Vintage Block MXR Phase 90 Mods

[DJPsychic]

Does any one have links or pics for the Vintage Block P90 Mods? (block to script, TB, depth pot, wave pot, output trimpot)

Most of the older links/pics are broken, and are for the newer Dunlop versions.

Hoping someone has them collecting dust somewhere 

Thanks!

[Mark Hammer]

The chief differences between block and script have to do with:
a) feedback amount
b) sweep width

Keep in mind that, as a single knob phaser, other parameters were fixed so as to provide an acceptable sound at all modulation speeds.

Varying the value of the feedback resistance will get you greater and lesser amounts of feedback/resonance.  I wouldn't go much below 15-18k and values above, say 39k will yield diminishing returns.

The 3.9M resistor coming off the LFO will provide narrower sweep from the "starting point" (set by the bias trimmer) is that value is increased, and wider/higher sweep if the value is reduced.  You probably don't want to go much beyond around 4.3M in the one direction, and 2.7M in the other.

I'llleave the output level to other minds.

[DJPsychic]

The chief differences between block and script have to do with:
a) feedback amount
b) sweep width

Keep in mind that, as a single knob phaser, other parameters were fixed so as to provide an acceptable sound at all modulation speeds.

Varying the value of the feedback resistance will get you greater and lesser amounts of feedback/resonance.  I wouldn't go much below 15-18k and values above, say 39k will yield diminishing returns.

The 3.9M resistor coming off the LFO will provide narrower sweep from the "starting point" (set by the bias trimmer) is that value is increased, and wider/higher sweep if the value is reduced.  You probably don't want to go much beyond around 4.3M in the one direction, and 2.7M in the other.

I'llleave the output level to other minds.

Thank you Mark!

I've read almost every thread on the topic, and you seem to be a recurring voice, so I appreciate you chiming in! 

Any idea the differences between the "modern" block and vintage circuit layout? From what I've read you can't go by the Dunlop when working on the older blocks. There was supposedly a site with the details but I believe the link is dead.

[Mark Hammer]

I have to confess that I'm actually pretty agnostic in such matters, and tend to expand what a given circuit is able to do, rather than focus on replicating a specific issue.  So, what I described in the earlier post HAS the qualities of both block and script "in there somewhere".  I don't know or think that either of them are perfect recipes to suit every conceivable song or tempo.  I mean, they are designed to "work" reasonably well, but aim for the middle ground, rather than flexibility.

[DJPsychic]

I have to confess that I'm actually pretty agnostic in such matters, and tend to expand what a given circuit is able to do, rather than focus on replicating a specific issue.  So, what I described in the earlier post HAS the qualities of both block and script "in there somewhere".  I don't know or think that either of them are perfect recipes to suit every conceivable song or tempo.  I mean, they are designed to "work" reasonably well, but aim for the middle ground, rather than flexibility.

I was speaking more on the location of physical components on a vintage block vs modern block circuit board.

There was a picture posted on an older thread that has been lost in time. It had the various components associated with the "script mod" labelled on a vintage block circuit board. You know, for noob dumb a**es like me 

[Mark Hammer]

A quick google image search suggests there are a great many different layouts.  I think your best strategy here might be to post a picture of the component and copper sides of the pedal in this thread, and we can tell you what part is what.

[DJPsychic]

1981 MXR Block P90

[DJPsychic]

Here's the thread I was referencing on the lost "Erik Hansen" site describing the vintage block to script mods

https://www.diystompboxes.com/smfforum/index.php?topic=86605.0

[Rob Strand]

There's like 5 versions of phase 90 out there all with subtle differences in PCB and circuit.

I don't have all my detailed notes but there's a couple of tell tale signs on your PCB.
- Notice no opamps are wired as buffers.   Most circuits on the web (vintage and non-vintage) show a buffer
  for the first stage.
- there is a 2k2 resistor on the PCB.

That makes me think your board not only has the feedback but also the pre-emphasis and de-emphasis circuits.

In this case you can just remove the feedback, or, you can remove both the feedback and the pre-emphasis and de-emphasis.

This circuit is an example of the version with pre-emphisis/de-emphasis + feedback
http://www.matsumin.net/diy/bunkai/MXR_Phase90/MXR_Phase90_1993.BMP

The first opamp has the 2.2k + cap  + 10k (R4, C2, R3) for the pre-emphasis and the cap 6.8n  (C11) for the de-emphasis.

You would need to
- trace the input socket through to the first opamp to find the pre-emphasis opamp.
- trace the circuit around the 2N4126/2N4125 transistor
- from the 2N4126/2N4125  trace which 150k goes to the last phase shifter, you should
  see a 24k, a 150k and a 10k going to the output an opamp.   The 24k is the feedback resistor.

You might find minor differences between your circuit and the schematic for example 470k input resistor and not 1M.
I can't remember all the details of all the versions there were some very fine points which determined the precise version.
 

