Mutron Phasor not phasing

BDuguay · July 10, 2010, 02:41:15 PM

Actually, I can hear the phasing action in the background but the signal is not affected. I've searched here and online for anything but no luck. Does anyone know where I could start troubleshooting?
This pedal is in really good condition and all the solder joints look fine. I've already replaced the Electrolytics.
Thanks,
B.

zombiwoof · July 10, 2010, 02:54:28 PM

Funny you should say that, as I had one of those for years, and it did the same thing, only intermittently. It was really frustrating, as it would work fine for a while, then all of a sudden display the problem you are experiencing. I took it to a pedal repair guy, and he couldn't find anything wrong with it, in fact he couldn't get it to replicate the problem at all!. It's like when you have a problem with your car, and take it to a mechanic, and the problem disappears. When I got it back from the repair guy, it started doing the same thing soon after. Back then, I knew nothing about pedals, so I ended up trading it off with some other pedals, and didn't get much money for it.

I'm thinking could it be a problem with the switch?. You might want to check that out. Try bypassing the switch and see what happens.

Al

Gus · July 10, 2010, 03:09:56 PM

I fixed a few mutron phasors it was never a bad part. It was a broken wire/circuit trace or solder connection or someone worked on it before and miswired it.
Check the PCB for cracks

Why did you change the electros before taking voltage measurements? Some advise from the web?

Measure your connections from point to point.

Measure the voltages at the pins of the IC

Do you have a oscilloscope?

Measure the switch connections before changing it

Read the sticky

FWIW I have seen DMMs on sale for $1.99 with a battery I bought a few they work OK at 9VDC levels. I have real meters like flukes but I need a some meters for a home made tube tester I am building.

I typed "mutron phasor" in and got a few hits

http://www.diystompboxes.com/smfforum/index.php?topic=69199.0

BDuguay · July 10, 2010, 03:44:03 PM

Quote from: Gus on July 10, 2010, 03:09:56 PM
I fixed a few mutron phasors it was never a bad part. It was a broken wire/circuit trace or solder connection or someone worked on it before and miswired it.
Check the PCB for cracks

Why did you change the electros before taking voltage measurements? Some advise from the web?

Measure your connections from point to point.

Measure the voltages at the pins of the IC

Do you have a oscilloscope?

Measure the switch connections before changing it

Read the sticky

FWIW I have seen DMMs on sale for $1.99 with a battery I bought a few they work OK at 9VDC levels. I have real meters like flukes but I need a some meters for a home made tube tester I am building.

I typed "mutron phasor" in and got a few hits

http://www.diystompboxes.com/smfforum/index.php?topic=69199.0

Bypass works fine but when the switch is engaged, all I get is straight guitar signal but i can hear phasing noise in the background. I'm sure the phasing part of the circuit is functioning because that phasing noise changes when I manipulate the controls. I actually have 2 scopes but sadly, I'm not entirely sure how to use them yet. I will get there someday.
I changed the electros as per advice from searching here. I only changed 3 and they are the same value as original. Yes, I've read the sticky but I know from experience that this is probably something very simple but I just don't know where to sart looking.
B.

zombiwoof · July 11, 2010, 11:27:24 AM

And you're positive the switch isn't the problem?. I have a RotoVibe that had a similar problem, and it turned out to be the switch.

Al

daverdave · July 11, 2010, 02:10:38 PM

It sounds like a problem with the allpass filter section, if you're getting the dry signal. I'd recommend audio probing through the allpass filters from input to output and see if you get a fault down the line.

MoltenVoltage · July 12, 2010, 01:20:10 AM

Although you might be sure you are hearing the "phasing", you might just be hearing the LFO.

I am in the process of repairing a Maestro Stage Phaser and thought I was hearing phasing, until I traced the signal and found the bad IC which wasn't passing the signal.

IC's do go bad, and so do electrolytics (I had to replace the power caps on my Phasor II to get it working again), but the way to figure it out is to send a test signal through and do it right. To properly repair the pedal you need to sharpen your oscilloscope troubleshooting chops - preferably on non-vintage gear!

