Morley Optical Volume Pedal Repair

[Unlikekurt]

Hi all.
I have a Morley PVO (http://morleypedals.com/pvoes.pdf)
I've had it about ten years and recently it stopped working.  There's nothing to the thing really.  I think the photocells must be acting up. 
Contacted morley to get information on the cells (the schematic didn't give me enough info to source a part) and they wanted ten bucks for two photocells.
Can't really justify that.  Can anyone offer some suggestion?
Also I was wondering what i could do to change the MIN volume setting.  I'd like to have a wider range. The pot is currently a B100K.
I'm also thinking of attaching a 1/4" piece of rubber to the chassis to raise the heel down position.  Where it is currently there is a good deal of travel from where the pedal reaches "silence" (even with the min vol setting all the way up) to where heel down is.  it is really not helpful when trying to set from full on to MIN with the rock of a foot.

[lonewolf]

does the bulb light up? thats the only problem I had with my morley pedal which I have had since 1979

[Unlikekurt]

they're LED's (it's a pcb from 1994) and they sure do lonewolf

[Mark Hammer]

IF its optical then you should be able to see the circuit in action.  Or are the leds sealed

[Unlikekurt]

Mark:
Yes, I am able to see it functioning.  And somehow it has begun to work again.  This is why I'd like to replace the photocells.  When the pedal was not functioning properly (ie: no signal passed through when set toe down) the LED's were still all lighting and their positions to the shutter windows were adequate to allow light through.  But no sound passed.  Now it is working once again and everything is the same.  I have done nothing more than analysis in the box.  This leads me to suspect something is awry with one or both photocells and I would like to replace them entirely.  The schematic only states "400-850 L ON" and p/n M79-211564-000.  A search for the part number turns up nothing and I do not understand what the quoted specification means in order to search one of the usual vendors for an appropriate replacement photocell.
I have however gained a better understanding of how the pedal works with relation to the shutter and can see how I could make a new shutter, adjust the position of the components or do both in order to a achieve a taper that i prefer more. 
Any help with determining replacement photocell specifications would be fantastic and appreciated effusively.

Thank you
James

[Mark Hammer]

You're fortunate that this is a volume pedal, since that makes the requirements for the photocell substantially less stringent, compared to a compressor/limiter, or envelope-controlled filter, where fast rise and decay may be critical.

Lift one leg of the photocell, and measure its off resistance in the dark.

The schematic shows that the pedal is basically a passive virtual pot formed by two LDRs.  As with any pot, the amount of attenuation is a function of the resistance from input to wiper, relative to the entire resistance.  So, when L2 gets brighter, and L3 gets dimmer, the input-to-wiper leg goes low resistance, and the wiper-to-ground leg goes high resistance.

As for the min resistance pot, it adjusts the current-limiting resistance to the LED/s in question.  Increasing the series resistance will dim them and decreasing it will make them a little brighter.  The foot treadle simply adjusts how much of that illumination lands on the LDRs, or is blocked by the shutter.

Rather than monkeying with the photocells, a better option might be to get a brighter (measured in mcd) LED for either L2 or L3 (or both) and play with series resistance to trim the brighteness back.  Alternatively, consider replacing P1 with two pots that provide independent control of minimum illumination for each LED, rather than the single reciprocal control you now have.

Incidentally, your query got me curious about the "Black Gold" series wah-volume I picked up a week and ahlf ago.  So, I pulled the bottom off and watched the LEDs.  The last 30-40% of the treadle movement gets me nice quack, but the first 60-70% is pretty useless. I think I may play around with the current limiting resistance or an LED swap  a bit myself to get a more pleasing sweep....once I replaced that godawful 741 with something that doesntT give me the constant sound of the ocean.

