Hekp needed with Vintage Script Logo MXR Dyna Comp Problem

**sohosteve** · 06-30-2019, 09:40 PM

I have a 1970s script logo MXR Dyna Comp that I can't get working.
I bought it a few years ago and it was working fine, though at some point someone has drilled a hole in the side to fit a DC socket though not completed the job.
So I thought I would add that mod and also convert it to true bypass.
Sadly, due to weak solder joints some of the connections came adrift - I found online resources that led me to where they should be connected but I couldn't get the thing working.
At this stage I thought the best thing would be to return it to standard - it still didn't work and it's been sitting on my shelf gathering dust ever since.
Until today when I decided it was way past time to fix it.
Several hours of research (I couldn't find one clear set of photos to show where every connection should be) and numerous attempts at re-connecting and checking for bad solder joints and I'm still at a loss!

This is the fault description
When the pedal is switched out I get signal through.
When the pedal is switched on I get no through signal whatsoever - no buzz, or hum, or crunches if I move any of the wires around to check connections.
So I wonder if there is either a dead component or I've just made a mistake with my connections due to not being able to find clear enough photos to copy.

So here I am begging assistance.
I must make clear that I am not an electronic engineer - I can't even read a schematic - but I can follow clear graphic instructions.

I've uploaded two gut shots that, hopefully, show all the connections clearly, so you can see where I am currently at.

In the first instance I just want to get the thing working again.
Though my end goal is to convert it to true bypass and run it from a PSU.
I'm willing to consider any advice that is offered.

Kind thanks in advance.

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**nosaj** · 06-30-2019, 10:02 PM

Originally Posted by sohosteve

I have a 1970s script logo MXR Dyna Comp that I can't get working.
I bought it a few years ago and it was working fine, though at some point someone has drilled a hole in the side to fit a DC socket though not completed the job.
So I thought I would add that mod and also convert it to true bypass.
Sadly, due to weak solder joints some of the connections came adrift - I found online resources that led me to where they should be connected but I couldn't get the thing working.
At this stage I thought the best thing would be to return it to standard - it still didn't work and it's been sitting on my shelf gathering dust ever since.
Until today when I decided it was way past time to fix it.
Several hours of research (I couldn't find one clear set of photos to show where every connection should be) and numerous attempts at re-connecting and checking for bad solder joints and I'm still at a loss!

This is the fault description
When the pedal is switched out I get signal through.
When the pedal is switched on I get no through signal whatsoever - no buzz, or hum, or crunches if I move any of the wires around to check connections.
So I wonder if there is either a dead component or I've just made a mistake with my connections due to not being able to find clear enough photos to copy.

So here I am begging assistance.
I must make clear that I am not an electronic engineer - I can't even read a schematic - but I can follow clear graphic instructions.

I've uploaded two gut shots that, hopefully, show all the connections clearly, so you can see where I am currently at.

In the first instance I just want to get the thing working again.
Though my end goal is to convert it to true bypass and run it from a PSU.
I'm willing to consider any advice that is offered.

Kind thanks in advance.

Need a shot of the front side of the pcb please. Here is an article on it.
https://www.electrosmash.com/mxr-dyna-comp-analysis
nosaj

**sohosteve** · 06-30-2019, 10:08 PM

Originally Posted by nosaj

Need a shot of the front side of the pcb please.

Now added

**DrGonz78** · 06-30-2019, 10:12 PM

I would spray some contact cleaner on that bias pot, wiggle it around and then leave it where it was set originally. If it looks like a bad idea to move the bias pot around (it looks soldered in place) then just a spray of cleaner would be fine. I would hit a few of those solder joints that look questionable. At that point those 4 maroon colored tantalum capacitors are suspect as they die of old age.

**sohosteve** · 06-30-2019, 10:16 PM

Originally Posted by DrGonz78

I would spray some contact cleaner on that bias pot, wiggle it around and then leave it where it was set originally. I would hit a few of those solder joints that look questionable. At that point those 4 maroon colored tantalum capacitors are suspect as they die of old age.

When you say 'bias pot' I assume that you are referring to the internal trim pot?
If so it is soldered in place.

As for the solder joints - I have done it already - but I shall have another pass.

So does the actual wiring all look correct then?

And thank you for your kind suggestions!

**DrGonz78** · 06-30-2019, 10:30 PM

Originally Posted by 52 Bill

Red wire (upper right in photo)= +9 volt battery Neg side of battery goes to ring terminal of the input jack

Yellow wire= pcb in Goes to one side of ft. switch and to input jack

Grey wire = Wiper of Sensitivity pot

Orange wire= CW term. of Sensitivity pot CCW terminal of sens pot not used

Red wire (lower right in photo)= ground Should be black wire

Purple = pcb out Goes to CW terminal of output pot

Let me know if there is anything that you don't understand or if there is something else you need to know.

