Yamaha CH-01 Chorus modification

[halegg]

hello everyone,

so i have this really warm, thick yamaha 80's chorus which i'm very pleased with... however there is a slight volume drop, say 5 decibels.
it's really hard to find schematics online on this, but here i have pictures attached of the circuitboard after opening the pedal...
later today i am going to put a 10k trimpot inside a boss tr-2, and i wondered if such a thing was possible with this yamaha pedal aswell?

here are some pictures showing the circuit board
http://imgur.com/a/O46e7

all help greatly appreciated!
halegg

[GGBB]

Hi and welcome.

I have one of these and have been searching for a schematic for ages. A couple of years ago i started tracing it out myself but I never finished. It is reportedly nearly identical to the Korg CHR-1 and I did find a schematic for that at the fsb forum which was posted about a year ago. Shouldn't be hard to find through Google.

If they are the same, fixing the volume drop should be possible, but adding a level trimpot would be a bit involved since there is no typical level voltage divider on the output. In the korg, the dry and effect signals are summed and fed into a non inverting opamp ic3 who's gain is set by feedback resistor r26 5.6k over r17 10k. Increasing r26 should boost your output. The only drawback of this is that the direct out is not changed so although both signals in the the mix out are now louder, the dry out alone is now softer than the wet out alone.

[halegg]

Thank you for reply GGBB.

I'm fairly new to all of this, but you explained it well, so I understand.
You ended up doing nothing with yours then I take it...?

[GGBB]

I haven't used mine in a while, but I don't recall that it had any volume drop. I replaced all the electro caps in it due to age and that's about it. The only thing I do notice (and don't like) about it is that it has a slight bit of bass suck - maybe it's the same thing we're talking about but describing it in different ways. I do like it the way it sounds though.

[halegg]

Yeah, maybe we are talking about the same thing. I'll check it out to make sure whether it's sucking tone/bass or drops in volume.
Anyways... It's gonna stay how it is me thinks.

[candlesolder]

Hi, sorry for "restarting" an old thread.
I recieved a Korg CHR-1 chorus and somebody seems to be f**king around with the circuit:
the direct ouput is disconnected, and only a mono jack is installed - looking at the pics of the yamaha / the schematic, it has to be a switchable mono jack.
The battery has also been disconnected.
Anyone of you having some better pics of the wiring? Especially to the  stereo out jack?
Due to the schematic, there has to be a connection from  R39 and R16 to the stereo output, but there seems to be no contact on the pcb. Is it wired directly to the resistors or am I missing something?

The thing itself seems to work, but I wanted to use the stereo-option of it.

Any help will be highly appreciated,
tia
regards,
Carsten

[GGBB]

There is no stereo out. The output jack is a simple mono jack, and the direct "dry" out jack is a switched mono jack. When you insert a plug into the direct out, the switch becomes disconnected. That switch is what forms the connection between R39 and R16 which then goes in to IC3a along with the wet signal from IC3b via R15, and then out to the output jack. When disconnected, the dry signal from R39 is connected only to the direct out, and IC3a is only fed wet signal, leaving R15 hanging.

To get "stereo", you would use both jacks simultaneously - dry-only in one channel and wet-only in the other like in a Roland Jazz Chorus. As far as I know that's how early stereo choruses always worked, but I could be wrong. Anyway, this isn't a trues stereo chrorus which would require two versions of the modulated signal which are opposite in phase. If you're curious about that, check out the stereo mod for the Small Clone Chorus project at tonepad.com. You could probably apply those principles to your Korg if you really really really want a true stereo CHR-1.

[Mark Hammer]

The Yamaha CH-1 is identical to the Korg CHR-1, shown below.