reducing phaser level

[space_ryerson]

Hi, I hate for this to be my first post (seems awfully simple, but...), but here goes:

I have two phasers, a phase 90 and a tokai tph-1. Both of them sound great in their own realm, but have a ridiculous level increase when turned on, regardless of what guitar or amp I use. I'm fairly adept with a soldering iron, have done some basic builds, and have a crude knowledge of basic circuitry. I was wondering if I wired in a trimpot or a resistor (like 5k or that neighborhood) where the 'phased' signal is mixed back in with the straight guitar signal, if that would work, or am I missing something?
I know I could put a compressor after the phaser to limit the level change, but I would rather not have to do that.

I do appreciate any insight that anyone has. I've been reading here a while now, and I must say that this is a great forum!

[Fret Wire]

What model Phase 90 do you have? Script, block, or reissue?

[space_ryerson]

it's a reissue. I'm more concerned with the tokai pedal, since it is being used for some recordings, but fixing either one (or both) would be great. I don't have a schematic for that tokai, unfortunately. A friend of mine and I were discussing this, and he apparently has the same problem with his mutron. Is this the norm for phasers?

[Fret Wire]

Some phasers do boost the signal, also depends where in the signal chain it is. On the phase 90 reissue, drop R7 (150k) to 130k and that should give unity gain. Removing R28 entirely will get rid of that nasty mid- boosted distortion.

[space_ryerson]

ooh thanks! I'll give that a try, and let you know how it goes.

[Fret Wire]

Remove R28 first, then check, if the volume is still boosted, then drop R7 to 130K.

[space_ryerson]

Sorry it has taken me a little while to get to this, but I realized that the schematic I have for the Phase 90 is for the original block logo, not the reissue that I have. On the PC board, only certain resistors are labeled. R28 is not, but R27 is, and it is a 25k/540M trimpot. Fret Wire, you seem to know this circuit well; which resistor would R28 be?

[Fret Wire]

On the block logo Phase 90, there are two 10k resistors coming off IC's two and three, that are connected by a 22k or 24k resistor. That is the one you want to remove. You could reference the Tonepad schematic to see what I'm talking about. I no longer have an older block logo 90 to check the pcb. You also may not find the resistor at all. It was added during the block logo period to distort and boost mid range, in response to EVH. He made the script logo popular with VH 1 & 2. MXR added the resistor to the ckt to simulate his sound at home with little practice amps. Problem is, it sounds like hell on a real amp, acoustic guitars, or vocals. Also, the 150k that connects the collector and base of the 4125 (usually) can be lowered to get rid of the volume boost. 130k usually gives unity gain. Don't confuse the transistor with the JFETS. I'll try to hunt down a pic of the block logo layout, so I can show you exactly where they are.

[space_ryerson]

Wow, you are quick. Thanks for the help.

How does the block logo differ from the reissue? I have the reissue's PCB right in front of me, and I only see 2 IC's, so there's a start. I didn't remember that the schem I have doesn't match the pedal, so once I took it apart, I realized that I could probably narrow down where R28 is, but I would still be taking a chance. Where is the tonepad schematic? Would it help if I took a pic of the front and back of the PCB?

[Eddie]

There are two versions of the block logo Phase 90.
6 Op Amps versus 3 Dual Op Amps.

harmony central review:

HOW TO TELL A “SCRIPT” FROM A “BLOCK” INTERNALLY.


Some of the mid ‘70s “block” logo stenciled MXR Phase 90 units contain the board and components of the most popular and expensive “script” logo units. The change of the logo markings and the internal electronics did NOT happen at the same time.


But how can you tell the difference between the two internally?


By looking at the components:


The older “script” used six (6) of the 741-style op amps (black, rectangular things with eight “legs” connecting it to the board). This is the most popular, collectable and expensive vintage version.


Around 1975, the board was redesigned to the board you’d see in a typical “block” logo vintage MXR Phase 90.


First versions of this new board were using Raytheon brand 4558 op amps. These had quality control problems. Only used for a short time.


So MXR then switched to Texas Instruments sourced op amps like the 4558 or TL062.


Because the newer version was now using dual op amps, you only find THREE (3) of them of the later version board in the “block” logo units.


The easiest way to be sure is to count the number of op amps.

