PEZ45 not *quite* working

[bluebunny]

I've just built and boxed my PEZ45 (Tonepad) and it's not quite working 100%...

I've tweaked the trimmer so that I hear the phasing effect, but it only happens at the far end (i.e. fast wobble) of the speed pot.  I've used the 500KC (reverse log) pot as prescribed.  And since I'm getting effect ("sometimes"), I'm pretty sure the JFETs and op amps are installed correctly and doing their things.

I'm using a pair of 2N5952, matched according to RG's advice (the only exact match from a bunch of 20!).  The only slight deviation from the Tonepad circuit is the substitution of more Europe-friendly values in a couple of places: the 0.05uF caps become 47nF, and the 250K trimmer is now 220K.  Other than that, it's quite "stock".  And as I say, working - albeit for only a very small bit of pot travel.

Anyone come across anything like this, or able to offer any clues to this strange behaviour?

[deadastronaut]

hmmm hi bunny,

ive built a few p45 tonepads..

for the 7.5k i used an 8.2k...(i didn't have a 7.5k)  but ive done that on all my p45' builds..

as for the pot i used a 470k lin...

trim i used a 100k...worked fine...found the sweet spot..

i also used 2x j201's..(try those out of curiosity)..i can't remember if i had to put em in backwards though..not sure!..but the j20'1s weren't as fussy...iirc.

but it all worked fine...

[Govmnt_Lacky]

Is the Speed pot wired correctly?

[bluebunny]

@lacky: Yep!  Three pads on the board to three pins on the pot, via a bit of ribbon.  Would be hard to short anything!  (More a reflection of FP's boards than my skills.)

@rob: Hmmm... sounds like quite a bit of latitude for parts without upsetting things...  (And spotted some of your earlier positive build reports.)

Thanks both for v. quick replies.  In the meantime I shall continue to prod an' poke an' inspect some more...  May have to resort to firing up the ol' DMM...

[R.G.]

 May have to resort to firing up the ol' DMM...

... which takes less time than typing requests here. 

[bluebunny]

Just looking a bit more at the relevant bit of the circuit...



...am I right in saying that the "fast" bit that I am hearing corresponds to the circuit where the 500K pot is largely (completely?) out of circuit?  Could it be I have a duff pot?

P.S. Just saw RG's comment come in while I'm typing.  I'm digging it out now, sir!  

[bluebunny]

Some preliminary results from those JFETs.  This is when speed is "fast" and the phasing is heard to "wobble":

D = 3.97V
S = 3.96V
G = 0.85 - 0.86V ("wobbling")

At the slow end it's 3.96V, 3.89V and 0.62V respectively.  I notice that as I back from off the fast end, into "no effect" territory, it's like the LFO is giving me a very slow down ramp - the gate voltages fall off very slowly...  (I guess if I waited long enough they might ramp up again?)

[bluebunny]

I've even dug out a little scope too!  Can clearly see a nice /\/\/\/\ LFO waveform on the end of the pot before it heads off to those JFETs.  And as soon as I back off the speed, it flatlines.  Actually, it looks like it's that very sloooow downward ramp.  Back to the DMM, this is settling at about 1.91V (compared to oscillating between 4.59 and 4.71V when it wobbles).

[R.G.]

Check your pot to see if it goes open at some settings.

[Govmnt_Lacky]

It really sounds more and more to me like you have either the wrong taper pot OR you have the correct taper but it's wired incorrect/reversed.

[bluebunny]

Check your pot to see if it goes open at some settings.

I've just been down to the lab (the kitchen table) and swapped out the pot with another that I've just tested and know to be OK.  The original one was indeed at fault.  Seems there is a discontinuity between the track and the "fast" pin (#3?).  When the wiper was sitting on the "landing area" (off the carbon track), I got the fast LFO.  Everywhere else, it was an open circuit.

Thanks to all the prods in the right direction: DA, for encouraging words with his builds; GL, for saying the word "pot" (not so much "pot wrong" as "wrong pot"!); and RG for making me delve a little deeper!  And the "Secret Life..." in the back of my mind helped me understand what was at fault and how the discontinuity could manifest itself.

Anyway, phaser now doing its proper job and at all speeds!  Thanks all.   

[deadastronaut]

great!...

[bluebunny]

Oh yes!  Just spent the last hour or so putting it through its paces...  

Snaps to appear in pictures thread shortly...

[bluebunny]

Epilogue.
I was just salvaging the plastic cover on the back of the errant pot, and that tricky pin and its little bit of phenolic just fell off!  I guess it must have endured some kind of mechanical stress during its short, troubled life.  Kinda explains the apparent break in the track...