Probably A Simple Fix
MXR 'Block' Logo Phase 90
First up in this series, I'll be looking at a ‘Block’ logo Phase 90 from MXR. Will it be an easy fix? Let’s see.
Some History
The MXR Phase 90 was first released in 1974 and, was in fact, the first pedal ever released by the company.
The first version of the pedal featured the distinctive ‘Script’ logo and was used by guitarists such as Edward Van Halen to great effect. By 1977, the ‘Block’ logo began to appear and the pedal remained in production until MXR went bankrupt in 1984.
A 'Script' Phase 90 (circled in red) on Edward Van Halen's pedalboard circa 1978.
Luckily for us, Jim Dunlop brought the MXR brand in the mid-to-late 80s and production of the Phase 90 resumed, this time with some modifications to the circuit and the addition of some user-friendly features such as a status LED and DC power jack.
A 'Script' versus 'Block' logo Phase 90. Note the addition of the status LED.
The Patient
I picked this Phase 90 up off Trade Me (the New Zealand-based version of the popular auction site eBay); the previous owner stated that it was working great until all of a sudden it wasn’t and that the pedal would happily pass a bypass signal but when engaged, nothing. No sound of any kind and the status LED refused to turn on.
The patient on the repair bench.
This was confirmed once the pedal arrived and I was able to plug it in and test it for myself. My initial thoughts were an issue in the power supply with the worst-case scenario being the dreaded ‘Whoops. I plugged in the wrong polarity power supply and it fried all of the semi-conductors inside.’ Cracking open the enclosure, there were no immediate signs of any catastrophic failures, in fact, everything looked fine (other than the badly corroded 9-volt battery snap). So it was time to break out the multimeter and test some voltages.
Inside the enclosure.
Powering the pedal up with my 9-volt DC power supply, I failed to measure any voltage whatsoever at a variety of points in and around the DC power jack itself. Having worked on a few Phase 90s in the past, I know the diodes located in the power supply section can often be the cause of this type of fault.
The corroded 9-volt battery snap.
I Have The Power
The two diodes in the power supply section each of which has a separate role. One provides reverse polarity protection; the other, a Zener diode, works in conjunction with the 10K resistor to provide a virtual ground that bias the op-amps in order for them to function correctly.
The two diodes in the power supply section.
Breaking out the multimeter again and setting it to the diode testing function, we should see approximately 0.5 to 0.7 volts when testing the forward voltage with the positive probe of the multimeter on the anode (the positive side of the diode) and the negative probe of the cathode (the negative side of the diode). With the probes reserved, the multimeter should read ‘0L’ for overload as current can’t flow in that direction.
The multimeter reading '0L'.
Testing the Zener diode first, the theory holds true. Moving onto the polarity protection diode, however, is a completely different story. As the diode testing function on the multimeter has a secondary function as a continuity tester i.e. used to determine if an electrical path can be established between two points, a resounding ‘beep’ let us know that the diode is running wide open and letting current flow in either direction, acting as a jumper wire sending the power straight to ground just after entering the circuit via the DC jack.
Removing the diode quickly restores power to the circuit, the status LED lights up, various voltages check-out at different points around the PCB, there is now even an audio signal when the pedal is engaged but now phasing effect. What gives?
The faulty diode removed from the PCB.
My audio probe tells me that the op-amps are functioning correctly and the LFO (or low-frequency oscillator) is 'LFOing'. That's when it hits me, maybe the previous owner had tweaked the trimpot on the PCB in a desperate attempt to try and get the pedal working again.
The new polarity protection diode installed.
There is usually a squirt of ‘Hot Glue’ on the underside of the PCB to prevent people from doing this as it is used as part of the calibration process done in the factory to make up for the known variance that exists within the JFET transistors used in the circuit.
An example of the squirt of 'Hot Glue' from another Phase 90.
It allows the manufacturer to use a broader range of transistors in regards to manufacturing tolerances that would otherwise be rendered useless for the circuit at the expense of having someone calibrating each and everyone as they come off the assembly line. This particular example was missing the telltale squirt of ‘Hot Gun’.
The trimpot located at the top of the PCB just above the potentiometer that controls the speed of the phasing effect.
Depending on what you’ve read, some people suggest only a technician should adjust the trimpot. I can tell you that if you have a Phase 90 that is working perfectly fine there is no need to touch it. It is not an extra control i.e. for controlling the depth of the effect or tone that’s hidden under the hood like some other pedals. It is simply used for calibrating the circuit.
Replacing the corroded 9-volt battery snap.
In saying that, you’re not going to break or damage the pedal if you do turn it. It will just stop phasing or sound different and turning it back will get it phasing again. And in this cause, doing exactly that got the pedal working as it should again.
Job done.
The new battery snap installed.
So Was It An Easy Fix?
Yes. Diagnosing the fault was fairly straight forward as was sourcing (it is just a common 1N914 diode) and replacing the faulty part in question. It did have the added wrinkle of needing to be recalibrated in addition replacing the corroded 9-volt battery snap but all-in-all, it was indeed a simple fix.
Ready to rock again.
Further Reading