Debugging a Mu-Tron Phasor ii

Luke51411 · September 08, 2015, 11:55:06 PM

So I have a Phasor ii that only works for roughly half of the speed range, the faster half and it doesn't seem like it quits at exactly the same spot every time. I've measured the resistance between lug 1 and 2 and between 2 and 3 while rotating the pot and it seems to be working properly. I repaired the solder on the lugs and that fixed part of the original problem which was that the rate was jumping without rotating the pot. It seems to me like maybe something is causing the LFO to lock up. I'm just looking for advice on what I should look for. I'll get voltages on the lfo section when I get a chance.
Schematic on page 7 and 8 here http://www.bruynooghe.fr/public/effets_vintage/Phaser/MutronMU-01PhasorIIServiceManual.pdf

Derringer · September 13, 2015, 12:06:55 AM

I had similar problems with mine. The way the board is mounted to the pots, there is a lot of flexing that occurs through regular use. I jumpered extra wire leads from the pot lugs to the pcb pads or to the exposed leg of the first component that a particular lug's trace went to so that even if the lugs flex and break the joint, which they will, the wire lead still makes contact and carries signal. Also, plenty of the opamp solder joints on my unit had also been flexed to death over the years. I touched them all up. Mine has worked flawlessly since doing this.

PRR · September 13, 2015, 01:48:13 AM

The circuit is good.

I like Derringer's joint-flex theory. (It wasn't made to last 35 years.)

Stalling at slow speeds *could* also be stray leakage. C16 is unlikely but possible. If the pedal has been years in a dry climate (lots of static shocks) there *could* be degradation of the Z6 chip (input leakage current). If it has lived in the damp, moisture in the PCB or flux could add more leakage than it can manage.

Luke51411 · September 13, 2015, 11:32:07 AM

Thanks for the replies. I looked over the solder side again and found a few questionable joints and a 1uf cap that had 1 leg completely removed from the hole. I installed a new one and touched up some joints. It's still doing pretty much the same thing. One thing it does is that when the rate is set to just above where it stops, the sweep can be kind of erratic, sometimes changing speed all together and sometimes altering the character of the sweep.

Derringer · September 14, 2015, 11:16:16 AM

the symptoms you're describing seem identical to what mine was doing. With mine in particular, if I turned the rate all the way down, nothing would oscillate and it seemed to be "stuck" in a phased position. As I turned the rate up to maybe 40% or so (liike you said, it wasn't always the same spot), the oscillation would kick in but it would still fluctuate.

When it was acting up, I could take my foot, gently apply pressure to the top three knobs and all of a sudden the rate would kick back in and it would phase. Relieve the pressure and it went back to misbehaving.

It wasn't till I had it apart and plugged in with a signal that I realized that my problem wasn't just where the pots are soldered to the board but that it was throughout the pcb. With the board out I just started flexing it - gently - by hand, pushing on opamps etc and found all sorts of places that needed resoldering.

fwiw, it did replace all the electrolytics as well while I was in there

Try flying some wire leads from those big pot lugs down to the first component leg that the individual trace goes to. That will cure any joint flex issues from the pots/pcb mount.

Luke51411 · September 14, 2015, 11:39:33 AM

Thanks for the advice Derringer I'll try your suggestions out and see what happens!

Luke51411 · September 14, 2015, 07:04:42 PM

Fixed! I went over the solder joints again looking closer (and redoing most of them). I found the IC near the in jack had almost all of the joints visibly cracked. Once I fixed all these joints it now works as it should. Thanks everyone!

Derringer · September 15, 2015, 05:39:34 PM

sounds awesome, doesn't it?

Luke51411 · September 15, 2015, 06:47:11 PM

Quote from: Derringer on September 15, 2015, 05:39:34 PM
sounds awesome, doesn't it?

Yes it does! I just finished a clone too so I can compare them too. I haven't done a side by side yet but the clone sounds really good but not quite the same I think.

Derringer · September 15, 2015, 08:16:13 PM

nice! I bet having it with a smaller footprint is a bonus (assuming the clone has a smaller footprint that is!)

Luke51411 · September 15, 2015, 08:27:39 PM

Quote from: Derringer on September 15, 2015, 08:16:13 PM
nice! I bet having it with a smaller footprint is a bonus (assuming the clone has a smaller footprint that is!)

Hah yeah it's in a 125B!

armdnrdy · September 15, 2015, 10:21:56 PM

I built this one about three years ago. 125B enclosure.
Sounds great!
Three VTL5C3/2 vactrols instead of the original six LDR/1 LED "photo mod."

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Debugging a Mu-Tron Phasor ii