MXR Phase 100 help please - Siren sound related to depth setting?

[Madrooskie]

Hello - full newb here.  I've built a DIY EHX Muff that worked straight away and have tinkered with a 70 silverface to get it back up and running, but after haunting this site for a bit, I am clearly no master, so I am looking to some of you for help.

Gear : 1997 MXR Phase 100

Issue: When set at what I understand is the high depth settings, meaning that the intensity switch with four settings is set at position 1 and 3 from left to right (long circles rather than the small circles) I get what I can only describe as a siren sound, and the modulation of the siren can be adjusted by increasing or decreasing the speed.  Playing the guitar makes no change to the sound, you can't hear the guitar through the siren.

Some background:  I bought the pedal very recently from a guy that said he used it once and put it in a closet.  The pedal has an inspection date of 10-9-97, has the TI TL062CP OP Amps, and VTL5C3/2 photocouplers.  All resistors/caps/diodes seem to be in at least good visual condition as well as the transistors.. although I know that visual is not a good way at all to tell, so do what you will with that info.  Its been great for about 2 months daily use, then a week ago I noticed what seemed like inconsistent volume while playing through the pedal, meaning that when picking at a string with the same attack over and over, the response at the amp was not the same, sometimes it would sound loud, other times it would almost decay or swell and then decay.  I thought it was battery but the battery was new and registering 8.9V while disconnected.  It happens when in line with other pedals, or if it is on it's own, and at any volume.

Question:  Why is does it sound like a siren when in the 1 and 3 position?

Ive been thinking about the circuit and took a look through some online schematics... now this is a total guess, but I am wondering if I have a bad photocoupler.  It seems that the photocoupler is the part of the circuit that provides the phase action?  I really don't know.  Any help would be greatly appreciated.  I love this pedal when it works!

Also, and I think that this was a no-no, but I did it so flame me if you like, I adjusted the trim pot after marking home position (factory setting as best as I can tell).  When I adjust closer to the 3 setting the sound goes away unless I really attack the strings.  When I go back to the factory setting, it comes back and remains constant.

Please help if you can, and I will try and answer any more questions that anyone has.

[Govmnt_Lacky]

Start here and repost after you run through it.

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[Funkmasterron]

Hi there!

I just saw this thread, although it is over a decade old.

I have the same pedal and mine is doing the same thing (i.e. siren sound)!

I was just wondering if you ever figured out what the issue was?

Thanks!

[antonis]

Hi there!

Hi & Welcome..

What are the chances for getting a reply from a single and only one post member a decade after..??

[theehman]

Well, if it's any help, I built one on a NOS MXR pcb and it sounded like crap on 2 settings (can't remember if it was 1&3 or 2&4).  I removed the optocouplers and matched a set with a multimeter and an led tester.  It fixed the issue.  I also had a Cheese Blocks Phase 100 that had the same issue and replacing the optocouplers with a matched set fixed the problem.

[Funkmasterron]

What does "sound like crap" mean??
My "siren" sound is mixed with the instrument signal, and comes and goes. Most of the time, the pedal sounds fine. Same waveform on both pins 3 and 5 (in and out) of optos.

So wondering if worth replacing optos.  They are about $15 each.

How did you match them?

[Rob Strand]

There's at least two circuit versions of the MXR 100.   There should be forum posts filling in some of the blanks.

The transistor circuit after the opamp LFO, including the intensity switch,  is very fickle.   It's not a great circuit for production and even less reliable for clones.

Take a look at,
http://www.generalguitargadgets.com/pdf/ggg_p100_sc.pdf

I seriously doubt opto *matching* is an issue for bad sound.  What you could experience is the Center Frequency trimmer hasn't got enough range in which case raise or lower R48.  If the general resistance values of you optos are all too high or all too low compared to the original. The way you compensate for that is the LED current.  The trimpot *should* shift the resistance back into the right zone.

You might find different switch settings need different trimmer settings.  The way around that is to mess with R41, R42 and maybe some other.    After all that the sweep might not be optimal in which case you need to reduce to widen the sweep range ... more mods.    When you change one thing you stuff up something else and you need to compensate.

And then ... once it sounds good to the ear, does it sound like the original?   It's all hit an miss.  That's why I don't like this circuit.  I don't like the small value of R54 either.

It's too long ago since I looked at the details.   I remember playing around with some mods but you are still left with the question what is intended sound of "the original".   The whole idea of the mods is to target the original sound but get there easier, more reliably and cope with some opto variations.

As far as the siren sound goes.  It could be a build error but it could also mean R1 (15k) needs to be increased.  Using 18k will probably fix it.  However you could also be hiding a mistake somewhere, for example R3 or R13 is the wrong value.   It not sure if extremely mismatched optos would cause the siren, or if the trimpot isn't or cannot be adjusted correctly.

[theehman]

What does "sound like crap" mean??


How did you match them?

Well, it didn't have a siren sound but 2 settings were pretty distorted.  I matched them by using an LED tester and a multimeter.  I would insert the LED of the optocoupler into the LED tester (10mA socket), then I would measure the resistance of one of the photoresistors.  I matched the resistance to within 1K of the other 3 optocouplers and that fixed the problem.  Fortunately, I had a good supply of original optocouplers on hand.

[Rob Strand]

Well, it didn't have a siren sound but 2 settings were pretty distorted.

In the modes where there's feedback you get response peaks.  If those peaks are too high it will clip.   Increasing R1 reduces the amount of peaking.   I put it out there though, what is the the correct amount of peaking on the original?

In the extreme, the peaks will cause the siren sound.