Vintage Morley Power Wah Boost Troubleshooting

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CheapPedalCollector
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Post by CheapPedalCollector »

I'm repairing a Vintage Morley PWB and it has a very weird compression happening when the Boost is engaged.

I've replaced the electrolytics in the unit as they were all failing, I've tested the transistors, replaced a few way out of spec resistors (4.7M and 2.2M). Checked voltages and I have 48V to the bulb, 28 Volts on the 25 Volts rail, and 65 Volts on the 55 Volts rail. I assume the voltage raise is due to AC power being 122 volts at my place and the units were designed for 117. I have checked the resistors and they are within spec. Is it possible the rectifier diodes are stressed? The light does not dim in any way when this happens so I don't think it's the power supply.

I have adjusted the bias trimmer pots so that it biases up correctly, they were only slightly off. The Wah circuit used to do this too, but now it works correctly. I'm still having this strange compression/pumping/breathing when really smacking chords hard with the boost circuit and I don't think it's supposed to do this. I am not sure what the issue could be at this point, and I never had a power wah in the past so I don't know if this is normal.

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Post by Manfred »

I have repaired and sold a PWB about 10 years ago for a friend of mine.
viewtopic.php?p=253530&hilit=pwb#p253530
There are obviously several circuit versions, my version did not have a bias trimmer.

I already mentioned in another post that I had contacted Scott at Morley about problems with the LDRs.
He told me that if the pedals with the LDRs were not used for a long time the LDRs passivate and then the LDRs must be exposed to light for a while so that they work again.

Setting the positions for the LEDs and the LDRs takes a lot of patience.

You can adjust the voltages for the circuit by changing the series resistance between the voltage doubler circuit and the filter circuit.
In the circuit of my pedal, this resistor had a value of 1.5 kiloohms.

Did you trace the circuit and draw the schematic of your pedal?
Could you post a shot of the circuit board?
Did the transistor types remain the same?

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Post by CheapPedalCollector »

I have a PDF of the schematic, I'll post it (I don't know if this is allowed or not?) It is identical as far as I can tell with the only difference being that the unit has MPSL01s in it instead of MPSA06s.

The LDRs operate fine, I also did some more troubleshooting and found the ceramic .01uf and .05uf caps were leaking and had very high ESR, so I replaced them with some WIMA polystyrene which made a *massive* difference in the sound, however I get some oscillating now. I also am going to replace some of the 10K collector resistors as they are reading ~13k which is more than 20% out of spec and can raise the gain considerably. I ordered some 560K resistors too as they are reading around 680K which is also out of spec.

Also attached photo, never mind the pulled up legs on resistors as those are the ones I measured as bad and in the midst of replacing them.

I have also since fixed the loose tape and adjusted the shroud so the wah/volume functions as intended.
morley_pwb_power_wah_boost_sch.pdf
(216.08 KiB) Downloaded 115 times
MorleyPWB.jpg

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Post by Manfred »

I see your PWB is different from what I had from my friend.
...however I get some oscillating now.
Is the oscillation present in all modes or only in some of them?

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Post by CheapPedalCollector »

It oscillates when the boost is engaged.

I checked some youtube videos as well, there isn't many but it definitely is not supposed to do the compression thing.

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Post by Manfred »

I ran a simulation of the boost circuit setting the collector voltage from the MPA06 to 27.5 volts DC.
The gain of the circuit ranges from about 150 to 930 depending on the setting of the boost potentiometer.
At the lowest gain of 150, the positive swing starts to round out at 660 millivolts peak-to-peak and the negative swing starts to clip at 720 millivolts peak-to-peak,
earlier at higher boost settings.
Does the boost control still work?
What is the frequency of the oscillation?
Did the oscillation occur after the capacitors were changed?

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Post by CheapPedalCollector »

Boost control works, the frequency seems to be in the wah circuit at the top end of the treadle range, I don't have a frequency counter atm so I can't say what the freq is. Yes it started after the cap change, but also volume of everything increased massively. It sounded very weak and pretty bad before I changed them. Perhaps the ESR is too low for the polystyrene caps, so I will order some MKT368 to put in there instead.

