Vox Beatle reverb issue

[goldstache]

Wrenching on a buddies vox Beatle amp, and have a head scratcher.


Massive overhaul.
All electrolytics.
About 12 transistor beans including all the switching fets.
Power switch/AC mains updated, etc.


After copious amounts of swapping, all functions restored, but for one thing.


On the normal channel, if I assign reverb to it, gets dirty and ducks volume.


If I assign the verb to the brilliant channel, it just drops volume but not dirty.


And…..
The normal channels trem depth ducks and distorts similar.
Wafer switch, appears to be good.


Any ideas.


Mixer transistors have been swapped.

[shooter]

Any ideas.

working on SS amps without a scope is like duck hunting blindfolded 
not really an idea, but it's early

[goldstache]

I have the tools.
And it traces out ok.
Just can’t understand why when the verb is switched in, loads down signal.
And why the normal channel attenuates when the trem depth is increased. That’s why I suspected mixer stage.
All lytics that stage isolate and filter have been swapped, and voltages are pretty close to spec, despite transistor subs.
I suspect that the switching fets are to blame despite the reverb being a mechanical switch.
If anything jumps to mind, please make mention.
Thank you

[PRR]

Instead of a blind link with "UNSIGNED-PAYLOAD" in the URL, let's host that schematic here.

[goldstache]

Struck from post.
Thanks!

[shooter]

when you switch RV on, does the 24vdc load down?

[goldstache]

Nope, rails are steady.


Some numbers to help.


Input 100mvac at 1K
Output of normal channel (clean, no distortion booster circuit in) 1.6vac after C113 (.0068uF) 600mvac
A tad less at lug 3 of channel volume 500mvac.


Switch reverb in series with that channel.
 47mvac as soon as switched at input to reverb.


Same lowering on brilliant with reverb engaged.


All coupling caps stage to stage are new. No dc leak present, that I’ve located.
Heck of a load presented when switching verb in series.


Then there’s the trem on normal channel ducking volume.


Can’t seem to get any swing out of oscillator. When depth maxed I get wiggle but barely any volume.
Voltage read pretty good.
But still not much wiggle to see or measure. Huh

[goldstache]

It does oscillate when depth is at 7 or above.
So my ac wiggle seems awry. And isn’t shaking the mixer and dry guitar gets mixed to near nothing.
First two stages of trem (dry buffer and wiggle feed transistors are 2n5088. All mixer section transistors are 2n5088 as well.

[goldstache]

Had wiggle up to the bridge rect. Only one leg oscillating.
Swapped diodes, despite testing fine.
Trem is back.


Which leaves only the dropout when reverb is assigned to a channel.

[shooter]

put channel 1 scope on VOL wiper, disconnect the tank input P201 and connect channel 2 to the tip.. REVERB OFF at FS
Inject a test signal
should see an amplified version of channel 1 on channel 2
Now RVB ON at FS
any signal change on ch 1 or ch 2?

[goldstache]

Thanks for contributing Shooter!


After much scoping, it turned out to just be classic loading.  I had to rebias the Input stage of the verb driver.
Still drops volume a tad with reverb selected inline. But adjusting that transistor gain stage, got me back the buffered dry (Emitter follower signal). And a good balance of wet return.
Before mod, it would turn a 300mvac input into a 38mvac input at verb blend knob.
After mod, 300mvac reverb input yields a 270mvac at verb blend knob.


I suspect the verb circuit is a little wonky yet, I get about 10VAC into the tank, and around 20mvac to the base of the recovery. That seems pretty par for the course.
At blend I now have around 300mvac per side (Dry/wet).  So when verb is switched in, it attenuates audio from 300mvac to about 270mvac (both channels, when loaded with verb). I can live with that.


Thanks very much.


Surprisingly sweet solidstate amp. Has nice sustain and harmonic content. Even blooms into feedback when notes are held.
Then, there's all the bells and whistles effects onboard.

[goldstache]

Been digging on the brilliant channel of that Beatle amp, so I decided to breadboard that channel (dry) from input to mixer section output.


I like it, got the percussions and MRB fashioned up as well.


One thing that has me puzzled is that the bass control comes on strong then does barely anything through its travel. I’m using 10ka currently to help with early onset of bass.


Any ideas?

[PRR]

Bass part works against R125 1K. So about 9/10 of the bass pot does very little. The "fix" is obvious and no-cost. I would say that is how the designer wanted it.

[goldstache]

Thanks, sorry I was speaking of the brilliant channel.
But it bears repeating….intended.

[PRR]

Yes, there also. Both BASS networks look dubious to me, unless VOX just needed "a knob which does something to the bass" and didn't care how useful it was.

However VOX was doing excellent keyboards and tube guitar amps, so you'd think they would know better.

Do VOX fans say these values are authentic?

[shooter]

question, Do the direction arrows on the pot wipers indicate "more"?

[sluckey]

question, Do the direction arrows on the pot wipers indicate "more"?

For those arrows to have any truly unambiguous meaning you need to indicate CW or CCW.

[goldstache]

After breadboarding it up, I found that both treb/bass were audio taper, to give even results. A1K/A10K.


Darlington feeds lug 3 of treble, and lug 3 of bass is the grounded lug.

[goldstache]

Here’s the brilliant channel cloned out.

eature=shared