could take awhile because I don't see any available anywhere for the 3 lead resistor config  (3/2).



 

[pedalbob]

 OK ... at the risk of sounding like a madman ...

So I put the 2 good original Vactrols back in.  Originally the bad one was the middle one in the chain of 3 ... so when I originally put the 1 new one in I put it in the middle.  At the time I hadn't figured out that the trimpot was toasted either.  When I tested it I goy very littel effect and the trimpot was not responsive.

Then I replaced the other two Vactrols and trimpot at the same time ... violating the rule of change one thing at a time and test

I just put the two original Vactrols back in but put them as #2 and #3 in the chain.  My thinking was that at least the 2 working together would be better than having the bad one in middle one in the chain.  I am now getting a pretty decent effect and the trimpot and speed control are much more sensetive.  Not completely getting that crazy wooshing sound when you rotate the speed control all the way CW but getting some.  The effect is actually pretty pleasing as is.

I think I have convinced myself that either the XVive are not perfect drop in replacements or that they really are more sensetive to matching than I would have thought.  I've read that the MXR stock ones got matched to put into each box so probably something to that.

Think I will keep my eye out online for a new old stock one and buy it if the price is right ... some guy in FL selling them for $29.00 plus $10.00 shipping which is a bit steep for me.

Hopefully my stumbling and bumbling will help some other sould trying to fix a Phase 100 ... still a very cool pedal. 
Thank-you everybody for your help !
 

[pedalbob]

Hi-

Finally got some time to mess with the Phase 100 some more and in the meantime was able to obtain a fully functioning one of about the same vintage (slightly older).
There is a lot of information about MXR "matching" the vactrols and I was wondering how big of an effect that could have on mine.  So I removed one of the vactrols from the working one (1st one in the chain which is the one I had confirmed had a bad LED from my non-working Phase 100) and replaced it with one of the brand new Xvive ones I bought and I can hear no difference in the effect (i.e. it still works fine).  The Xvive ones have a slightly higher forward voltage on the LED side by about 0.2 volts compared to the old Vactec Vactrol I took out).

So apparently something else not working in my LFO of the bad unit.  Now that I have a working one too I will go point for point in the schematic on both units and figure out what has gone bad.  Stay tuned I will post my progress.

I also was messing around with the vactrol with the bad LED and used my Dremel tool to carefully cut away a bunch of the epoxy.podding in the middle.  I was able to extract the LED without messing up the Resistor side.  The LED in the old Vactrol is a 3mm one and clear glass but I can't tell what color it is because it is broken.  Does anyone know what color LED's they used in the old Vactrols ?  I was going to mess around with it and see if I could repair it and get it to work.  I can post some pictures if anyone is interested.

Cheers and happy holidays !

[ElectricDruid]

I'm not surprised that the newer Xvive vactrols aren't *identical* to the old ones, but any two vactrols out of the same bag aren't that close to each other anyway (hence the tales of matching) so I doubt it matters.

Either way, an unmatched vactrol wouldn't be a go/ no go issue. It'd work fine with completely unmatched vactrols as long as they're working parts. OK, *perhaps* you could get a *slightly* better effect by going to the trouble of matching the vactrols, but I know I wouldn't bother.

Since you've eliminated the vactrols as the source of the problem, you've probably got an LFO failure.

Have a check on the working unit to see if you can see the LFO voltage on the LED side of the vactrols, and then look for similar on the dead pedal. My bet is there'll be nothing there, just a static voltage.

HTH,
Tom

[pedalbob]

Thanks for the advice.  I spent sometime messing with it this weekend checking voltages to ground all over the circuit.  I was seeing significanty different voltages at Q3 of the bad pedal vs. good.  I pulled Q3 from the bad pedal but it tested ok out of the board.  So now I will go backwards thru the circuit from there.  I can definitely see the LFO working at various points in the good pedal.  So I think you are right about the LFO not working.   

[Jarno]

I think I built my Phase 100 from the tonepad layout, did not match vactrols, and it is a thick and lush woosh

Maybe splice in a function generator (careful with the amplitude of the signal!) somewhere to see if that solves it.

You do not have an oscilloscope? I sold mine a while ago (big and bulky Tek unit), on the lookout for something digital and handy, but feel a little bit helpless without an oscilloscope around, it is really worthwhile to see what is going on in a circuit.

[pedalbob]

Thanks but no I don't have a scope.  I've been looking too at the moderate priced digital ones which would do everything I need and more. I've really gotten into these pedals during Covid as something to do in my free time.  I have a crude signal probe I have used successfully to troubleshoot a few pedals.  That will probably be my next step if I can't find something with my multimeter.  Will post an update if I find something.  Thanks !     

[pedalbob]

OK so I did a back to back with the good pedal and the "bad" pedal measuring the voltage variations at the LED side of the 3 Vactrols.  The overall voltages are slightly different but the voltage variation range seems consistent (it is very slight a little less than 0.1V overall delta).  So I think the LFO is working ...

I had taken Q3 out to check out of the board because I was getting some odd measurements and when I put it back in before doing the above test the pedal is working a little better than before.  I would say I'm getting about 1/3 to 1/2 of the effect that the good pedal gives.  The Speed pot changes speed and the intensity switch changes depth of effect.  It's almost like I'm only getting a single stage of phasing.  Going to attack it with my audio probe and see if I can find where the two differ.

I'll post an update.     

[Govmnt_Lacky]

At this point (and from previous experience) I would suggest checking all of the resistor and cap values in your pedal circuit. You never know if some tinkerer replaced a resistor or cap and mis-read the value. It will take some time but worth the sanity check.

[pedalbob]

Thanks I did that and everything looked correct.  I have been using the MXR schematic which is online.

I also checked voltages at each end of each resistor to ground and may have found something but I don't understand how the LFO works well enough to interpret.  The voltage on the good pedal at R18 (resistor between OP Amp A1 pin 7 which is the Op Amp at the end of the LFO circuit and input to Speed Potentiometer)  oscillates significantly on the "good" pedal I have (on the order of volts) and only slightly (tenth of a volt) on the "bad" pedal.  Any one of the capacitors or transistors in the more likely to control this ?

I did swap out C-3 tantalum capacitor previously with no effect.     

[Govmnt_Lacky]

Thanks I did that and everything looked correct.  I have been using the MXR schematic which is online.

I also checked voltages at each end of each resistor to ground and may have found something but I don't understand how the LFO works well enough to interpret.  The voltage on the good pedal at R18 (resistor between OP Amp A1 pin 7 which is the Op Amp at the end of the LFO circuit and input to Speed Potentiometer)  oscillates significantly on the "good" pedal I have (on the order of volts) and only slightly (tenth of a volt) on the "bad" pedal.  Any one of the capacitors or transistors in the more likely to control this ?

I did swap out C-3 tantalum capacitor previously with no effect.   

Hmm.. looks like you are getting somewhere on this. Unfortunately, the only MXR schematic I am seeing on Google is from FSB and I don't feel like going there 

If you are seeing the small voltage swing from Pin 7 of the IC, then I suggest checking Pins 5 & 6 and comparing with your good unit. Keep following the schematic back from those points until you see a likeness between the 2 pedals.