phase 90 trimpot

[mung]

I've got a phase 90 that I haven't been able to get working for anything.  I tried the trimpot while fiddling around and found that it doesn't have a stop or start point.  It just goes around continually.  Would this indicate it's broken or do some trim pots just go around endlessly?  What I get is sound when bypassed, when on the LED lights but I get no sound at all.  If I poke around on the back of the board it'll make some noise but nothing from the guitar.

Thanks!

[Govmnt_Lacky]

First of all.... you either have a Multi-turn trimpot or a broken single turn.

More than likely, you have a multi-turn. Is this a stock pedal you are repairing or a DIY pedal?

As for the "no audio when poking around," Are you using an audio probe? Where are you poking at?

Could you link the layout/schematic you used if you built this pedal?

Also, looking at this link is always useful: http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[R.G.]

I've got a phase 90 that I haven't been able to get working for anything.  I tried the trimpot while fiddling around and found that it doesn't have a stop or start point.  It just goes around continually.  Would this indicate it's broken or do some trim pots just go around endlessly? 

Simplify your life. Take out the trimpot, and replace it with two equal-value fixed resistors, one from each end, both meeting at the wiper. It is very unusual for this circuit to really need a trimpot.

Concentrate on the other circuit problems. Read "Debugging : what to do when it doesn't work"

[Govmnt_Lacky]

It is very unusual for this circuit to really need a trimpot.

RG... I don't understand. The trimpot is used to dial in the sweet spot for the phasing effect. What do you mean by the above quote?

At the very least, mung will have to find that resistance value to make his Phase 90 start phasing.... THEN he could replace the trimpot with a fixed resistance... right??

[R.G.]

Sorry. I read that in way too much of a hurry. 

Yes, the trimpot is necessary on the P90. To all - please ignore my silly remark.

Shoot, that's mistake number 537,286, and that's just today!! 

[mung]

Thanks for the replies everyone.  It's a stock phase 90, not my build... I've read through the debugging thread but thought I'd make sure it's not the trimpot (since I'd never encountered one that goes around endlessly) before I went through all the measurements, etc.  I think I've got my answer, though... if I understand correctly it's possible that it could have a "multi-turn" trimpot?  Thanks again!

[Govmnt_Lacky]

It COULD be a multi-turn trim pot. The thing to remember is that even multi-turn pots have limits!

If you can turn the pot more than 15-20 rotations and it will still turn then you DEFINITELY have a broken pot.

Once you are passing a signal in the NON BYPASSED mode, you need to sloooooooooowly turn the trim pot and listen for the sweet phase spot.

If you never get any phasing to occur and you find that your trim pot is good, you may want to post some voltages of the FETS and maybe even some pics for further examination.


Good Luck! 

[elshiftos]

Multi-turn pots usually have a rachet mechanism at the end stop. The adjuster will continue to turn and you should hear (or feel) a feint click on each rotation.
Stick a voltmeter between wiper (one side of 1M resistor) and ground - you should see the voltage change as you tweak!

As for no audio, well I would start  by checking the voltage accross the zener diode.

Today I was debugging my latest P90 board. It turns out I had put the fets in back to front. All these years of building and repairing stuff and fundamental errors continue to be made! 

[soupbone]

This is kind of a crude way of doing it,but..put on Van Halen 1,and fast forward to the end of "Eruption".Set the speed knob to 9 O' Clock,and turn the trimpot until it matches the speed of the recording.Just a thought :icon_smile:I've actually done it myself,and it actually worked!

[bob barcus]

that is not a trimpot it does not change the phase I thought of doing it and someone at metroamp.com forum/ EVH .told me not to touch it

[Govmnt_Lacky]

that is not a trimpot it does not change the phase I thought of doing it and someone at metroamp.com forum/ EVH .told me not to touch it

On a WORKING unit I would agree with you 100% however, .... since the OP said that the unit DID NOT WORK I would have to disagree.

That pot is used to adjust voltages on the FETs and "tune" them so that proper gating is occurring. When the "sweet spot" is found, then you should LEAVE IT AS IS.

If the trim pot is spinning endlessly (as in more than 20-30 turns one way....) then that trimpot is broken or past its stops. It needs to be replaced with a 250K trimpot.

Good Luck 

[mung]

Thanks again to everyone... I haven't had much chance to mess with the phase 90 and have become distracted by a dynacomp board I got recently.  I'll post my results once I get back to the phaser.