[lonewolf]

before you start replacing parts you might want to check for cold solder joints/loose connections,etc

[Unlikekurt]

thanks for your feedback mark.
I'll measure the off resistance of the LDR this evening.  And I'll probably fiddle around with the current limiting resistors as well come later in the week.
I did however look into the shutter some more.  When the heel reaches that seemingly off position 1/4" from being full heel down, the shutter allows no light from L1 to LDR1.  Therefore it may be prudent for me to adjust the positions of L1 and LDR1 to best utilize the shutter in regard to the position of the pedal.
It may be worth mentioning that L3 on the schmeatic doesn't really seem to contribute much at all to the circuit except to gobble up current when the MIN volume pot is set so as to shunt less signal to ground.  L1 faces LDR1 with the variable shutter in between.  L3 faces the rear of LDR1 and I believe provides minimal to no light to the face of the photocell.  I will try blacking it out entirely in order to see if my suspicion is correct.  If so, am i correct in assuming it's sole purpose is to utilize the current which L2 is not?  If this is correct, a) why do this instead of just shunting the voltage to ground of this leg of the potentiometer?  b) why not have the LED off in some banished corner of the enclosure?
Good luck with that wah!  Let us know what happens.  The only time i like the sound of the ocean is from all small stone through a cranked amp with no notes being played.

[Mark Hammer]

 b) why not have the LED off in some banished corner of the enclosure?

Because it may be the LDR used requires the LED to provide at least some illumination.  Remember that the pedal is not absolutely light tight, and the shutter does not occlude any and all light.  I've done experiments where I wrap and LED and LDR in electrical tape and black heat-shrink, and still find the LDR resistance changes depending on whetherI'm holding it under my desk or on my lap cupping my hands around it.

So it may be that their strategy is to use an LDR with far greater resistance than they really need (e.g., to mimic a 500k or 1M pot) and tune the LDR up to a desired resistance with illumination.

My wah-volume has a stompswitch that selects between volume and wah functions, but nothing that bypasses the pedal entirely.  I have to fix that.

[merlinb]

The volume pedal is very tolerant of the LDRs you use. You can get a bag of cheap 'uns on eBay that will work fine. You want the dark resistance to be a couple of hundred k-ohms or more, and the light resistance as low as possible, ideally less than 1k. It should be easy peasy to find LDRs in that category.

If you look at the schematic:
http://www.morleypedals.com/pvoes.pdf
For better offness you need LDR1 to have higher dark resistance, and/or LDR2 to have lower light resistance, than what you have now.

[romans10:9]

How does the optical sensor work to select the volume intensity? Is it in proximity with the under side of the pedal? Thanks for you time y'all. God bless.-pc

For God so loved the world that he gave His one and only Son, that whoever believes in Him shall not perish but have eternal life. John 3 :16

[deadastronaut]

morley vs 2: chapter 46:

and so it was that the morley came forth with a little bit of plastic card with a slit in it

and that card obscured the heavenly red led light entering the kingdom

of the photocell, thus changing the resistance and brought peace and joy  upon the amp...

[duck_arse]

Luton 3:1 Chester - let there be light!

[R.G.]

And somehow it has begun to work again.  This is why I'd like to replace the photocells.  When the pedal was not functioning properly (ie: no signal passed through when set toe down) the LED's were still all lighting and their positions to the shutter windows were adequate to allow light through.  But no sound passed.  Now it is working once again and everything is the same.  I have done nothing more than analysis in the box.  This leads me to suspect something is awry with one or both photocells and I would like to replace them entirely. 

As Lonewolf says, you're not thinking this through.

It seems that you have decided that the photocells are the problem, and are adjusting each bit of information to support that.

There is only one situation I can think of where a photocell is the major problem causing this; that's if there is a mechanical crack or other flaw making the photocell itself be intermittent. Could happen, but it's going to be very rare. I did - once! - find a resistor with a cracked lead, and - also once! - a capacitor with a break in the body, but that's over several decades of working with electronic parts.

It is vastly more likely to be a cracked solder joint, wire, bum jack or connector, something like that.

[merlinb]

As Lonewolf says, you're not thinking this through.

[duck_arse]

As Lonewolf says, you're not thinking this through.

..... but what else have the romans done for us?