Fender M-80 Chorus clipping.

[Cuetek]

Hi, folks.

I have a Fender M-80 Chorus amp that I have personally brutalized for close to 40 years without a hitch. Well I was blazing away recently with nearly everything it had and noticed something didn't sound right. Investigation proved there to be a faint clipping much more noticeable at low gains and pretty much the same no matter which channel (clean or distortion) I used. Even with all the volumes turned way down there is a noticeable background clipping. It's like I can hear the full clean signal getting through, only something has added a faint clipped version and mixed it in at about 10% of the signal strength and remains at that level no matter what volume you play at, even though it's less noticeable if there is a lot of clipping from the gain channel. One more thing, it plays in the headphones too, so I wonder if that removes the power amp from the equation? (don't have a schematic).

-Cue

[g1]

Near the bottom of each thread, there is a section called 'similar threads'. For this one it leads to a schematic here:
http://music-electronics-forum.com/a...-80-chorus.pdf

40yrs. already? For some of us we think of these as the 'newer' Fenders.
The fact that the problem is in the phones at least rules out the speaker. If you plug your guitar into the FX return, do you still hear the problem?

[Cuetek]

Near the bottom of each thread, there is a section called 'similar threads'. For this one it leads to a schematic here:
http://music-electronics-forum.com/a...-80-chorus.pdf

Copy that. Thanks, G1.

40yrs. already? For some of us we think of these as the 'newer' Fenders.

More that I personally put all 40 into it (original owner). It's my kick around amp.

So, yeah, it's still there in both the "Mono" and "Stereo" return jacks (pretty sure they are the same as FX return). My power stage?

Also, not certain about this symptomology, looking back, but it seemed like the problem started only when the chorus was kicked in, but now it's there no matter what.

-Cue

[g1]

Can you see if it is just on one side or both? I think if you go halfway into the stereo return you will get one side, all the way in you will get the other side.
If it's in both sides, it's probably power supply related.

[Cuetek]

Can you see if it is just on one side or both? I think if you go halfway into the stereo return you will get one side, all the way in you will get the other side.
If it's in both sides, it's probably power supply related.

Thanks for this suggestion, G1. Very interesting outcome. I pushed in the mono plug from my guitar into the stereo return all the way in, and I could hear the corrupt signal in both ears but louder in the left. I took it out and pushed it barely in at all and got a clean signal in the right ear and the noise only in the left! If I push a little further, there was a sweet spot halfway where the corrupt signal was in both ears equally (both hot contacts in the jack touching the mono plug tip). But most interestingly, if I held it carefully enough, inside the parameters of the sweet spot was another sweet spot where the noise disappeared and both channels were clean! Also, the noise doubled in volume for all inputs if I activated the chorus effect.

Something to do with the switch contacts in the jack, but looking at them on the schematic, it looks like it should work in the opposite way. That all the way plugged in should be clean of all preamp influences and fooling around with the sweet spot should cause the noise to appear via the jack contacts connecting the outputs from the preamp buffers. So, I'm perplexed (not uncommon for me).

-Cue

[g1]

This is rather confusing and perhaps will mislead us.
Maybe try this: disconnect the left speaker, is noise present?
Now reconnect the left and disconnect the right, is noise present?

[Cuetek]

This is rather confusing and perhaps will mislead us.
Maybe try this: disconnect the left speaker, is noise present?
Now reconnect the left and disconnect the right, is noise present?

The noise is present in both channels whether only the right, only the left or both speakers are plugged in.
Also, the noise is a beat frequency of the input signal and some other signal, not a direct clipping of the input signal. That is, if I bend a string the frequency of the noise increases or decreases depending on the note I'm bending. The lowest B on the neck seems to be the frequency at which the beat frequency slows almost to a stop. (is that B at 60Hz?) Also, if I put the headphones in the mono send jack, there is no noise.

-Cue

[g1]

I pushed in the mono plug from my guitar into the stereo return all the way in, and I could hear the corrupt signal in both ears but louder in the left. I took it out and pushed it barely in at all and got a clean signal in the right ear and the noise only in the left! If I push a little further, there was a sweet spot halfway where the corrupt signal was in both ears equally (both hot contacts in the jack touching the mono plug tip). But most interestingly, if I held it carefully enough, inside the parameters of the sweet spot was another sweet spot where the noise disappeared and both channels were clean! Also, the noise doubled in volume for all inputs if I activated the chorus effect.

Sorry for the delay, I'm still trying to wrap my head around this. Can you verify that you get the same results as above with the speakers rather than phones?

Also, if I put the headphones in the mono send jack, there is no noise.

And with the phones in the stereo send is the noise there? Both sides or just one?

[Cuetek]

Sorry for the delay, I'm still trying to wrap my head around this. Can you verify that you get the same results as above with the speakers rather than phones?

I did the noise test with the speakers themselves, and the noise is present in each separately and in both together.

And with the phones in the stereo send is the noise there? Both sides or just one?

Both ears of my stereo headset hear the noise in the stereo send jack.

Since the mono send is clean and the noise worsens when the chorus is activated, I'm pretty sure the problem is somewhere in the chorus circuitry. As a last resort I can isolate it and split the mono signal into the U6 output buffers and just lose the chorus. But behind the noise, the chorus is working fine. Be sad not to have it.

-Cue

[olddawg]

I did the noise test with the speakers themselves, and the noise is present in each separately and in both together.



Both ears of my stereo headset hear the noise in the stereo send jack.

Since the mono send is clean and the noise worsens when the chorus is activated, I'm pretty sure the problem is somewhere in the chorus circuitry. As a last resort I can isolate it and split the mono signal into the U6 output buffers and just lose the chorus. But behind the noise, the chorus is working fine. Be sad not to have it.

-Cue

I don't know what it's like in your area but I see these amps on CL all of the time for around $100. Less with no speakers. If you really like it you might want to consider a donor amp.

[mozwell]

I may be wildly wrong, but this would be easy to check. It may be that some of the chorus LFO is getting into the power supply, and then into the pre-amp signal. I would suspect any low value electrolytics used for power supply bypassing.
We can check be soldering another capacitor in parallel. If the problem goes away, we know what the cause was.
Around the chorus section we have C35, C8, C24, C30, C32, also try C61, C62, C40, C42, C43
Other low value electrolytics could be changed if you have the amp open anyway. They dont cost much.....

[Cuetek]

It may be that some of the chorus LFO is getting into the power supply, and then into the pre-amp signal.

That is the best (because only) diagnosis so far. Definitely worth a try. This one has gotten to be one of those that I'll either find out what's wrong or die trying.

Thanks, Moz.