[DJPsychic]

hey thank you thats super interesting. I found this site which I believe confirms what you said about the pre-emphasis/de-emphasis

http://www.fxdoctor.com/phase-90-circuit-board-revisions/

The first opamp has the 2.2k + cap  + 10k (R4, C2, R3) for the pre-emphasis and the cap 6.8n  (C11) for the de-emphasis.

Is removing these components typically part of performing the script mod?

[Rob Strand]

hey thank you thats super interesting. I found this site which I believe confirms what you said about the pre-emphasis/de-emphasis

http://www.fxdoctor.com/phase-90-circuit-board-revisions/

Looks good.  The site captures the main changes (there are some minor changes in between).

Quote
    The first opamp has the 2.2k + cap  + 10k (R4, C2, R3) for the pre-emphasis and the cap 6.8n  (C11) for the de-emphasis.
     
Is removing these components typically part of performing the script mod?

There's a bit of semantics at play because the "script mod" is fairly unambiguous when applied to the old Block Logo model.   When you get to the later pre-emphasis/de-emphasis models it's not so clear.     I'd say it's more common to remove those components as well.   You need to leave in the 10k (R3) or replace it with a shorting link.   The least intrusive way to remove the pre-emphasis/de-emphasis would be to just lift one leg of C2 and C11 and leave R3 and R4 in circuit.   It's very easy to un-mod.

FWIW,   the schematic I posted the link to has some errors.  I'm fairly sure C2 is 10n not 100n and C11 is 680p not 6.8n.

The pre-emphais/de-emphasis does have a purpose.  It reduces noise/hiss.  The TL062 devices can be noisy although original LM741's aren't exactly low noise.    The  pre-emphasis/de-emphasis frequencies on that unit aren't particularly aggressive.  They are just enough to shave off some hiss.

What I can't remember is how well the pre-emphasis and de-emphasis match.  If they don't match it can affect the tone a small amount - most people wouldn't pick it up.

[Mark Hammer]

The pedal itself dates from a time when it was assumed the user would run it on batteries.  That's why it uses TL062 dual op-amps, which are low current (i.e., longer battery life).  While not notorious for noise, TL062 chips are not as quiet as their cousin TL072.

It's always a little risky to pursue desoldering of existing chips if one's technique and supplies are in the early stages of development.  But perhaps down the line when your chops are up to doing it flawlessly, consider swapping the 62s for 72s for a quieter pedal, with a jack for external power.

[DJPsychic]

The pedal itself dates from a time when it was assumed the user would run it on batteries.  That's why it uses TL062 dual op-amps, which are low current (i.e., longer battery life).  While not notorious for noise, TL062 chips are not as quiet as their cousin TL072.

It's always a little risky to pursue desoldering of existing chips if one's technique and supplies are in the early stages of development.  But perhaps down the line when your chops are up to doing it flawlessly, consider swapping the 62s for 72s for a quieter pedal, with a jack for external power.

Thank you Mark. So the pedal should not be powered using 9v jack without first swapping chips?

I've only considered doing a jack because I've read 9v batteries don't last long in phase pedals.

And FYI, my soldering skills are fine, I'm just in the early stages of learning schematics, circuit analysis. It's a foreign language to me but slowly getting there 

[Mark Hammer]

Thank you Mark. So the pedal should not be powered using 9v jack without first swapping chips?

I've only considered doing a jack because I've read 9v batteries don't last long in phase pedals.

The stock or modified pedal is absolutely fine to run on a battery.  Indeed, the zener diode was used to make sure that whatever trimpot setting produced decent phasing would still be valid even as the battery started to get old.

My suggestion of a power jack was really to avoid having to buy a battery for it ever again, that's all.  Nothing more serious than that.

[DJPsychic]

What I mean was, if I add a 9v Jack, do I have to change the IC's? Or can I add power jack and run with stock IC's?

Sorry for the confusion 

[Mark Hammer]

No, the circuit that's in there will run just fine on whatever 9V source you have, be it battery, wallwart, solar power, or a hamster running-wheel generator.

Many pedals of that era were built without a 9V jack.  That wasn't because they couldn't or shouldn't use external power.  Rather it was because, when people only had 2 or 3 pedals (none of them digital), buying 9V batteries wasn't a budget-breaker, even though there weren't dollar stores with cheap 9V batteries at that point.  So most companies simply didn't built in the capability.  In some instances, even if it could operate off a 9V battery, but tended to eat them up quickly, the manufacturer might simply include an onboard transformer to power it from the wall.  For instance, I made an EHX Hot Tubes clone wayyyyyy back when.  Electro-Harmonix built it with an onboard power supply and power cord.  It ran just fine off a 9V battery but when through them quickly if you weren't careful.  So, rather than deal with complaints, they just made it AC-powered.