AC-15 Vibrato Issues

[Jacobysmalls]

Hey Folks,
Wondering if anyone here has experienced a similar issue to what I'm experiencing:
I've got a home-built Sluckey AC-15 someone else built and brought to me to solve a weird issue with that stumped them. It sounds amazing, except for when the vibrato is engaged. Wether or not the oscillator is engaged, the Vib/Trem channel sounds choked and bad with the switch on Vibrato.
I scoped it out and found a nice sine wave up until after the Modulator stage, at which point the signal gets strange. At lower frequencies it appears to do a 'frequency doubling' effect when the Vib switch is on, at higher frequencies not so much. But at 1kHz, the sine wave does a funny thing. Here's what the 'scope is showing, can anyone tell me if this looks like what I should be seeing here?
Thanks so much
Pics are of a 1kHz signal at the anode of the modulator stage with the Oscillator off, first is Trem mode second is Vibrato mode.

[Jacobysmalls]

Sorry, here's the photo of the scope in Tremolo mode.
Thanks for any help!

[sluckey]

Here's the theory on that vibrato circuit. Lot of phase shifting going on...

     http://thermionic.info/dorf/Dorf_TheWurlitzerVibrato.pdf

[Jacobysmalls]

Thanks Sluckey! I read through that and it was really helpful. What I'm seeing on the 'scope appears to be around 90 degrees out, as it should I guess.

What I'm wondering at this point is: is the amplifier performing normally? I haven't ever played this circuit before, maybe this is just how it sounds. I have to say in Vibrato mode, it sounds choked and distorted in a digital way, like the back end of every note is being chopped off. when the oscillator is engaged the effect covers this up a bit and it sounds better, but with the switch on Vibrato and the oscillator disengaged it is pretty bad.

Otherwise this amp is a dream! Tremolo works and sounds great, and the EF86 channel is really wonderful.

[sluckey]

It's been a while since I've played my AC15. I do remember the vibrato channel not being as loud as the normal channel. The normal channel has always been my favorite. The vib channel was always more of a novelty to me, ie, I was curious about how the phase shifting vibrato would sound. It sounds OK to me, but nothing great. I have other vibrator circuits that I like better. I recall the tremolo mode was a bit clearer than vibrato mode, but that's about all I recall. Like I said, I prefer the normal (ef86) channel for the Vox sound. This is the only AC15 I have ever heard so I can't really say how it compares to the original.

I'm hoping Ed Chambley will chime in. He still has good ears and has built several of these AC15s. I bet between him and the other people he has built this amp for, you can get a better description of how the circuit should sound.

I don't see anything alarming with your scope pics. In this case, I think some sound samples would be more telling, especially to Ed.

[Jacobysmalls]

Thanks Sluckey.
Yeah, the normal channel in this thing is one of the best circuits I've ever heard.
I'll work on getting some sound clips up.