I'm going to order MPSA06 and MPSA12 and see if that fixes the compressing issue if the 560K replacements don't fix it. I seem to always get these basket case pedals :D

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Post by Manfred »

I found an error on the PCB-shot the coupling electrolyte capacitor after emitter follower is installed upside down.
According to the schematic, the positive lead is on the emitter.
What solder did you use leaded or lead free solder?
I ask because the PWB was certainly still soldered with leaded solder and there are connection problems when using lead-free solder.
I recently had the experience again that both types do not fit together.
Despite the desoldering of a switch contact this was still wetted with leaded solder.
It was not possible to solder a wire with lead-free solder, there was no proper solder joint and it looked more glued than soldered.
MorleyPWBDetail.jpg

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Post by CheapPedalCollector »

Oh wow, good catch! That's a factory defect as I never removed those caps, I only lifted one end to test them. Thank you!

I am using leaded solder (Kester 60/40), I can't stand lead-free solder, it always makes cold joints unless I have my shop heater cranked up to uncomfortable levels. I am aware that it can also be dangerous to mix leaded and lead free as it can make alloys that melt at too low temps and cause catastrophic failures. I have lead free for when I absolutely need it, but I also don't like having to change my soldering iron tip and I haven't bought a second iron yet.

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Post by CheapPedalCollector »

Still working on this one.

Q2 was replaced as it was noisy and making snap crackle pop sounds. The 560Ks were replaced but this made no difference. I've also replaced the emitter bypass caps as I just didn't trust them especially when one was installed incorrectly at the factory. I tried replacing the diodes in the power supply as well with no effect. I got rid of the oscillation by using MKT368 polypropylene caps instead of the polystyrene which I guess have too low or high ESR or something, and they are also a bit less bright sounding (save the comments about different caps sounding different, they clearly do and act different in this circuit).

I've now observed after I got some clip leads for my meter that the power supply is being stressed somehow by the circuit and the voltage drops when this compression happens by 1-2 volts which seems odd. I'm starting to wonder if the power transformer is bad too. This repair has traveled into not worth the cost territory already, so I'm wondering what to do.

I really hate doing shotgun repairs, but I'm at a complete loss as to what the hell is wrong with this thing at this point.

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Post by Manfred »

Read the voltage drops across the 1.5k and 220R resistors of the power supply,
so you can calculate the currents and add them together to see if the transformer is overloaded.
I do not think that this is the cause of the oscillation.
Do you have an oscilloscope?

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Post by CheapPedalCollector »

I'll check that, and I wonder if the voltage being too high according to spec is the issue, I'll have to order some 1.5K and 1.2K resistors to see if I can get it down into the 55V territory its supposed to be in.

The oscillation is gone now, different caps solved that. I'll record a video too about what I'm talking about is going on so it's more clear. I've fixed many hundreds of pedals and never had such an issue as this before, other than that DOD Flanger that doesn't work also for no reason at all. It's frustrating, but if it can be figured out I think it will be valuable lesson.

I do not have a scope atm (hope to buy one by the end of the year).

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Post by CheapPedalCollector »

14.5v across the 1.5k (which reads 1.8k) which is .008 and 8.2v across the 220R (which reads 229) which is .036 roughly which is .044, so 44ma total. The transformer is a Stancor and its 28V CT with a rating of 85ma so I think that should be more than enough. The plot thickens...

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Post by CheapPedalCollector »

So I've determined the sag is just caused by the poor design of this pedal. Increasing the feedback resistor on the gain stages to 4.7M pretty much eliminated it. I note this only occurs with modern high gain pickups and doesn't happen with older pickups with 500K or less volume controls. Anything higher than that and using any modern high gain humbucker will introduce this issue.

The oscillation is a bit of a mystery, I just assume its due to the stupidly high gain of the boost circuit running at a quite high supply voltage, I'd have to change the circuit significantly to eliminate this problem. Again a higher feedback resistor or split pair with an audio shunt, at this point I would just be redesigning the circuit so I'm calling it good and its working as intended.

I have learned I can exploit this type of circuit with carefully chosen parts to get a bit of a tube like spongy/compressive feel out of it.

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