Vintage Morley Power Wah Boost Troubleshooting

Forum dedicated to helping people debug and troubleshoot non-functional pedals or builds. Please use an clear and informative title, indicating circuit and basic problem. Don't forget to mark the issue as fixed if this is the case.
Post Reply
User avatar
CheapPedalCollector
Solder Soldier
Information
Posts: 183
Joined: 19 Oct 2020, 15:15
Has thanked: 153 times
Been thanked: 63 times

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.

User avatar
Manfred
Opamp Operator
Information
Posts: 1465
Joined: 04 Apr 2009, 23:42
Has thanked: 1283 times
Been thanked: 915 times

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?

User avatar
CheapPedalCollector
Solder Soldier
Information
Posts: 183
Joined: 19 Oct 2020, 15:15
Has thanked: 153 times
Been thanked: 63 times

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 15 times
MorleyPWB.jpg

User avatar
Manfred
Opamp Operator
Information
Posts: 1465
Joined: 04 Apr 2009, 23:42
Has thanked: 1283 times
Been thanked: 915 times

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?

User avatar
CheapPedalCollector
Solder Soldier
Information
Posts: 183
Joined: 19 Oct 2020, 15:15
Has thanked: 153 times
Been thanked: 63 times

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.

User avatar
Manfred
Opamp Operator
Information
Posts: 1465
Joined: 04 Apr 2009, 23:42
Has thanked: 1283 times
Been thanked: 915 times

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?

User avatar
CheapPedalCollector
Solder Soldier
Information
Posts: 183
Joined: 19 Oct 2020, 15:15
Has thanked: 153 times
Been thanked: 63 times

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

Post Reply