Help with repair - Jen Phase Shifter

[redbagy]

Hi! I'm attempting to repair this Jen Phase Shifter for a friend of mine which basically passes audio both when engaged and not but there is no pronounced phaser effect.

It's the same model like the one in this video. Here is a gut shot.


Since I could not find any schematic to properly probe any DC voltages and audio signals, my first instinct was to replace the ICs however I could not properly identify them. I believe that the 2 same ones are a 4136 op-amp but not sure about the other one.

Any help is appreciated! Cheers

[GGBB]

The third is an AM97C09 current switch.

[redbagy]

Do you know where I can buy one online or a suitable replacement please?

[GGBB]

Sorry no. I don't think they are made anymore. Are you sure the one you have is bad?

[redbagy]

Truth be told is I'm not sure how I can test it. The circuit is made up of passives and these 3 ICs only so my first gut feeling was that an IC went bad...

[GGBB]

But you have sound so something works.

If it were me I might:

Try adjusting the trimpot.

Measure voltages (especially on power pins of ICs) and look for oddities.

Pull the ICs and test independently (breadboard).

Trace the circuit and draw a schematic.

Pull/test/replace the electrolytics.

...

[redbagy]

Thanks for the tips and I agree I'll probably have to go down a more logical route now.

[StephenGiles]

I would change those jack sockets for a start!

[Mark Hammer]

The only other phase shifter I'm familiar with that uses the 97xx chip is the orange Ross phaser.  Who knows, the Jen might even be a clone of the Ross (or vice versa as was common with Ross pedals).  The Ross uses a 97C11, rather than 97C09.  The 09 is 14 pins while the 11 is 16 pins, but I imagine the internals are similar in most respects.  Either way, as FETs, they require biasing in order to be modulated by the LFO, so that's why the trimpot is there.  Consider gently tweaking that trimmer to see if you can get some phasing back.

[jorg777]

The third is an AM97C09 current switch.

Holy crap.  I never heard of that one before.  Tragic that it's discontinued!  I wonder if Alfa RPAR or CoolAudio would consider cloning it?  That chip would be incredibly useful for DIYers.

[Mark Hammer]

A quartet of matched JFETs?  You betcha.