BDuguay · July 12, 2010, 08:54:18 AM

I metered the switch yesterday and it's fine. I guess the next step, as has been suggested, is to audio probe it. The problem with that is, I have no schematic to refer to.
And yes, I must learn to use my scope. As soon as we institute a 36 hour day, I'm all over it

B.

MoltenVoltage · July 12, 2010, 09:00:25 AM

http://www.experimentalistsanonymous.com/diy/Schematics/Phasers/Mu-tron%20Phaser%202.jpg

BDuguay · July 12, 2010, 09:39:12 AM

Thanks MV, but that's the schematic for the PhasorII. The pedal I have is, I believe, the first generation, plain old, straight up, plane jane, basic Phasor.
Does anyone know what the differences are between the 2?
B.

daverdave · July 12, 2010, 09:51:35 AM

Don't hold me to this as I don't know for sure, but I think the first was ota based whilst the second was optical.

BDuguay · July 12, 2010, 10:42:32 AM

Oh I will hold you to that Mister cuz you're right! The one I have is loaded with the older 'can' style CA3080's and the linked schematic of the PhasorII shows photo couplers.
While I pride myself in knowing at least that much, I'm still unsure how to use that schematic to help with troubleshooting.
B.

daverdave · July 12, 2010, 01:32:17 PM

I doubt the phasor II schematic is much good, but all phasers generally follow the same building blocks.

Input buffer, allpass filter stages / dry signal split, output mixer, then the lfo. It's just a matter of plodding through and trying to figue out what's what. You could trace the schematic as you go through the circuit, start at the input, that would be good practice for ya as well

MoltenVoltage · July 12, 2010, 07:39:16 PM

http://www.beavisaudio.com/img09/MutronPhasorFactorySchematic.jpg

The original used transconductance amps:
http://www.mu-tron.org/list.html

Mark Hammer · July 12, 2010, 08:02:44 PM

Thanks for that Karl.

First time I've seen it.

The unit uses a buffered bypass. Does it uses single op-amps all round or are doubles used? The schematic labels each op-amp section individually and provides no pin numbers. So, it could be that duals are used...or maybe not. So how many chips all together?

Rodgre · July 12, 2010, 10:20:49 PM

I have fixed three Phasor IIs for the same problem:The PC board is stressed where the Rate pot is soldered and just reheating the joints in that area fixed all three. Might not be your problem, but it's probably not a coincidence that I had three with the same problem.

Roger

BDuguay · July 13, 2010, 07:41:35 AM

This particular example is in remarkable condition - inside and out. I've thoroughly inspected all of the solder joints with my magnifying glass and nothing looks dodgy.
Mark - all of the opamps in this one are RC4558's
Thanks for all of the replies and interest folks. Keep 'em coming.
B.

Mark Hammer · July 13, 2010, 08:36:27 AM

Okay, then if 4558s are i use AND you have good bypass signal, then that would suggest thatthere is nothing wrong with the chip handling the input. That chip, in turn, is likely to be the basis of the output mixer stage as well. So, we'll assume that a fried mixer chip is not the issue....for now.

BDuguay · July 13, 2010, 09:24:14 AM

Now that I have a schematic to follow (thanks very much M.V.!) I can have my co-worker help to pinpoint where to start. Mark, you're saying it's not likely the opamps though?
B.

Mark Hammer · July 13, 2010, 09:57:33 AM

Well, it is common for such circuits to use the same dual op-amp for the input stage and output stages. While not labelled as such, it makes perfect sense from a layout standpoint, that Mu-Tron would have done the same thing. Is it possible for the half of the 4558 that forms the input buffer to be fine and the half that forms the output buffer to be fried? I suppose it is remotely possible, just not likely. To me, in my youthful naiveté, I would take that as a sign that there is an issue with continuity on the input side of that mixer stage, OR the utput side.

Another avenue which I don't think has been explored is whether the Depth switch is making good contact. In the absence of a scope, crank up the speed to max, so that you get a good steady peak voltage, and measure the AC voltage at the common of switch 2 over the various depth settings.

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Mutron Phasor not phasing