70's Power Wah Repair

[beneharris]

​Well I've been snowed in again for 2 days now. 140 something inches this year. I've had this Power Wah for several years now, I just never got around to getting it fixed.

It would turn on, but it was farting out like it was starving the transistors for voltage. I could get a small response changing the light to the LDR, but those transistors were just being starved. Most notably the photocell that it uses as it's variable for the wah had pieces missing off of it (pictured).

I pulled data sheets for the transistors up, and they just weren't getting the voltage they needed to turn on and off properly, so I ended up having to pull out every resistor and check it for tolerance, and I ended up replacing 2 of them - a 220k, and a 4.7m. I replaced some capacitors, though I don't think those were needed (I do need to trim and finalize placement of the last one pictured). To finalize the fix, I need to order the correct sized resistors, it looks stupid having some 1/4w in there. I have no idea what I am going to do about the shade that goes over the LDR as it was missing when I got the pedal. It doesn't affect the sound, though it does affect the range of the sweep.

But crap, this thing sounds amazing. If it weren't so huge, and needs it's own 120v power, I would be putting this thing on my board for sure.

[peccary]

Thanks for sharing your experience with this, and it's great to see these gut-shots, too.

A cousin of mine is sending me one of these (one of these days...) that he says works intermittently and I'm eager to get in there and see what's what and get it going again. I'm guessing it's something to do with the curtain/box around the bulb. Do you know the voltage coming out of the transformer?

[beneharris]

Thanks for sharing your experience with this, and it's great to see these gut-shots, too.

A cousin of mine is sending me one of these (one of these days...) that he says works intermittently and I'm eager to get in there and see what's what and get it going again. I'm guessing it's something to do with the curtain/box around the bulb. Do you know the voltage coming out of the transformer?

Actually, I have no real problem other than the sweep range without the shield, but the curtain is absolutely needed because it is what blocks the light. (not meaning that to be patronizing).

I have a good copy of the schematic, I would be happy to send it to you, it has the important voltages in it. Off the top of my head its 55v and 25v coming off the power section.

[peccary]

Thanks for sharing your experience with this, and it's great to see these gut-shots, too.

A cousin of mine is sending me one of these (one of these days...) that he says works intermittently and I'm eager to get in there and see what's what and get it going again. I'm guessing it's something to do with the curtain/box around the bulb. Do you know the voltage coming out of the transformer?

Actually, I have no real problem other than the sweep range without the shield, but the curtain is absolutely needed because it is what blocks the light. (not meaning that to be patronizing).

I have a good copy of the schematic, I would be happy to send it to you, it has the important voltages in it. Off the top of my head its 55v and 25v coming off the power section.

Ah, no worries, man. I wasn't attempting to give you advice, I was just speculating as to what might be wrong with the pedal I'm supposed to be getting (not yours). It seems like you were able to get yours fully functional, well done.

I'll take you up on the schematic for sure, I'll send you a PM in a few minutes.

[lars]

I've never understood why back in the day Morley chose to solder their components on the wrong side of the board. It's like they were trying to do surface mount with through hole.

[WonkoTheSane]

I fix these all the time, the caps are 50 years old and die, check em with ESR meter. The issue is these were made for 115-117 mains and modern mains can be upwards of 125 volts. It uses a voltage doubler circuit to achive 55-60v, but with an extra 10v going in, it's 20v coming out and the caps aren't rated for that and die quickly.

I usually replace them all with Vishay 105C since the pedal can get pretty warm. It's easy to retrofit a 3 prong cable and fuse in it too to make it safe. I also add a 56v zener to keep the power at a reasonable voltage, and I add some MLCC .1uf bypass caps across the electrolytics to filter out RF racket. I use an 18ga power cable with a Heyco PG7 strain relief, no drilling needed.

Something else that happens is the bulb draws more current than it should and the resistor for it can begin to burn, I replace them with 220ohm 2watt vishay and raise them off the board a bit so they won't damage it if they get a little warm.

The shroud is just some vulcanized paper that's about 1/2mm thick. It's square. They are taped down with DUCK brand black masking tape.

Attached a photo of my PWF (yes I know the fuse is attached to neutral instead of hot, I'll fix it later).

[beneharris]

I fix these all the time, the caps are 50 years old and die, check em with ESR meter. The issue is these were made for 115-117 mains and modern mains can be upwards of 125 volts. It uses a voltage doubler circuit to achive 55-60v, but with an extra 10v going in, it's 20v coming out and the caps aren't rated for that and die quickly.

I usually replace them all with Vishay 105C since the pedal can get pretty warm. It's easy to retrofit a 3 prong cable and fuse in it too to make it safe. I also add a 56v zener to keep the power at a reasonable voltage, and I add some MLCC .1uf bypass caps across the electrolytics to filter out RF racket. I use an 18ga power cable with a Heyco PG7 strain relief, no drilling needed.

Something else that happens is the bulb draws more current than it should and the resistor for it can begin to burn, I replace them with 220ohm 2watt vishay and raise them off the board a bit so they won't damage it if they get a little warm.

The shroud is just some vulcanized paper that's about 1/2mm thick. It's square. They are taped down with DUCK brand black masking tape.

Attached a photo of my PWF (yes I know the fuse is attached to neutral instead of hot, I'll fix it later).

Nice, that is super helpful, thank you.

I'll probably go through and do the couple caps that I didn't replace, I only did the ones I identified as bad, but I just don't like the look of it half done. You really go all out on these, that's great.

This is my first one, but from what I can tell, the transistor bias is what was causing all of my issues, and at the voltage this runs at, it is much more of a problem than a normal stompbox would be.

[WonkoTheSane]

Yeah the voltage can be a problem, that's why I added a zener to keep it in the original spec.

Some of them have trim pots on the collector resistors to adjust them but I found that doesn't always work very well. I did one that no matter what I did it did a kind of compression thing that I could never solve, even with new transistors. I think the transformer was going bad and just couldn't supply the current anymore. Probably from someone at some point putting the wrong kind of bulb in it.