scripted mxr phase 90 help!

[pontiacpete]

Hi all, , , this pedal is the original 70's one and arrived here with the battery clip destroyed and disconnected. I spent way too long a time finding the points to which to connect it back in. Anyway, I now have it making sound when the switch is engaged but it's not doing the phase. The volume is at about the same, there's a ticking noise and a little distortion. Seems like it's trying to work.
I changed out the 1N919, the 1N5231, and all the IC's - LM741cn. Swapped the 1m pot and pulled the funky looking 250K trim pot. I think there use to be a cover or something on it(take a look below). It's seems to be functioning according to the test with the ohm meter. I tested all resistors(in circuit, though).
Next are the caps, the FETS and the one 2n4126
I haven't seen any schematics of this pedal with the LM741 or the 2N416.

I'm getting voltage on the IC pin 7- the battery, pin 2/5 1.6vdc, pin4 -1mv.

Does anyone have any thoughts on this?
I hate to keep throwing new parts at it.

thanks in advance

[TheEHMan]

Check your tantalum cap (upper right corner of pic 1).

[Mark Hammer]

Do you know that the trimpot has been reset to provide the proper bias? The JFETs need to be biased up to the right point or else you won't get any audible sweep.

[pontiacpete]

No I didn't know . I have tried turning the trimpot to see if I can dial it in, but I don't detect any phase like sound or any thing close to it.
Is there something I'm not understanding here?

[Mark Hammer]

Is there something I'm not understanding here?

Maybe.

The P90 feeds a bias voltage to the gates of all the FETs so that they are set to vary their drain-source resistance appropriately when the LFO voltage is added on top of that bias voltage.

The bias voltage is derived from the trimpot, which, in turn gets it through a zener diode. I believe the zener is situated just to the right of the 470k resistor that is just to the right of the rightmost turquoise capacitor in that first picture. The other diode in the picture (lower right hand corner) is likely just there for power to the overall circuit.

The zener drops the +9v down to 5.1v or thereabouts. Note that it will do this even as the battery weakens and drops from 9v to 8.5v to 8v, etc. Since that 5.1v is stable, that means that wherever you set the trimpot, the divided-down version of the 5.1v will remain valid independent of the state of the battery. That lets you set the trimpot for optimal phasing at the factory, and leave it.

I don't know what the actual optimal voltage at the trimpot wiper needs to be, but you should be able to hear it when you've nailed it.

[pontiacpete]

Are you saying that i should have 5.1 vdc at the top of the zener? because I don't. There's only .5v.
I tried turning the trimpot extra slow to see if there were any signs of phasing and nothing.

I don't have any tantalum caps on hand, but how likely is it that those could have gone on me?

[Mark Hammer]

Ah, well if the trimpot cannot be set for anything higher than 0.5v no wonder you can't get any phasing.

Unless I have misidentified which diode is which and the one we want to look at is the one in the corner.

Postscript: and it seems that is exactly what I have done. Looking at the 3rd photo of the copper side of the PCB, I can see that the diode I thought was the wrong one in fact connects directly to the trimpot, making it the right one.

You should be able to measure 5.1v, or thereabouts, where it contacts the trimpot, and variations below 5.1v at the trimpot wiper.

[pontiacpete]

I can set the trimpot from 0v to .5vdc

Do you know what ;might be causing the low voltages?

The only thing I can think of is that the zener is shot or has been replaced with a different type of diode.

[Mark Hammer]

Not sure how it was that ended up editting your post, but there it is. The zener may be fried or not a zener.

[pontiacpete]

hey that was weird.
I replaced the zener 2 times. thanks for the help, I'll probably let this one go, spent too much time on it and the traces are looking sad. I figure I'd better quit now before it falls apart on me.

[R.G.]

Zener put in backwards could make the bias voltage be 0.5V.

[pontiacpete]

I thought about that but the symbol on the board (>l) kept me from trying it the other way. Since you mentioned it, I'll give that a try, thanks!

[pontiacpete]

weird, I reversed it and it still gets only .5vdc.

The owner told he used to get it working by wedging a plastic wall anchor betw the board and the bottom plate. So, I resoldered the board and I tugged and pulled on the FETs feet.

[Mark Hammer]

That board is uhg-LEEE. I recommend taking an old toothbrush, and a bit of methyl hydrate, and giving the copper side a bit of a scrub (soaking up the excess with a paper towel). It is a rather trivial suggestion, but I find that sometimes it opens up doors for me, by removing the excess flux that might be otherwise obscuring a solder bridge or a crack in a trace.

Like I say, a rather trivial thing, but it's nice to be able to say "Nope, it's not that".

[cjlectronics]

You might try replacing the red 10uf cap. If the cap is shorted (which I've seen many tant caps short rather than open) this would pull the zener voltage to gnd. Pull the red cap and use your DMM to measure across it. If you get a dead short the cap is the problem.

CJ