Morley PWV burnt LDR ?

[isophase]

Hi all,

I'm trying to fix my Morley Wah/Volume model PWV and need some help.
This is the very first pedal i owned, bought it in 1993. It was stored for many years without use.

I opened it and made a visual inspection and clean it, all looking ok. But when i put a fresh 9V battery and test it there was no sound or only very little signal was passing on the wah setting.

I took some mesurements of the original TL071 and replace it with a new one. I also installed an IC socket and replaced all three electrolytic capacitors C3/C7/C8.

Power it up again and took mesurements with the freshly installed TL071. Inverting input and output of the TL071 now reading 5VDC, the original TL071 was giving 1.3VDC. i guess the original TL071 was faulty.

I plug in my guitar for testing, and so now there is some "clean" sound on the wah setting but no wah effect (i covered the LDRs with a piece of black tape while testing)
When i switch to volume mode, there is almost no sound and no volume change when i rock the pedal. Intensity does change a little when/if i remove the piece of black tape and let some light in.

When i short the LDR1, signal passes through.

So now I am suspecting the LDRs... i tried testing them "onboard", i get infinit resistance on my ohmmeter. Are the LDRs fried?
I trying to find some equivalent LDRs online but i can't find the same value.

Original LDRs are 400-850-L-ON and 150-250-L-ON.
Almost all LDRs i find are in the Kilo-ohm range, i could only find one LDR sold by Banzai music as a "morley replacement" but even this one is not in the correct range > 200-400 ohm.

Do you think i can replace the LDRs with the "Morley replacement" sold by Banzai music? it will work ok?
Thank you for your help.
Jonathan 

Schematic attached for reference.

[11-90-an]

if I may ask... do the LEDs light up? if they do, then it's most likely the LDR.

If the LED is the one that's broken, replace the LEDs...

[isophase]

Wow fast response! 

Yes the LEDs are lighting up correctly.

On the schematic one pair of LEDs is "Red diffused" (L1,L2) and the other pair seem standard RED LEDs.
You can see on the photo that LEDs L1 and L2 on the left side are more pink than red. but mabe this is normal since they are supposed to be "diffused" type? I first thought the pink color was a bit suspect when i first had a look until i read about the diffused type so mabe the pink color is normal.

So do you think the Morley replacement LDRs sold by Banzai music will work correclty even if they are not in the same range as the original? 

Thank you Nathan!

 

[11-90-an]

If it would be possible, the only way to make sure the LDRs are fried it when we really see it at infinite resistance consistently (while rocking the pedal, for example)

Especially with this..

Intensity does change a little when/if i remove the piece of black tape and let some light in.

That means that at least 1 LDR works... i guess..

Could you try connecting your multimeter to each of the LDRs and try rocking the pedal? some alligator clips would help...

When you originally got it.. was it working?

[isophase]

The pedal was working perfectly when i bought it new in 1993, but it was stored and not used for almost 2 decades

I did a new test feeding 1V RMS@440hz into the pedal.

Volume mode : almost no signal passing, no change in intensity when i rock the pedal. When i move the black tape on top of the LDRs there is a slight change in intensity but signal is always present (and very low) and does not disapear.
When i place aligator clips of my ohm meter across the legs of LDR1, it shorts the signal and sound now passes through the pedal at unity. I can read 3Mohm across LDR1 on my ohm meter but the wierd thing is i can only read when the range on the multimeter is at maximum (200Mohm range) if i switch to the 20Mohm range i read infinity, strange. Reading doesn't change when i rock the pedal.

Wah mode : the signal passes at unity but nothing happens when i rock the pedal. i can measure 50Mohm across LDR3 when the pedal is at maximum and about 160Mohm when the pedal is at minimum. but the reading keeps moving a lot on the multimeter.

[Mark Hammer]

This may have nothing to do with the pedal's current state but I am perplexed by the purpose of LED3.  What is served by having an LED pointed at the back of an LDR?  That's a headscratcher.

More relevant, I'm wondering if the issue may be the footswitch itself.  The schematic shows that the footswitch enables/disables the LEDs, and engages L1 and L2 at once.  If these remain "on" when you switch to wah, then LDR1/LDR2 would provide an always-on low-resistance path to ground, attenuating the input signal to the wah.  A simple test would be to lift one end of R27, such that L1/L2 donot light up.  You can also see if, with R27 left intact, they stay lit all the time, whether you're in wah or volume mode.

