Peavey Stereo Chorus 400 - No Vibrato

[lowell]

Hey folks,

Hope everyone is doing well! I have this amp in and the chorus works, but the vibrato does not work. I cannot find a schematic to link to although I have a paper copy. Hopefully someone (enzo?) has it. The bmamps link I kept coming across no longer seems to work.

The vibrato is toward the upper right corner of the schematic and I believe U17 is the chip. It says 8038 on the chip.

The footswitch DOES turn the chorus on/off, but no vibrato when chorus is off.

Thoughts on where to poke first?

[nosaj]

schematic and datasheet on ic.
nosaj

[The Dude]

Here's the schematic.

Peavey-Stereo-Chorus-400-Schematic.pdf

Edit: I see nosaj snuck in on me.

[lowell]

Links don't seem to work for some reason... The Dude your link just opens a new tab/window to the ampage.org site.

Nosaj the ic link works but the MIT.edu link just takes me to their website.

[The Dude]

Click the link at the bottom of the post in "Attached Files" not the link in the post. It works for me.

[lowell]

This is all I see

[lowell]

[The Dude]

This is what I see. It seems you are missing the "Attached Files" box. I'm not sure why. Try refreshing the page and/or clearing your cache.

[Enzo]

The 8038 is a waveform generator IC, I used to use them in another industry.

Got +16 on pin 6? Got a waveform on pin 2?

Doesn't the chorus have to be on to get vibrato?

[The Dude]

I see only one shared FET controlling chorus and vibrato on/off, so I think Enzo's right. The chorus must be on to get vibrato.

[g1]

This is all I see

Go into your user settings. (click on your avatar at top right of page). Click on the 'account' tab. Scroll down to 'conversation detail options'. Make sure that 'display mode' is set to 'posts' (NOT 'latest activity').
Then click on 'save changes' at bottom of page.

[J M Fahey]

FWIW THIS link worked for me:

https://music-electronics-forum.com/...etch?id=941264

the other one did not.

[J M Fahey]

As I see it, chorus mixes 2 signals, a flat one and a delayed one, the delay being modulated by an oscillator so itīs being both frequency and phase modulated.
]The mix creates peaks and notches t differet frequencies and since modulation is changing delay all the time, *we* perceive that as "Chorus".

They found that a broken Chorus where flat signal is missing sounds like a crazy Vibrato.

A real one (frequency shifting), not the Fender and others version which is actually Tremolo (volume modulation), no matter what they call it.

I suppose they do exactly that, but if you fully cut the delay/modulation block which is within the Chorus module then you get nothing.

In a nutshell: I expect the Chorus footswitch to cut both of OFF.

They might have added an extra button so they could both be used alternatively (but not together) but since nobody complained since when? 1979? Earlier? , they never did.

[Enzo]

This was a real chorusing amp. A typical chorus pedal mixes the dry signal and the phase shifted signal to make a chorused sound. This amp is stereo, two power amps, one per speaker. In these peavey amps - and other brands are similar in some models - in chorus mode, one channel continues the dry signal, and the other channel has the phase shifted signal. The two mix in the sound field, they are not mixed in the circuit. If you mute one speaker or the other you can hear that one is dry and the other has the phased signal only. And indeed that signal alone is a wobbly weird sound.

[lowell]

Go into your user settings. (click on your avatar at top right of page). Click on the 'account' tab. Scroll down to 'conversation detail options'. Make sure that 'display mode' is set to 'posts' (NOT 'latest activity').
Then click on 'save changes' at bottom of page.

Thanks! That worked. And now the posts are in order too! I had been frustrated as the entire forum seemed to change months ago and was difficult to navigate. All fixed now.

Thanks for the info folks, the schematic link worked.

What I'm hearing, is that if the chorus is on, the vibrato should work. Does the vibrato sound more like chorus than Tremelo (per Fender amps)?