Zener diode question

[aab0mb]

I have a Phase 90 with a bad zener diode.  Can I just leave it out or is that detrimental to the pedal?  I only have a 1W 1N47 zener which has a test voltage (Vr) rating of 16.7v and I'm not sure if it's a suitable replacement.

Thanks,
Aaron

[mdh]

If this is the zener that biases the gates of the FETs, then no, you can't do without it.  I'm not familiar with the original MXR boards, so I suppose it's possible that there's a diode in there that's not in the versions we DIY around here.  If it *is* the diode I'm thinking of, you'll probably want something more in the range of 4.7 to 5.1V.

[aab0mb]

Thanks for the reply Mdh.  I should have posted this right away.

http://www.aronnelson.com/gallery/v/Schematics-etc/MXR_Phase_90.gif.html?g2_imageViewsIndex=1

I only see one zener.  We must be on the same page (i hope).  So would the absence of this diode cause the pedal to not phase the sound?  That's the problem I encountered with the wrong valued diode and the diode position open.  Turning the one trim pot does change the sound like a manual phasing.  I hope they have what I need at RatShack.  I guess I'll find out in an hour.

[e178453]

I have repaired a couple of these pedals. They don't have proper protection against reverse DC supply. This can fry the IC's. It can also damage the diodes. On one, someone else had tried to repair it and got the zener mixed up with the signal diode next to it. Was this pedal working when you got it? Let us know what you find, I like these pedals a lot, I even like fixing them.
scott

[mdh]

Yeah, that would be the one, although the associated resistors are different values than the ones in the schematic on Tonepad.com.  You know, if you're really desperate, it's also possible to use a 3 terminal 5V regulator (e.g., 78L05) to provide that reference voltage.  It might take some creative soldering and not be worth the effort, but you would connect the input directly to V+, the output to where the cathode of the zener is, and the ground terminal, of course to ground.  The only problem I see (other than the flying leads) is that I don't see a cap across the supply rails.  I suppose you could make a tiny piece of perf with the regulator, a 10uF or so cap and three leads to connect to the board if you're feeling adventuresome and desperate.  This is all assuming that you have a 5V reg, but it's a pretty common item, so I'm just putting it out there.  An adjustable reg like an LM317 would work as well, but you'd need resistors to set the voltage correctly.

[aab0mb]

Ok, I got the last two 5.1v zeners that RatShack had.  Put one in but no phasing.  I started to inspect the rate potentiometer a little closer and the wafer was cracked!  All I had on hand was a 50k linear pot form RatShack so I tried it out.  It's now phasing just fine but the rate adjustment is all smashed at the high end of the control.  The original pot was board mounted so it took a little working to get that big pot to fit but I got it all buttoned back up and working at the moment.  Does small bear or anywhere else sell the phase 90 style board mount 25k reverse log potentiometers?  Or, can I make the 50k linear function as a reverse log?  I think there's a thread about that...

Thanks again for the support and the ideas guys.  This debug is number 6 in two days.  Fixed a ts9, ts9dx, 2 boss tu2 tuners, Line 6 MM4, the Phase 90, and a morley volume pedal. I got all the broken pedals that a semi-pro musician I know had and said I would try to get them working for free.  I never could have done any of these fixes w/out the info I've gained from this forum.   If only I could get my ROG tube reamer and my AD80 to work. 

Aaron

[mdh]

I haven't seen the inside of one of these things, so I don't know what kind of pot it is.  In any case, shouldn't the rate pot be closer to 500k?  That could make a big difference, no?  Looking at the schematic you linked, R36 is the rate pot, and is labeled 470k.  The 25k pot at R27 should be the bias trimmer (and incidentally, in the Tonepad.com version that I referred to before, the trimmer is a 250k).  For the rate pot, a 500k linear should do the trick, although 500k reverse log might be a bit better.  Of course, if it's really a 25k reverse log, then maybe some other details in the LFO are different.

If you could post a picture of the old pot (or even describe it in words, e.g., round vs. square, how big it is, etc.), we might be able to identify a replacement.

[aab0mb]

Aaaahhhhhhh. Color me red.  In my haste I didn't think about the trim pot on the scheme and just ASSumed.  Sorry about that and good eye.  I think there is a 500k linear around here somewhere although I'm not looking forward to rewiring a giant pot back into that little box...again.  Lesson learned.

[e178453]

I bought a switch for one of these direct from Dunlop, maybe they stock pots as well.
scott