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Author Topic: fender ultra chorus distortion  (Read 2905 times)

ilyaa

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fender ultra chorus distortion
« on: August 13, 2020, 03:00:10 AM »
hey!

got one of these:

http://www.electronicstudio.net/schematics/Fender/Ultimate_Chorus_Schematic.pdf

works great UNTIL i turn the chorus then -

the chorus itself works and sounds great, EXCEPT if i hit a note hard enough it gets this nasty sounding distortion. like the chorus effect is clipping - not good.

speakers are good - when amp is cranked w/o chorus on, sounds crisp and strong and clear.

any thoughts? not troubleshot an issue quite like this before.....

thank you!

ilyaa

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Re: fender ultra chorus distortion
« Reply #1 on: August 13, 2020, 02:09:38 PM »
its Clear the distortion is coming from the chorus effect -

If I mess with the chorus bias, I can hear it getting better or worse, but I can’t get it to go away. It kind of sounds like the chorus effects internal gain or makeup gain is too hot - but what causes this and how to repair?

joecool85

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Re: fender ultra chorus distortion
« Reply #2 on: August 14, 2020, 10:00:41 AM »
its Clear the distortion is coming from the chorus effect -

If I mess with the chorus bias, I can hear it getting better or worse, but I can’t get it to go away. It kind of sounds like the chorus effects internal gain or makeup gain is too hot - but what causes this and how to repair?

I would verify connections on the opamp going into the chorus.  My guess, either a shorted resistor or a cold solder has let go.  One way or the other, the voltage divider has been broken and it's sending too much signal and getting distorted.  Or the opamp isn't get proper voltage, also causing distortion.
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ilyaa

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Re: fender ultra chorus distortion
« Reply #3 on: August 18, 2020, 04:40:25 PM »
what a strange problem -

i replaced all the chorus-related ICs - checked for loose/cold stuff but didnt see anything

thought problem went away but it didnt.

so just to reiterate: amp works great but once you turn on the chorus effect any loud-ish transients break up in a very non-musical way. more like bad connection/busted speaker cone kind of way. but speakers are good and connections seem fine, too.

the chorus controls do not affect the issue - even with depth all the way down the distortion is still there. as soon as chorus is off it goes away.

the square waves at TP34 and TP35 have a little squiggly oscillation right at the top of each square.

when i scope the input to the power amps, it looks fine on both channels. clean sine wave on clean channel and chorus-y, wobbly sine wave on FX channel, but NO distortion. BUT the distortion is triggered by transients - so maybe i should put a square wave through the amp and see what that looks like...

the +/- DC bias to the op amps is a little high - almost +/- 18VDC when should be 16, but everything else in the amp works fine so im not really suspecting that....

any thoughts???


g1

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Re: fender ultra chorus distortion
« Reply #4 on: August 18, 2020, 08:57:44 PM »
The +/- 18 sounds a bit high for 16V zeners.  Check for ripple on those supply rails.
Also, the schematic does not seem to show where the 3007 & 3101 get their supply from.  Check for ripple on those supply pins too.

Is it possible that this is normal for this model?  It sounds like there is no problem unless the chorus chips get too large of a signal?  You might want to check around and see if other people have complained about this issue for the same model.

joecool85

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Re: fender ultra chorus distortion
« Reply #5 on: August 18, 2020, 09:23:10 PM »
The +/- 18 sounds a bit high for 16V zeners.  Check for ripple on those supply rails.
Also, the schematic does not seem to show where the 3007 & 3101 get their supply from.  Check for ripple on those supply pins too.

Is it possible that this is normal for this model?  It sounds like there is no problem unless the chorus chips get too large of a signal?  You might want to check around and see if other people have complained about this issue for the same model.

If all checks out but this is just poor design, he could always do a voltage divider running into the chorus circuit so that it didn't get quite so hot a signal.
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ilyaa

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Re: fender ultra chorus distortion
« Reply #6 on: August 19, 2020, 02:32:17 PM »
Haven’t found any similar complaints -

I’m not too familiar with chorus chips - here is what seems odd maybe:

The P-P square at pins 2 and 4 of the 3101 and 2 and 6 of the 3007 should be like 15V peak to peak but it’s more like 30V peak to peak -

I’ve got about 1VDC at pin 8 of the 3101 at pin 4 of them 3007. That ok?

It’s confusing because data sheet says pin 8 should be 14/15 of pin 3, but pins 1 and 3 are swapped in this design - with 1 being the supply instead of ground and vice versa with 3....

And at pin 5 and 6 - the input of the 3101 - there is some weird looking oscillation - but thats supposed to be there according to data sheet

Oh, and no noticeable ripple of low voltage rails

This whole thing feels like I’m barking up the wrong tree - but this noise is def chord related and it’s def bad!
« Last Edit: August 19, 2020, 02:44:26 PM by ilyaa »

Enzo

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Re: fender ultra chorus distortion
« Reply #7 on: August 19, 2020, 04:52:16 PM »
Look at the data sheet for the 3101, note it runs on -16v, that is a negative supply.  SO if they connect +15 to ground and ground to the VDD terminal, the IC sees a negative supply.   The VDD terminal is 15v more negative than the +15 supply.

Your MN chips are probably fine.   Remember the chorus circuit path is a lot more than the MN chips.  Certainly U6, and Q6 could be leaky as well.

ilyaa

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Re: fender ultra chorus distortion
« Reply #8 on: August 20, 2020, 01:22:37 PM »
No dice -

U6 replaced, Q6 and Q7 replaced, C41 and C42 replaced, C89 and C88 replaces -

No change - man what is behind this?

The reason I suspect the effect itself is that if I turn the chorus trim pot I can hear the distortion getting worse as i turn it away from the center position in either direction -

g1

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Re: fender ultra chorus distortion
« Reply #9 on: August 20, 2020, 01:58:03 PM »
Schematic shows VDD pins of chorus chips going to ground.
Tough to say if that's the only error in the chorus part of schematic.
If schematic is wrong, that's not going to help.  :(

(edit: my error, schematic is correct)
« Last Edit: August 21, 2020, 03:13:26 PM by g1 »

ilyaa

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Re: fender ultra chorus distortion
« Reply #10 on: August 20, 2020, 02:11:00 PM »
Schematic I posted is correct!

g1

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Re: fender ultra chorus distortion
« Reply #11 on: August 20, 2020, 02:25:47 PM »
delete
« Last Edit: August 21, 2020, 03:14:24 PM by g1 »

Enzo

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Re: fender ultra chorus distortion
« Reply #12 on: August 20, 2020, 03:24:02 PM »
g1, see my post two above your earlier one, is my explanation ffaulty?  I don't think the drawing is wrong.

ilyaa

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Re: fender ultra chorus distortion
« Reply #13 on: August 20, 2020, 06:06:32 PM »
it's all moot now!

i had a FEELING i was barking up the wrong tree - joecool's early hunch was right!

after a ton of pointless tinkering, turns out C47 had come just evverrrr so slightly desoldered. argh! anyway, secured it and we are good to go - i knew it would have been something like that - whenever i find myself too deep down the 'doesnt make sense' rabbit hole, it turns out to be some mechanical nonsense -

thanks, guys!!

g1

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Re: fender ultra chorus distortion
« Reply #14 on: August 21, 2020, 03:11:02 PM »
g1, see my post two above your earlier one, is my explanation ffaulty?  I don't think the drawing is wrong.
No, you are correct.  I got confused expecting a positive supply requirement.  I'm not sure I've ever seen any other chips that run exclusively on neg. supply.
Now that I look at the datasheet, your explanation makes sense.

ilyaa, glad you were abe to get it fixed!