MXR auto Q schematic? And is this thing even fixable?

[BillyJ]

I have one that lights but no sound. I bought it broken thinking it would be cool to try and fix.
It is made with SMDs....ugh!
I wonder is this thing is even worth trying to mess with?
I guesss I can use the enclosure....

[Peter Snowberg]

SMD isn't that hard to deal with if you don't mind killing the components you remove.

As long as the parts aren't custom, give it a shot.

Use a tiny tip, some desoldering braid, some liquid flux, and/or a solder sucker. Sometimes it's easier to lay down some additional solder blobs at each contact and quickly switch the iron back and forth to melt them all at the same time. It's pretty easy with resistors and caps. Once both sides are molten the component will almost jump off the board and stick to the iron UNLESS it was affixed with a dot of epoxy before soldering. In that case, use needle nose pliars to twist the part and it will pop off. For chips and transistors, I use an exacto knife to lift the legs one at a time as I touch them with the iron.

If there is any extra room in the case, you can just use small regular parts as a replacements everywhere except chips; unless you are a masochist.

Good luck with it no matter what happens.  

-Peter

[Mark Hammer]

What are the part numbers on the semis?  Curious to know how much this is simply a modded Envelope Filter vs a complete redesign.

[BillyJ]

Hey Mark,

Here is what I can find:
all ICs are 16 pin. Here are the numbers:
MC35174D..... underneath on the same chip is the word 'ON' with a circle around it next to that is PAJ122.
There are 3  of the above mentioned chips.
next:

             HEF40498T
Next line:9071600
Next line: Hm94094

next ic:
14016B
XAA115

Ooops found an 8pin one:
33178
2BNXD


Did you want the number for the trannies or fets? Not sure what is what on here but I can read the number on them too if you want them.
LMK I am happy to oblige. You have hooked me up dozens of time already. :O)

Thanks!

[Mark Hammer]

A quick search reveals the following:

MC35174D are low power quad op-amps with the same pinout as a 324/TL074/etc.

33178 is a dual lowpower op-amp (ON is the manufacturer's abbreviation)

The other chip is likely a 4016 quad CMOS switch.

The HEF4049 is likely a 4049 hex invertor.

So, in total, we have 14 op-amps across the various chips), 6 invertor sections, and 4 switches.  Enough to make something pretty complicated.

The original Envelope Filter had a dozen invertor sections (using them for filters and buffers as well as the envelope modulated follower and PWM clock, and used 2 switch sections off a 4066 quad switch).  So, it seems the basic filter structure is likely intact, and my guess is that a number of the pins on the 4016 aren't used.

The on-board LFO wil take at least 2 op-amps, the filter sections will take 2, the input and output buffers take another 2, and my hunch is that the envelope follower may use op-amps instead of invertors because a) there aren't enough invertors available to provide both the PWM of the switches AND envelope follower, and because b) there needs to be a means for mixing the envelope signal and on-board LFO, which will be done via an op-amp.

At this point that is about as much advice as I can offer.  I wouldn't say it is a wholly new design, but it is obviously much more than the original with a few tweaks.  In particular, the use of lowpower op-amps which were not available in 1978-80 allows MXR/Dunlop to produce filter sections using something other than CMOS invertors.

[Gus]

Does it pass signal in bypass mode?

Was anything spilled on it

Check the switches first.

Check any wires to make sure they are not broken or unsoldered.

The fact that you have a light is a good thing.  If the wrong power was applied in the past you might not have a light.

Most effect I fix don't have bad ICs Often the wrong power is applied and the protection diode shorts or opens or there is a bad solder joint or wire.

One "good" thing about SMD builds is the solder is often better than hand built board: However any place parts get attached to the PCB like pots, wires, switches etc the connections can go bad.

Check the coners of the dual opamps for power pin 4 ground pin 8 +9V

Gus

[BillyJ]

Ok I got the datasheets downloaded. Just need to get some more time to poke around with it. I wish I had a scope and the knowledge to use it. I bet this woudn't take long at all.
I did seem to get power at those pins but I will get back with exact details.
I may try to trace it out to make debugging easier.
Thanks for the insight guys. I will make this darn thing again even I have it shelved forever or it takes me forever.
Thanks!!

[Andrey]

I have one that lights but no sound. I bought it broken thinking it would be cool to try and fix.
It is made with SMDs....ugh!
I wonder is this thing is even worth trying to mess with?
I guesss I can use the enclosure....

Hi Billy!Didi You fix Your pedal MXR auto Q?I have same problem .its not working and cant find any diagram for 7 years. .did You have some diagram or service manual ?Will be Great to hear from You!   Best Regards! Andrey

[TinyQuarks]

Hello again, did someone fixed their problems???
having exactly the same issue and searching for a schematics, all IC seem fine but no sound, just a very low "click" at every LFO period
THANKS!