phase 90 mod fail :o(

[overpoweredbyfunk]

so, this weekend i attempted my first pedal mod
i took a perfectly good mxr phase 90 (block reissue circa 2006) and did the R28 removal mod i read all about--sounded simple enough.

i was worried there'd be a chance i wouldn't like the mod, so i only unsoldered one side of the resistor and put the other end through a SPST and back to the board, that way i could flip a switch and get the old tone back if i hated the mod.

oh, and i put a blue LED in, cause theyre prettier than the stock red.

i do the mod, plug it in and it sounds like a police siren. but hey, the LED works.

frustrated, i took the resistor and the SPST out all together and now i get nothing. the thing lights up, and the signal passes through it, but there's no phase.

tweaking the trimpot does nothing
none of the solders seem to be loose or cold
MY desoldering is pretty clean--no gunk crossing traces

any help would be hugely appreciated!

[Robert M. Martinelli]

Hi,
the resistor you removed is a PFB ( positive feedback ) resistor, and removing it is the most common mod for a Phase 90. The symptoms you describe are typical of self-oscillation, but that would occur only it the resistor's value is too LOW.....Is it possible that you somehow wired the switch wrong and created a short circuit in place of that resistor? Sorry for asking such a question, no offense intended, but that would explain the problems you're facing....

I bet at least one ( or more ) op-amp inside your Phase 90 are gone south for a LONG holiday....replace them one by one starting with the oscillator IC which is your prime suspect ( or all of them at once, if you're not interested in discovering which ones failed, as they're pretty cheap ) until the unit starts to work again.

Oh, and throwing in DIP sockets would be a good idea to make future ICs replacement easier.

Hope this helps

Best regards

Bob

[oc disorder]

Hi and welcome to the forum
While you are waiting for some info I see some one else had the same problem...
http://www.thegearpage.net/board/sho...d.php?t=470119

Is yours the smd one ? I gather the resistor R28 numbering is different
on different models.

Is this where you got the "mod" from ?
http://www.erikhansen.net/?page_id=31

Can you restore it to what it was ?

You need to check all the traces near where you were for continuity. Some are plated through holes which may have broken. This is where a trace connects through to the other
side of the board.

Good luck !


Help the economy Pay a Technician to fix it !

[overpoweredbyfunk]

hey everyone--and thanks so much for your replies

last night i went through pretty much the entire board with the multimeter checking for continuity and values hunting and pecking.

as im doing this i bumped a probe into the pot . . . and the pot kinda breaks in half! i guess all the stress from taking it apart and putting it back together a few times was more than it could handle.

nobody really stocks 470k audio taper pots with right angle PCB mounts, so im gonna go for a 500k--hopefully that will work! i'll let you know!

thanks again
todd

[Mark Hammer]

470k = 500k

There's something we talk about at the diystompbox forum that I labelled the B.U.M. syndrome - blind urge to mod. Nothing wrong with modding commercial pedals. I recommend people to do it all the time - quite often cheaper than building from scratch, and I've contentedly modded some vintage pedals.

But:
- you gotta know what you're doing
- you gotta be gentle and methodical
- you gotta have the right schematic and/or parts layout diagram
- you gotta be able to troubleshoot just as much as with from-scratch pedals
- there's gotta be room to do the mod properly

The P90 comes in a small chassis. Is it at all possible that the solder lugs of the toggle switch are shorting thngs on the board when you put the pedal back together, or are you getting the problem even with the back off?

[overpoweredbyfunk]

thanks mark
believe it or not for a newb i was pretty careful about locating the toggle. i threw some paint on the lugs and loose fit everything back together to see if the paint transfered and rubbed anywhere on the board and i was all clear. i located it above the part of the board with the lowest profile.

i even covered the lugs with hot glue after i soldered the wires on to make sure there'd be no shorts. (i recently opened a pedal and saw the hot glue method--not sure if it's good practice, but monkey see, monkey do)

i really just think i wasn't gentle enough and when i tightened everything back up i tightened TOO much, breaking the pot--or tightened the button side tighter than the pot side and the uneven tension fractured it?

as soon as radio shack opens im off to buy a new pot--ill let everyone know how it goes.

and again, i really appreciate everyone takin' the time to give me the input.

[Mark Hammer]

I've been making pedals for over 30 years now, and something I find myself doing more and more this last few years is slipping a small piece of heat shrink over the solder joints when toggle, stompswitch, jack, or pot lugs are involved. It's cheap, easy to do, and it provides both strain-relief and continuous insulation. I've had more than my fair share of cases where I was certain the wire was secure but it fractured, as well as cases where I tightened up a nut on the outside, only to find out that a pot had twisted a bit and a solder lug was shorting out against the chassis or another component. Same thing with toggles.

Hot glue has its limits. It is easy and cheap, and I have used it to secure things like battery leads to the PCB (to provide strain relief). But it is way too easy to accidentally remelt when diving into the box with your iron, and the resulting goop can be a royal P.I.T.A. Consequently, I would suggest using it in places/ways where you can be fairly certain you won't be going back or that won't have anything hot near them.

Many people don't realize that a lot of the component and layout choices MXR made in the 70's were specifically so that the components could have as low a profile as possible and fit behind the large pots in that teeny enclosure. Things like tantalum caps were used not for their sound, but because they were smaller than electrolytics available at the time, and could be easily bent over on their sides to lie flatter.

[EffectsGuru]

Keep in mind that any time you handle electronics without anti-static devices (via an anti-static bracelet hooked, or anti-static lotion) you risk causing static to go through any of the parts and they may not work anymore... it actually happens a lot, more so in the winter in dry climates, so watch out for that static. Your best bet is to make or buy a Signal tester (i believe thats what it's called) and run through the circuit one part at a time... until you find the problem... you can buy a signal tester from BYOC.com (they make some sweet pedal kits too... I sell them on my website. Hopefully that helps you out at least a little bit.