Copicat Mark IV Echo - 100% Wet Mod

Hi,
I would like to modified this echo unit so I'll be able to mute the dry signal and stay with 100% wet signal.
I was thinking to install a SPDT switch on the dry signal line and send it to ground / to the output transistor.
Does it make sens?
If so where would you add the switch?
Here's a link to the schematic:
http://wem-owners.com/wp-content/uploads/2015/07/copicatmk4.jpeg

I think you can break the connection between the cap and resistor shown here:
https://www.dropbox.com/s/ngg3y0cxuextml2/copicatmk4%20dry%20cut.jpeg?dl=0

With the connection open, there is no dry.  Closed, dry mixes in.  Do it carefully in case I am wrong, the schematic is a bit confusing.  If it is wrong it will not harm anything, but it won't do what you want.  If it is right, please let us know!

That's the spot but I wouldn't "cut" it exactly. I would insert a 1M there and use the switch to bypass it. Otherwise the shift in DC will cause a loud pop.

Thanks, I'm waiting for some capacitors to arrive and then I'll procedure this mod while recaping it.

squarewave said:

That's the spot but I wouldn't "cut" it exactly. I would insert a 1M there and use the switch to bypass it. Otherwise the shift in DC will cause a loud pop.

Click to expand...

Hmmm, thought I picked that spot to avoid DC, but I could be reading it wrong.  Either way, your suggestion is a good one.

There's no DC current but that net has a DC path to the base of the output transistor which has a bit of gain. If you break it at the cap, the tiny leakage current will cause the open end to charge up / down and generally drift depending on which way the wind is blowing. Then when the switch is closed, it will discharge / charge through the transistor base and cause a pop. With a 1M across the switch, it gives the tiny leakage current somewhere to go and holds the cap at the same bias voltage.

Thanks for the explanation, Mike.

I've finished by adding a 100% wet output using one of the input jacks with the signal coming from the foot switch with a coupling cap before the jack.
I've also recaped the electrolytics and the echo sounds much quite.
Now the only problem with this unit is after about 30 minutes it's getting hot, the echo disappears and there's some strange noise.
I'm suspecting that this  120R/5W (in my unit it's a 220R) resistor is getting too hot and making problem with the bias.
Would you think that replacing it with a 10W resistor will keep it cooler?
Also should I install a 120R (as the schematic refers) or keep going with a 220R resistor?

That might be the problem. But that doesn't bias the signal. That's the oscillator. I would think if the oscillator quit you might still get some signal. So it's not obviously the problem.

To test your theory, wait for the problem to occur and then blow some "air duster" on that resistor and see if the problem goes away. Put the straw right up to it and blow slowly on it for 90 seconds or so.

I've found the problem, the former owner replaced the back grill with a MDF and closed the ventilation holes for the motor and this 5W resistor.
I've removed this MDF and now the machine working good for over an hour.

smilan said:

Hi,
I would like to modified this echo unit so I'll be able to mute the dry signal and stay with 100% wet signal.

Click to expand...

The signal that goes to the footswitch is the 100% Wet signal.
To have a 100% Wet is just a matter of cutting the footswitch cable and wire a female Jack plug there.

I found a replacement plastic grill for my cat at my local hardware store ,  MAP air vent products .
Its the same one John Beer sells on his copicat website ,as he says very close but not identical to the original ,

https://amp-fix.com/copicat_spare_parts.htm

Whoops said:

The signal that goes to the footswitch is the 100% Wet signal.
To have a 100% Wet is just a matter of cutting the footswitch cable and wire a female Jack plug there.

Click to expand...

worth sticking a little opamp buffer circuit on the wet output. I did a simple mod on a mates, i'll see if i can find a pic.

mrclunk said:

worth sticking a little opamp buffer circuit on the wet output. I did a simple mod on a mates, i'll see if i can find a pic.

Click to expand...

I never felt the need for that, just worked pretty well into a DI and it's interchangeable with the footswitch.

Why did you use an opamp?

Just a small update, after the copycat worked well for about a year the 120R/5W (in my unit it's a 220R) resistor got too hot and made the oscillator to fail after about 10 minutes.
So I've replaced the 220R resistor with a 110R / 10W and it seems like the problem has gone (at least for now...). I let it run for over one hour with no problems.