MXR Phase 100 Script Logo issue - help appreciated...

[tubelectron]

Hello Fx Pedal Diyers,

I may not have posted my question at the right place, or this subject may have been already solved - So I apologize for it, and I will be happy to be redirected where to go - I apologize too for my approximate english because I am French - Thanks !

I have a circa 1975 Script Logo MXR Phase 100, all original it seems. Pot reads 1377532. S/N° label : 701524. I would like to... resurect it !

It doesn't phases. I got dry signal of equivalent volume both when the effect is engaged or not. When engaged, the phase selector position lightly affects the tone.

The footswitch works well. I cleaned the speed pot.

When testing the voltage to the led of one optocoupler, I sometimes found an average of 1.45VDC with a sweeping from 1.42 to 1.47VDC following the rate of the LFO - So I think the LFO is not dead... But sometimes I found a steady circa 1.2VDC on the 2 first  photocouplers and a steady circa 6.5 VDC on the third one.

The Q's are 2SC1849.
The phasing IC's are 3x Texas SN72L044 quad op amp.
The 3 optocouplers are 21L312 (3 pins photoresistor side, 2 pins led side).
the PCB wears the following indications :
P 100 6
MXR
107 3001 101

a schematic can be found here : http://www.generalguitargadgets.com/pdf/ggg_p100_sc.pdf

I am akeen to think that :
- one or all of the 21L312 optocouplers are maybe dying or faultly,
- the 2x 2SC1849 LFO output darlington circuit which dirves the optocouplers leds is maybe dying or faultly.

Have you experienced and maybe solved a such problem on a phase 100 ?
Have you simply an advice about that issue ?
Do you know a replacement for the 21L312 or the specs (I found nothing), or where to find original ones ?

Thanks for you help & best regards,

tubelectron.

[ammalato]

So this is exactly the same problem I'm having.  Vintage Phase 100.  Equal dry signal when engaged and not.  The rotary switch does affect the sound slightly (when turning to one particular position one sweep of phase can be heard...from top of the sweep to bottom...and then it's dry again).  Trying to figure out how exactly to trouble shoot.  Test points, etc. 

[BDuguay]

Going way back to resurrect. I have an older quad op amp version that is demonstrating the same symptoms as these 2 posts describe. So, essentially, I'm calling on all Phase 100 gurus to chime in please, I'm not sure where to begin...
Thanks,
B.

[Fender3D]

...I'm not sure where to begin...

In phase 100 the 3 Vactrols are connected in series, hence if one fails, noone works anymore.
Lift one LED pin and see if you can measure some kind of resistance (test it like a diode).

[BDuguay]

I feel very stupid now for not having thought of that . Thanks and I'll report my findings!
B.

[BDuguay]

1 vactrol has a shorted LED and the other 2 give no reading so are probably open. I'm going to attempt to take one apart.....
B.

[charmonder]

Hi, I know this is old thread but I have a vintagey phase 100 with the same problem.

So I'm pretty sure its one of the vactrols has its LED blown as suggested. Should I just replace the faulty vactrol OR was this just a symptom of a real problem on the board that will inevitably blow my other vactrol? (they are relatively expensive bits!)

[Kipper4]

Well before you start removing bits. Test your theory and croc clip an led // with the vactrol led, to see if it lights up. At least you'll know if the vactrols are getting lfo signal.

[charmonder]

thats a really good idea Kipper4! Thanks 

[Kipper4]

Observe the polarity...mind.

[Atodovax]

Sorry to resurect this old thread. How should i meassure the vactrols? i have exactly the same issue on mine!

[Kipper4]

Ohm meter on the ldr.

[Fender3D]

Ohm meter on the ldr.

It's unlikely the ldr fails... usually it's the LED and since they're in series here just one burned LED stops the others from functioning.
Then:
poke the LED's terminals and check like a diode.

[Atodovax]

Thanks for replying. So i can meassure a resistance change in the ldr part of the vacrol. From middle leg to the ones on the sides. But they only slighly change. ( From 100k to 93k) is that ok? I was expecting a fluctuation from 0 to 100k or something wider

Ohm meter on the ldr.

It's unlikely the ldr fails... usually it's the LED and since they're in series here just one burned LED stops the others from functioning.
Then:
poke the LED's terminals and check like a diode.