JCM 800 4210 FX loop

Techref

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Hi all

The story goes like this,
I hooked up a EQ via the FX loop.
Engaged the EQ, no sound. Totally.
When EQ is disengaged, there is sound, as per normal.

When it is bypassed using a cable from the Send to Return, there is sound, as per normal.

I'm thinking of replacing the jacks, but it seems like there is something more than just replacing those jacks.
Anyone had similar problems with their FX loop?
I searched the forums and the closest I came across was from the gear page about FX loop with no sound, however without solutions.

Another was here, but also no solutions regarding "noisy FX loop"

I hope to post a solution just in case in the future someone would ask the same question.

and yes, the pedal works. Nothing is wrong with the pedal.
Amp year made: 1982

thanks guys
 

South Park

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I looked at the schematic the power the power from the heaters powers the foot switch and the FX you have some transistors in the circuit that should be look at does the foot switch work .
 

Techref

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Thanks so much for the replies.


The loop on the 4210 is always active. It's not the loop or the jacks.
If it works with your fx in bypass but not with fx active, that suggests to me your fx is the problem

I understand.
I just changed tubes and noticed that it is connected in series to a pot next to the DI Jack.
I have not tried to fiddle with that pot yet. Is it a volume control for the FX loop?
 

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johan.b

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This is your loop... in red.. that's all it is.. the blue components are setting the level but expecting hi z input on your fx.. this makes it noisy and not consistent with different pedals/ brands.
I've suggested on other threads on this topic that you install a modern loop just were the original loop sits... it would sort the impedance issues and probably most noise issues too ... but that's a semi educated guess from my part...
Screenshot_20211107-140033_Samsung Internet.jpg
 

Techref

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I looked at the schematic the power the power from the heaters powers the foot switch and the FX you have some transistors in the circuit that should be look at does the foot switch work .

yes
The foot switch works.
 

Techref

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I've suggested on other threads on this topic that you install a modern loop just were the original loop sits... it would sort the impedance issues and probably most noise issues too ... but that's a semi educated guess from my part...
View attachment 97442

Thanks.
I got it.
That is actually what I plan to do if all else fails.
 

Tatzmann

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I can attest that the loops on 800
Channel switchers are very picky
about what you try to feed it with.

One Rackmount or floor device
works perfectly without noise
whatsoever, the next one is
noisy to unusable noisy, even
when there is impedance select
on the device!

But i only have experience with
the later 80s switchers, they differ.

Try a different EQ Device.
 


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