*****************************************************

The script Logo uses 6. I own the early Block Logo version. The circuit is the same as the script version. The only difference is the feedback loop resistor. You will find that one easily it`s the only resistor connected on the side where the traces are (opposite of the parts side of the PCB).
Connects to the first 741 Op amp. Sorry bad english...

I have pictures of the script/Block logo/Dunlop R28 Mod !

http://www.starguitar.net/st01008/st01008.html

http://www.starguitar.net/Script_Logo/script_logo.html

http://www.landrocket.com/sled/MXR%20Phase%2090%20Script%20Logo/mxr%20003.jpg

But no picture of the feedback resistor of the Block logo.


I like the block logo more WITH the feedback loop the effect is a lot stronger.

Eddie

[Mark Hammer]

The two phaser use different switching systems and so will require different fixes to the volume issue.

The MXR uses a mechanical stompswitch, where the Tokai appears to use FET-switching.  It is common for older pedals using a SPDT stompswitch to provide some tone-sucking in bypass mode because the input resistor on the circuit board (which is *always* tied to the input jack when using a SPDT switch) loads down the signal.  In many instances, the designers would try and produce a circuit that would yield good bypass/effect level matching, but when people would do a true bypass mod with a DPDT or 3PDT switch, the bypass level would be much louder than effect mode *because* the loading effect would be eliminated with the true bypass.

In the case of FET-switched pedals, it is a very different problem.  Note that a phasing effect relies on having w wet+dry mix.  If you remove the wet/phased signal from the point where dry and wet are mixed together, the "effect" is cancelled.  So, the majority of manufacturers making phasers will use a single FET between the wet signal path and the mixing stage as their "switch".  Hit the treadle and the flip-flop circuit either turns the FET on (low resistance) or off (high resistance) to mimic a closed and open mechanical switch.  Remember, however that nothing changes about the dry signal path in such an instance.  Curing a level imbalance under these circumstances is much more difficult.

[Fret Wire]

Wow, you are quick. Thanks for the help.

How does the block logo differ from the reissue? I have the reissue's PCB right in front of me, and I only see 2 IC's, so there's a start. I didn't remember that the schem I have doesn't match the pedal, so once I took it apart, I realized that I could probably narrow down where R28 is, but I would still be taking a chance. Where is the tonepad schematic? Would it help if I took a pic of the front and back of the PCB?

If you only see two IC's, chances are you have a reissue, the third is under the pot. Does your board match any of Eddies pics? The first is the Dunlop reissue, the other two are script logo. The Tonepad schematic should help you find both resistors. If you can take a pic of both sides of your board that would help.
http://www.tonepad.com/getFile.asp?id=42

[Hal]

_or_ you could put a 100k trimpot on the output jack...if you dont wanna mess with the board.

there was a different thread about this a while back.

[Fret Wire]

_or_ you could put a 100k trimpot on the output jack...if you dont wanna mess with the board.

there was a different thread about this a while back.

A trimpot won't address the "R28" or "feedback resistor" mod. Removing that resistor cuts the mid-boosted distortion the later units had. Removing it also lowers the perceived volume level. The 150k mod is strictly a volume level mod.

[space_ryerson]

If you only see two IC's, chances are you have a reissue, the third is under the pot. Does your board match any of Eddies pics? The first is the Dunlop reissue, the other two are script logo. The Tonepad schematic should help you find both resistors. If you can take a pic of both sides of your board that would help.
http://www.tonepad.com/getFile.asp?id=42

I have a reissue. Eddie's first pic showed me where R28 is on a reissue board, so when I get home tonight, I'll be able to remove it. I think I confused you when I mentioned that I have an original block logo schematic.

As for the Tokai, after reading up, I'm unsure how (or if) the level problem could be easily addressed. The volume jump on it is really severe, no matter where I put it in the signal chain, and no matter what type guitar I use.

[Fret Wire]

After you remove R28, try it and see how it sounds. If the volume is still too boosted for your liking, I'll show you where R7 is, so you can lower the volume.

[space_ryerson]

It seems to have worked. The level is slightly increased, but not in a way I find objectionable. I would like the effect to be a little more pronounced, but I think that may be the battery being old.

Either way, thanks for your help guys.