"Effects Loop Level toggle" only works when half-depressed, just like amp techs

[Mr_bibbles]

Hey folks....

Had a question about an issue I've found on two different amps and wondering if there is "common knowledge" about this failure that I'm too inexperienced to know;

On both a Peavey XXX and a Crate BV60h, The FX loop level toggle switch has no effect EXCEPT for when it is halfway through it's push cycle;

Meaning when I push the button in halfway, even before the "Click", I can hear the level drastically increase... But then, before it locks into the next toggle position, the level drops back down. The actual output level is the same in the "in" and the "out" positions, meaning that it doesn't actually toggle the pad.... It only releases it for the split second that the switch is moving BETWEEN "on" and "off"

This means obviously that the amplifier is always acting as though it has a pad engaged, so we have a malfunctioning amplifier that is always on "pad".

In your experience, is this usually just a simple "replace the switch" job? Or does this have to do with some strange issue with the controlling transistors?

Thanks!

For reference:

https://www.thetubestore.com/lib/the...-Schematic.pdf

https://www.thetubestore.com/lib/the...-Schematic.pdf

[eschertron]

If I knew what I was talking about, I'd suggest that the 'loop level' control adjusts for unity gain in either position. What it actually is doing is not changing "level", but changing headroom. if a pedal is distorting in one position, press the switch to get more headroom. If the pedal sounds thin and noisy, press the switch to feed more signal to it (less headroom). That's my edu-ma-cated guess. You're welcome.

Oh, and you're right about techs

[Enzo]

Esch got it, the switch does not make it louder or softer to the ear, it changes the level that the send AND return use. If the switch makes the send louder, it then also reduces teh gain of the return to accommodate the hotter signal. And if teh send is a weak signal, then the return raises the gain to compensate for the lower level. In other words it is working right. When you game the system by pushing the switch half way or something, you are managing to get the send louder without going all the way to reducing the return too.

There is nothing wrong here, there is nothing to replace, nothing to test. The transistors are fine. It is not a failure.

Well, it is a failure of sorts. It is a failure to understand the function of the circuit. Look at your Crate schematic, it is fairly clear. As drawn the switch is in the OUT position. look at the left half.- the SEND side. Note it loads the signal with a large 100k resistance, not much of a drag. That means the SEND will be the hotter mode. On RETURN to the right, at the same time note we have padded the signal down with a 3.3k resistor in parallel with R63. That draws the signal level down substantially. So hot signal out, padded return. Now push the switch IN. Now the SEND is dragged way down by a 330 ohm resistor, but over at RETURN, that 3.3k is switched out, so the level goes higher. So low signal sent, higher gain on return.

The switch was never intended as a gain boost. It is there to match your effects units to the loop. Your floor toys expect a weak signal from a guitar, so if you use them in the loop, the signal to them needs to be weak, and the return fairly gainy to compensate. But if you use rack effects - line level effects - in teh loop, then they expect line level, a fairly hot signal. And they return a hot signal which the return pads back down.

[g1]

Same as said above, there should be no change when amp used as complete system. I think if you used send to another device only, you would hear a difference when throwing the switch. Or if you just used the return jack sourcing from some other preamp.
The boost you hear momentarily while throwing the switch would be when the contacts break before reconnecting to the other set of contacts.

[Mr_bibbles]

There is nothing wrong here, there is nothing to replace, nothing to test. The transistors are fine. It is not a failure.

Well, it is a failure of sorts. It is a failure to understand the function of the circuit. Look at your Crate schematic, it is fairly clear.

Oh my lord I'm in stitches over here. May I congratulate Enzo on probably the sickest burn ever delivered with the MEF! Even so, this is extremely helpful information, I do really appreciate your diligence and generosity when it comes to explaining the finer points. I'll take another longer look over the schematic and suss out exactly what's going on with the routing. Appreciate you guys, especially you Enzo for the in -depth explanation.

cheers!

[Enzo]

Well I certainly didn't intend it to be mean. I was just going for a clever turn of a phrase.

[Mr_bibbles]

Well I certainly didn't intend it to be mean. I was just going for a clever turn of a phrase.

I know! I found it funny and endearing rather than mean. Your posts always leave me with a sense that I don't know a damn thing about my profession, and honestly, that's the best thing you could ever ask for in a teacher, so KUDOS!