[11-90-an]

This may have nothing to do with the pedal's current state but I am perplexed by the purpose of LED3.  What is served by having an LED pointed at the back of an LDR?  That's a headscratcher.

Mark, please correct me if I'm wrong, but from the schem it looks like a double sided LDR... if that even exists

[duck_arse]

more odder - the backface led operates on the volume circuit ldr, but switches on in wah mode.

[Mark Hammer]

You're correct in that the schematic shows an LED on both sides of the LDR, but look at the board and you can see that the leads of that photocell stick out the back, just like any normal unit.

Note that while photocells are most sensitive to light on their front, they are sensitive to light, albeit less so, from all sides.  The world is full of lots of things that can seem opaque, but still let light through.  Next time you measure an LDR's resistance, try measuring it covering the LDR with your finger, and then measure it with your finger on it, but under a table.  Under the table, the "off" resistance will be much larger, because less of the ambient light is coming through your finger.  Put the photocell "face" down on a table and hold it in place with a piece of masking tape.  Now measure the resistance.  It will be low-ish, because light permeates both the tape and the seemingly-opaque backside of the LDR.  This is why vactrols are larger then you'd think they need to be. It takes a lot of black epoxy to keep the light out and allow them to reach their highest possible off-resistance.

The real mystery for me is why Morley felt the need to light up the LDR from both sides.  Was this in an effort to push the LDR to its lowest possible "on" resistance? 

Oh wait, I think I get it now....or maybe not.  In volume mode, L3 is turned off, leaving only L1 to shine on LDR1, and LED2 to shine on LDR2, presumably in reciprocal fashion (i.e., the shutter blocks either LED1 or LED2, but not both at once), which would be required to produce volume changes.  In wah mode, LED1 and LED2 are turned off, which would make LDR1 and LDR2 go high, and function like a large-value input terminating resistor.  Is their combined dark resistance too high to provide a desirable tone from the wah?  Maybe.  In which case, lighting up LDR1 dimly from behind might result in a suitable LDR1+LDR2 resistance to ground.

IN any event, that simply dances around the logic of their use of front and back illumination, and doesn't really tackle why the pedal isn't working properly.

BUT, having said that, is the shutter itself aligned properly?  I know it is misleading to judge from the single photo provided, but it looks like the shutter might be rubbing against, or even maybe blocked by, the PCB on the "lower" edge, where L2 and LDR2 are situated.  I don't expect anything to be burnt out during 20 years of storage, but stuff CAN get jostled and repositioned, and as delightfully crackle-free as Morley's system is, the positioning of the shutter, light source, and photocells IS critical to their proper functioning.

[PRR]

I agree the back-side LED_LDR is very strange; but it is far too late to second-guess Morley on this. It worked that way in 1993 and unless the LDRs have been cracked and soaked in water I doubt they could be "burnt".

Measuring LDRs for basic function is trivial but you MUST control the light on them. With pedal power OFF and normal workbench lighting (don't strain your eyes) a plain ohm-meter across the LDR will read 500-5,000 Ohms. If you have a small bright flashlight, beaming the LDR should drop the Ohms even lower.

Now rig the bench for BLACK-out. Wait for midnight, kill the lights, the PC, and any nightlights. You probably need one SMALL light to set your probes and read your meter, but cover it as much as possible. Repeat the measurements. If the Ohms changes a lot then the LDRs aint dead.

The measured Ohms will be in parallel with the rest of the circuit. For LDR3 there is nothing else. LDR1 is shunted by the input jack so stick an open plug in it to break the switching.

[anotherjim]

Not countering anything said, but it does like the original opamp got fried so maybe something catastophic has happened in its life? While musicians still use 1/4" jacks for speaker connections, could it have been plugged into the wrong place?

I wonder what the shutter is made of? Some plastics breakdown over time and it may not be as opaque as it was.
But as PRR just said, you have rule out stray light falling on the LDR.

[willienillie]

It was stored for many years without use.

I would bet it was the electros, not a bad opamp, causing the low voltage readings.  LDRs can die for no apparent reason, but it would be strange if they all did.

[isophase]

I desolered all 3 LDRs and tested them:

LDR1_light=7Mohm_dark=infinity
LDR2_light=3.2Mohm_dark=infinity
LDR3_light=8Mohm_dark=infinity

All 3 LDRs vary with light but only in the Mega ohm range. When i put the face of the LDR against the table i read nearly 200 Mega ohms (max reading of my multimeter) when i cover the back of the LDR with a large black tape i read infinity on all three LDRs.