--- Relates to this picture here I believe...
https://music-electronics-forum.com/...7&d=1228093104

This was a quote from from 52 Bill from an old thread that looks to be a similar or exact pedal. Perhaps the color of wires might differ but relate them to the function. And yes I saw that trimmer pot is soldered but I would still spray it a bit to clean it up a little.
https://music-electronics-forum.com/...ad.php?t=10108

**sohosteve** · 06-30-2019, 10:38 PM

Originally Posted by DrGonz78

--- Relates to this picture here I believe...
https://music-electronics-forum.com/...7&d=1228093104

This was a quote from from 52 Bill from an old thread that looks to be a similar or exact pedal. Perhaps the color of wires might differ but relate them to the function. And yes I saw that trimmer pot is soldered but I would still spray it a bit to clean it up a little.
https://music-electronics-forum.com/...ad.php?t=10108

Yes, I saw the directions from 52 Bill earlier today and followed them as best as I could but still ended up with silence when the effect is switched to 'on'.
I shall try giving the trim pot a good sluice out and give all the solder joints a gentle melt and report back

**sohosteve** · 06-30-2019, 11:07 PM

No joy.

What would be a huge help is a clear graphic or photograph that shows every wire connection from jack sockets to switch to pots to pcb.
But sadly every photo I have found on the web (and I've been searching all day - and have made numerous searches in the past) are all unclear.
Some connections are easy to see but then there's one or two twisted wires of the same colour and no obvious sign of what should go where.

This thread has the clearest photos that I could find but even so, there is still a tangle that has me totally confused...
https://music-electronics-forum.com/...ad.php?t=15979

**nosaj** · 06-30-2019, 11:11 PM

Originally Posted by sohosteve

No joy.

What would be a huge help is a clear graphic or photograph that shows every wire connection from jack sockets to switch to pots to pcb.
But sadly every photo I have found on the web (and I've been searching all day - and have made numerous searches in the past) are all unclear.
Some connections are easy to see but then there's one or two twisted wires of the same colour and no obvious sign of what should go where.

This thread has the clearest photos that I could find but even so, there is still a tangle that has me totally confused...
https://music-electronics-forum.com/...ad.php?t=15979

Wires all look correct to me, Do you have a multimeter?
nosaj

**sohosteve** · 06-30-2019, 11:18 PM

Originally Posted by nosaj

Wires all look correct to me, Do you have a multimeter?
nosaj

Yes, I have a cheapo Tandy multimeter, though I'll require a little assistance in adjusting the setting

**olddawg** · 07-01-2019, 04:08 AM

Is there power to the circuit when the switch is toggled? And I don’t want to sound condescending.. are you sure it’s plugged in correctly? We have all done it. A sure sign of a pedal plugged in backwards is it only works in “off” mode. See if the rail voltage is there when it’s on.

**52 Bill** · 07-01-2019, 05:52 AM

In your photo, I can't tell where the black ground wire from the circuit board goes. It seems to head behind the volume pot.

It should be connected to the CCW terminal of the volume pot which is connected to the input and output jack grounds.

**DrGonz78** · 07-01-2019, 08:09 AM

Originally Posted by 52 Bill

In your photo, I can't tell where the black ground wire from the circuit board goes. It seems to head behind the volume pot.

It should be connected to the CCW terminal of the volume pot which is connected to the input and output jack grounds.

Yeah I noticed it too but I thought it might have been the negative side that leads to the battery. Either it just looks like it's down there flopping in the wind.

**sohosteve** · 07-01-2019, 05:04 PM

Originally Posted by 52 Bill

In your photo, I can't tell where the black ground wire from the circuit board goes. It seems to head behind the volume pot.

It should be connected to the CCW terminal of the volume pot which is connected to the input and output jack grounds.

Hi there - sorry that was a slightly earlier photo and the black wire was disconnected.
Now I have put a newer inline photo attempting to show absolutely every connection as clearly as possible.
Hope that's helpful

**sohosteve** · 07-01-2019, 05:07 PM

Originally Posted by olddawg

Is there power to the circuit when the switch is toggled? And I don’t want to sound condescending.. are you sure it’s plugged in correctly? We have all done it. A sure sign of a pedal plugged in backwards is it only works in “off” mode. See if the rail voltage is there when it’s on.

Not condescending at all as I suspect that I'm mildly dyslexic - often I get things back to front - so good call! However, in this instance I did make doubly sure that I didn't muddle my ins with my outs!

Sorry to be a plum but where would I look for voltage on the circuit and what setting should I have my Tandy/Tesco/Poundland Multimeter set to?

**sohosteve** · 07-01-2019, 05:08 PM

Originally Posted by DrGonz78

Yeah I noticed it too but I thought it might have been the negative side that leads to the battery. Either it just looks like it's down there flopping in the wind.

It was indeed 'flopping in the wind' as I'd used a slightly earlier photograph - a more recent one is now uploaded 'inline'

**olddawg** · 07-02-2019, 03:20 AM

Set your meter to the correct scale for DC volts. Connect the black lead to ground. Use the red probe. Start at your power supply. You should have around 9vdc. Follow it into the circuit. Can you read a schematic? At this point you need to no if the power is getting to the device at all and not open or shunted to ground.