MusicMan Phasor

johnhostetter · Post by **johnhostetter** » 25 May 2014, 04:12

I apologize in advance if this is not the place for posting this. I realize this is not technically a boutique pedal, but maybe it could be one!

I own a MusicMan 212-75 with a phasor (phaser) effect. It has recently stopped working, and so I have two missions: the first is to try to isolate the phasor circuit from the amp schematic, breadboard it, duplicate it in a pedal enclosure. Hopefully after accomplishing this, I will understand the circuit enough to fix the inboard effect on my amp, plus we will have a new phaser project to play with!

My problem is that I'm just starting to become familiar with amp schematics. I'm having difficulty isolating the circuit from the amp schematic. Any help is greatly appreciated!

https://www.dropbox.com/sh/qxjuxy6wtoj3 ... YHnf2g-bWa

johnhostetter · Post by **johnhostetter** » 25 May 2014, 04:29

Would I be right in thinking that in a stomp box, the input would be inserted just before C34, and the output would come after C38? I'd sub TL072s for the ICs, and I think I have a couple LDRs lying around. What would happen if it was run using +9v and -9v rather than 16v. I would need a Max1044 right.

Somebody please fill me in on how far off I am on this.

Ice-9 · Post by **Ice-9** » 25 May 2014, 08:47

johnhostetter wrote:Would I be right in thinking that in a stomp box, the input would be inserted just before C34, and the output would come after C38? I'd sub TL072s for the ICs, and I think I have a couple LDRs lying around. What would happen if it was run using +9v and -9v rather than 16v. I would need a Max1044 right.

Somebody please fill me in on how far off I am on this.

Yeah that's about right but I would look at the output being after R49.

johnhostetter · Post by **johnhostetter** » 25 May 2014, 23:12

Ok, great. I think I can handle this as long as I figure out what's going on with the power. Is it running from -16v only? I don't understand.

blackbunny · Post by **blackbunny** » 26 May 2014, 00:49

The LFO oscillator which drives the optocouplers LDR1 & 2 uses the -16v rail to enable and disable it, which starts and stops the phasing. The whole Phasor circuit is based around IC5, IC6 and IC7 which are standard workhorse 1458 / 4558 dual op amps. Pin 8 of each op amp is connected to +16v supply rail and pin 4 is connected to -16v supply rail.
A split supply from a MAX1044 resulting in +/-8v or +/- 9v should be OK for a pedal version of this circuit, but you might need to lower a few resistor values in key areas to allow similar drive currents with the lower supply rails.

For example, the LFO circuit around IC7 may work fine with the values on the diagram, but if it doesn't oscillate you may need to try halving the values of R54-R57, but that will also affect the fastest and slowest rates, so you will then need to double C41 to 0.22uF.

I would try building it as it is shown on the diagram, and only start tweaking the values if it doesn't work as you want.

You could also consider powering the pedal from from a 12-18v plugpack and get higher +/- supply voltages using the MAX1044.

johnhostetter · Post by **johnhostetter** » 26 May 2014, 14:36

blackbunny, this is very helpful! This is what I thought as far as the power supply goes, but I would not have thought to change resistor values and the cap. I'd love to run the phasor on 16v, but the Max1044 fizzles above 10v, doesn't it?

blackbunny · Post by **blackbunny** » 27 May 2014, 00:24

johnhostetter wrote:blackbunny, this is very helpful! This is what I thought as far as the power supply goes, but I would not have thought to change resistor values and the cap. I'd love to run the phasor on 16v, but the Max1044 fizzles above 10v, doesn't it?

Yes, you're right, the 1044 'maxes' at 10v input.

I should have said that you could use a 12-16vAC plugpack and derive +/- DC rails using a similar method to this schematic:

DrNomis · Post by **DrNomis** » 27 May 2014, 06:43

I seem to remember coming across the Musicman Phasor effect while servicing a "Hybrid" (Solid-State Preamp/Tube Power Amp) Musicman amp one time, the amp was making crackling and hissing noises, and the Phasor effect wasn't working like it was supposed to, I ended up fixing it by replacing all the original IC sockets and all the original JRC4558 ICs with TLO72 ICs, when I looked at the original JRC4558 ICs under a magnifying glass I noticed that all the pins were dirty and each IC had a small crack on the underside of it......

johnhostetter · Post by **johnhostetter** » 27 May 2014, 12:53

I'm not savvy enough with power yet to fully understand that circuit, blackbunny. So there is an input of 12vac. and the diodes, capacitors, and voltage regulators then work to produce +18v, +12v, and -12v. Am I right so far? But are these resulting values AC or DC? I'm assuming DC.

Either way, I'd rather the end result be a pedal that can run off a daisy-chained 9v DC power supply. Any predictions about what would happen if I tried it with a positve/negative 9vdc? I bet I'm going to find out!

DrNomis, I love this amp! The problem you describe sounds different from whats happening with mine. Everything works great on the amp, but when I try to engage the phasor, absolutely nothing changes. There is the faintest hint of a phasing sound, but it could just be my imagination. I've replaced the ICs with TL072s, but that didn't change anything. Could it be the LDRs are bad? I believe I have a couple lying around that I could try...but I'm not familiar with any reports of these things failing...

blackbunny · Post by **blackbunny** » 29 May 2014, 00:56

The Rotor Spear (H&K Rotosphere clone) power supply diagram shows one proven method of deriving various DC supplies from a single AC source. You're getting a handle on it.

The fastest way to find out what's going to work would be to go ahead and breadboard it according to your original idea, deriving +/- 8-9vDC rails using a MAX1044. If it doesn't work, after careful double checking to make sure you have breadboarded the circuit correctly, you can start changing some values in the LFO circuit.

The opto couplers are VTL5C which have dual LDRs in each black epoxy package fed by a single LED. These are made by Vactrol and are still available from some online electronics suppliers - although they will be expensive.

johnhostetter wrote:DrNomis, I love this amp! The problem you describe sounds different from whats happening with mine. Everything works great on the amp, but when I try to engage the phasor, absolutely nothing changes. There is the faintest hint of a phasing sound, but it could just be my imagination. I've replaced the ICs with TL072s, but that didn't change anything. Could it be the LDRs are bad? I believe I have a couple lying around that I could try...but I'm not familiar with any reports of these things failing...

I repaired an MM RP-112-100 a few years ago and I loved the sound of the Phasor after it was done. In a similar manner to Dr Nomis I removed the ICs, installed high quality 8 pin DIP sockets and replaced all the ICs in the preamp section with TL072s, including IC5 and IC6 which handle audio in the Phasor circuit. I replaced IC7 with a 4558 and all worked well. I then decided to try some low noise bipolar input ICs in IC5 & IC6 sockets, firstly NE5532s and then LM833s, which I ended up leaving in there as I thought they made the nicest phasing sound.

The Vactrol opto couplers could be faulty. Establishing whether the phase shift stages are being modulated by the optocouplers can be tricky if you don't have much troubleshooting experience. Have a look at the MM service bulletin that describes calibration and compare it to the schematic to gain some understanding of how the Phasor circuit operates. Try the test procedures in the Service Bulletin and write down the resulting voltages & resistances for comparison with the target results.