Mu-Tron Phasor II - blows LEDs?

[drasp]

I'm working on a Phasor II for a friend. It was a bit of a basket case when I first started sorting it out, tons of bad solder joints, power switch was inop/sticking, bad/dirty rate pot, dead LED. . .

Touched up ~2/3 of the solder joints on this thing, cleaned up the power switch which seems to have come back just fine, cleaned the rate pot which tests okay on my DMM but the thing only functions intermittantly. If rate is above ~2:00 it sounds great, below that it falls off & the effect stops - not dry, I can turn the depth knob & get the effect, just w/o any rate. May have to resort to a standard panel mounted pot w/ leads as the original pot has four legs to attach to the PCB and I haven't found a direct replacement.

Original LED was DOA. Replaced it & the new one died w/in ~5min of playing. It didn't glow any sort of crazy color or get hot, just stopped lighting up. Tested & found 20v across the LED pads. ???

Any ideas on this? Should I try wiring a resistor into the LED? Suggestions on the weird rate issue? Any help very welcome & greatly appreciated.

[cjlectronics]

What are the power supply voltages? You should have +/- 11vdc according to the schematic here. You might want to check the wiring...especially the ground and the negative supply voltage.

CJ

[drasp]

Thanks! Will check & report back. Anyone got a larger version of that schematic? I've seen that one before & can't make any details out. . .

[drasp]

Wow - sorry for the long wait on an update/more info. Just had a chance to tear the pedal back down & test the output from the transformer.

There are 3 wires on the secondary side of the transformer, two are green, one is yellow-ish white (old, cloth wire). I'm not 100% sure how to probe them, but from the green wires or yellow-ish to the big ground field on the PCB, or ground at the input jack, I only get ~1.85VDC each. However, when I probe between the (+) and (-) sides of the LED's traces, I'm still reading 20VDC. Also found that the LED is still working just fine, but the traces on the PCB that provide it power & ground have lifted from the board (I'd already re-soldered them as I found them bad).

Really not sure where to go from here - hoping for some direction!

[cjlectronics]

Based on your original problem, blowing LED's, and now it is not blowing LED's...what is the problem? Is the LED working or not working? Is the problem just 20vdc across the LED leads and everything else works OK?

[drasp]

Sorry, I guess my first post was pretty rambling. There are a number of issues with this pedal, some I thought I'd solved.

1) Power switch inop - this seems to be solved now after treating with DeOxit.

2) LED inop - I thought I'd solved this by simply soldering in a new LED, but after only a few minutes of using the pedal the new LED also went off and it turns out that the LEDs themselves are fine, the traces on the PCB are actually lifting off the board causing the LED to go out. As noted, when I test for voltage at the LED, my multimeter reads 20VDC - now it seems the LED may only be a symptom and not the problem.

3) Rate pot tests good with a meter, but does not function below 2:00. I can manually sweep the effect by turning the depth or feedback pots, but there is no sweeping effect unless the rate pot is at the top half of its travel.

Since I have zero experience with electro-optical devices, I wasn't sure if there could be some kind of power supply issue causing the rate issue AND the LED issue. Hoping you can point me in the right direction. . .

THANKS!!!

[cjlectronics]

I still don't get a warm and fuzzy feeling that the power supply is being measured correctly. Can you measure the voltage between ground (the chassis should work fine) and pin 8 and 4 of one of the 4558 IC chips. The 4558 is an 8 pin IC chip. Pin 8 is positive and pin 4 will be negative.

CJ

[drasp]

I still don't get a warm and fuzzy feeling that the power supply is being measured correctly. Can you measure the voltage between ground (the chassis should work fine) and pin 8 and 4 of one of the 4558 IC chips. The 4558 is an 8 pin IC chip. Pin 8 is positive and pin 4 will be negative.

CJ

Thanks! I really appreciate the hand-holding. I'll report back w/ what I find.

[R.G.]

Long ago, in a galaxy far, far away, I wrote down what I thought the best drill would be for resurrecting an elderly dead pedal. That's here.

With power problems, especially with an ac powered pedal, I'd replace all the diodes as well as all the electro caps, verify the power supply was putting out the right voltages before doing any more. If the power supply isn't right, it isn't going to work even if everything else is perfect.

[drasp]

I still don't get a warm and fuzzy feeling that the power supply is being measured correctly. Can you measure the voltage between ground (the chassis should work fine) and pin 8 and 4 of one of the 4558 IC chips. The 4558 is an 8 pin IC chip. Pin 8 is positive and pin 4 will be negative.

CJ

Just measured voltage @ pins 4 & 8 on all the 4558s. Wasn't surprised to find that they're all correct w/ pin 4 measuring ~ -10.75VDC & pin 8 measuring ~ +11.00VDC on all the chips. Re-soldered the old LED which still doesn't work & still measures +20VDC. When I hold the LED I'd replaced it with across the traces, it lights & doesn't visually appear overdriven, doesn't get warm, etc. Is it possible that +20VDC is normal for the LED & that the traces simply lifted as a result of my repair? Doesn't explain the death of the original LED, but maybe I'm getting stuck on the LED and missing something else.

Going to keep digging.

[trevorus]

+20 on an led is too high. It's time to check back from there and see where the failure is tracing back to the power supply. Also, you could be getting a lot of ripple in the power supply if the filtering caps have gone.

[52 Bill]

+20 on an led is too high.

How are you getting the +20 volt measurment?

Are you measuring across the LED terminals or are you measuring terminal to ground?

[drasp]

How are you getting the +20 volt measurment?

Are you measuring across the LED terminals or are you measuring terminal to ground?

I was measuring across the LED. . .

[drasp]

I'm ordering all new electrolytic caps from Mouser right now - anything else I should be replacing? I see 8x diodes, think its necessary to replace them, or should I try caps first, then diodes?

Also, I want to order a replacement rate pot, just in case, but I can't read any of the low-res schematics that are floating around (same as the one that used to be linked to earlier in this thread) I've attached the version I have, but its darn near useless to me, even after trying to make it more clear in Photoshop. Anyone know the value of the rate pot + a manufacturer that builds the strange 4-leg pots?