FX Loop in Ceriatone TW Clone

[BasementJames]

So, what do they mean when they say the loop has its own input coupling capacitor?

[sluckey]

It means it's safe to connect the input as you did.

[BasementJames]

Right, this is the contradiction.

When I show them what I did, they said "no, you can't connect there, it is 200 volts."

Ok, then why do the instructions say there is an input coupling capacitor?
I have asked them that question about the coupling capacitor several times and they just won't answer it.
It is a little frustrating.

Am I just not understanding something?

[sluckey]

A schematic of that FX loop would make everything crystal clear.

[pdf64]

So far, it seems like guesswork trying work out whether anyone is referring to 200V AC (peak) or DC

[Guy77]

Here is a schematic of an earlier version I found on my hard drive.
Any changes would have probably been minor.
Where it says optional Send trim, that is the blue 1 Meg trimmer that is on the board with version 3 & 4.
It lies before the .1uf 400v cap

Mojo Fx Loop #2.gif

[Guy77]

This version of the schematic maybe easier to read. This shows the send trimmer which is blue on the boards and the return pot shown here is extra and not part of the Mojotone boards.

mojotone loop2.GIF

Cheers

[jbrew73]

Forgive me if I’m not following this whole thread correctly and have missed what’s already been posted but wouldn’t a high b+ voltage smoke the internal trim pot? It seems their coupling cap is to protect the bias circuit for the first stage. It is not providing any protection for dc voltage coming into the trim pot.

I do agree that the instructions are misleading. They show the effects loop being inserted after the tone stack,which would be a safe place to install it, but they don’t make it clear if any other location is acceptable.

[pdf64]

Right, this is the contradiction.

When I show them what I did, they said "no, you can't connect there, it is 200 volts."

Ok, then why do the instructions say there is an input coupling capacitor?
I have asked them that question about the coupling capacitor several times and they just won't answer it.
It is a little frustrating.

Am I just not understanding something?

From what I can make out, you didn't do anything contrary to Mojo's instructions.

From Mojo's documentation https://cdn.shopify.com/s/files/1/0604/ ... 1755282510

A coupling capacitor is necessary between the effects loop
and the phase inverter. This means that the signal path must
be broken there to install the loop in series.
Next, find the pads that are labeled ‘IN’ and ‘GND.
’ Connect
these pads to the preamp side of the circuit where the signal
path was broken. The loop features its own input coupling
capacitor, so no additional capacitor is necessary for
installation.

[Guy77]

Forgive me if I’m not following this whole thread correctly and have missed what’s already been posted but wouldn’t a high b+ voltage smoke the internal trim pot? It seems their coupling cap is to protect the bias circuit for the first stage. It is not providing any protection for dc voltage coming into the trim pot.

I do agree that the instructions are misleading. They show the effects loop being inserted after the tone stack,which would be a safe place to install it, but they don’t make it clear if any other location is acceptable.

Yes I agree 200v dc would most likely destroy the tiny trim pot on the board.

Guy

[Ten Over]

This version of the schematic maybe easier to read. This shows the send trimmer which is blue on the boards and the return pot shown here is extra and not part of the Mojotone boards.

mojotone loop2.GIF

Cheers

Where's the bypass switch?

I think that little blue pot on the board is the Return level, not the Send level.

[Ten Over]

Here is my guess from looking at pictures.

FX LP Mojotone Ver 4.jpg

[Guy77]

Hi, yes that is where the bypass switch is. After examining the traces on the ver 3 boards the control trimmer on the board actually does sit as a return control trimmer as you mentioned! The 100n cap on the input is 630v on the ver 3 board
The version 4 boards should have the trimmer in the same position as well.



Cheers
Guy

[Ten Over]

Hi, yes that is where the bypass switch is. After examining the traces on the ver 3 boards the control trimmer on the board actually does sit as a return control trimmer as you mentioned! The 100n cap on the input is 630v on the ver 3 board
The version 4 boards should have the trimmer in the same position as well.



Cheers
Guy

I couldn't pick up any voltage ratings in the pictures and the instructions gave a limit of 400V, so I just put 400V on all of the capacitors. I ordered one in so that I can reverse engineer it and post a confirmed schematic.

[BasementJames]

Thanks for all the replies, I appreciate everyone chiming in.

I'll post all the information I have gathered so far in my conversation with MojoTone about this loop for the benefit of the community. I do feel that the instructions leave a lot to be desired and I still don't have all the information I am looking for.

Here is what I can say so far:

The blue trim pot is the return level. Instead of using the blue trim pot, one can install an external pot to control the return level without having to get at the trim pot inside the amp. I have turned the pots in every direction twice and tried every possible combination of settings. I can rule out the pot settings.

According to MojoTone, the loop is designed to work with signal level voltage. Although the directions say "The loop features its own coupling capacitor, so no additional capacitor is necessary for installation", it seems as though one should not interpret this to mean that the input has a coupling capacitor that will block the DC voltage to the loop. I asked MojoTone about the input coupling capacitor and didn't get an answer. I see a schematic posted here that seems to indicate the presence of an input coupling capacitor (looks like 400 volts, which is maybe on the low side). So, I still don't know. Does this loop have an input coupling capacitor and what is its purpose if not to block DC voltage? I can confirm that MojoTone does not recommend sending 200+ volts into the input of the loop, regardless of how the instructions are interpreted.

If you follow the instructions to the letter in a Trainwreck circuit, the loop will not work. The instructions seem to work just fine for a Marshall style circuit, but also imply (to me) that the same approach will apply to any circuit. I don't believe that is the case and this is why we need more information about this loop to properly apply it to various other circuits. I did what it said "Connect these pads (OUT and GND) to the coupling capacitor just before the input of the amplifier's phase inverter circuit". That doesn't work. In fact, if you do that, you will send 200+ volts to the input of the loop and people will make fun of you.

The loop works by tapping into the amp's B+ voltage. The instructions say, "Do not exceed 400V". MojoTone did say that the loop is happiest with about 300 volts and 200 volts might be a bit low. 300 volts provided ideal headroom. The instructions don't say anything about a minimum voltage, just the 400-volt maximum.

I really don't know if the loop is damaged or not and if anyone knows how I could check to find out, I would take any suggestions. I asked MojoTone if I could send it back to be tested but didn't get a reply to that.
I can say that with the loop engaged, I was getting a 1-volt signal from the send. It seems like the problem was on the return side. It is possible that the reason it wasn't working is the location, but I'm not really convinced of that.
I tried the loop both before and after the .1uF coupling capacitor, with the same results.

Their last suggestion was to install the loop between the middle tab of the treble pot and the right tab of the volume pot. I thought putting the loop that early in the signal path didn't really make sense and didn't bother trying. Maybe it would work, but I have basically run out of patience with this and have scrapped the loop idea.

James