I guess i'm good to order some new ones... I will order the replacement part from Banzai.
Lets see if the new LDRs will fix the problem.
Many thanks to all for your help!

The LED on the back of LDR1 is strange indeed, the pink color LEDs L1 and L2 also caught my eye on first inspection, but all LEDs do light up, and the pink "red diffused" ones are indeed glowing a "red" light.

The LEDs, shutter holes and LRDs are all aligned correctly. The shutter seem to be made of very dense carton/paper material.   

I will report back with the new LDRs installed
Thank you!

[Govmnt_Lacky]

I desolered all 3 LDRs and tested them:

LDR1_light=7Mohm_dark=infinity
LDR2_light=3.2Mohm_dark=infinity
LDR3_light=8Mohm_dark=infinity

If you are reading in the MegaOhms when any kind of light is being placed on the LDR then something is wrong with them. Even putting a small amount of light onto the LDR should result in them going down to 1 MegaOhm or below.

Replace all of the LDRs. It will cost you about $3 at most.

[isophase]

Hello,
I finally replaced the LDRs, problem fixed!
Thank you for your help!

[garcho]

So what happened to the original LDRs? I have the same Morley (mine is *only* 20 years old) but I ripped the guts out and used the shell for my own project. Like everything they've ever made, built like a tank. I remember thinking it sounded pretty damn crappy but it definitely worked, all the way up to a couple years ago. Where did you store it for those decades? In a non-climate controlled warehouse in Miami? In the shed behind your home in the Yukon? At the bottom of your pool? 

[danfrank]

Every LDR I've seen that's gone bad is because the two wires going to the resistive element stop making good contact with said resistive element. This could be due to weather, corrosion, hairline cracks in the epoxy encapsulation, but it's usually right where the wire meets the resistive element.

Funny because I've always thought the Morley Wahs sounded best out of all the wah pedals. Maybe it's because the first time I heard a wah live was an old Tel-Ray Morley...

[garcho]

Every LDR I've seen that's gone bad is because the two wires going to the resistive element stop making good contact with said resistive element. This could be due to weather, corrosion, hairline cracks in the epoxy encapsulation, but it's usually right where the wire meets the resistive element.

That makes a lot of sense to me, considering in my experience with audio gear, devices usually become faulty because of either age (electrolyte dries up, plastic or metal degradation, etc) or because of human user interfacing, in this case, the leads. I have a glitchy Mutron phaser on the bench (behind a couple other projects), I wonder how the LDRs are. I haven't had any problems with LDRs in my own builds, none of which are from the 1970s

I've always thought the Morley Wahs sounded best out of all the wah pedals

At the time I played out of a Hot Rod Deluxe with a Strat, mostly "dirty clean". Maybe I didn't have my chain dialed in right. Or maybe it just sounded bad to my ears. Kind of whistle-y, like a tea kettle, and harsh, like there was a noisy 1kHz wash over everything. Anyway, I have a few "non-working" Morleys I've bought off eBay for 20 bucks, just for the shell. As opposed to cry babys, there's plenty of room for big PCBs and extra knobs!

[isophase]

So what happened to the original LDRs? I have the same Morley (mine is *only* 20 years old) but I ripped the guts out and used the shell for my own project. Like everything they've ever made, built like a tank. I remember thinking it sounded pretty damn crappy but it definitely worked, all the way up to a couple years ago. Where did you store it for those decades? In a non-climate controlled warehouse in Miami? In the shed behind your home in the Yukon? At the bottom of your pool? 

The pedal was stored in a carton box with other pedals for many years without use. That box was moved quite a bit and was stored in a cellar for a couple of years at some point, so i guess humidity from the cellar helped with degradation..
The original LDR's had a very blackened surface compared to the new ones, so i guess they got oxydized ?

Thanks again to everyone for your help! very happy now with my recuscitated pedal! i'm using it mainly as an parallel feed on my pedalboard, its really perfect and absolutely noiseless.
i will have to post some photos of my new pedalboard with the new power supply, sounding really good! i am very happy and satisfied about it. 
Cheers
   

[anotherjim]

Very Cool you got is back to glory.
Humidity is a devil. We love gear in metal casings, but it's far worse in this respect than plastic. Even gear with plastic upper but a metal bottom plate can be damaged unless you store it the right way up.