Fender Blues Jr III - FX Loop negates reverb

[ggjgjhkihkjhiuyiy8788]

This is a follow-up to a previous thread on installing a Blues Deluxe FX loop into a Blues Jr III circuit: http://music-electronics-forum.com/t24108/

I have the loop running great- balanced send and return, low noise, no tone loss, etc.- but one problem I am having is that using the loop kills the reverb. The reverb works fine until the FX return is used- then it cuts out completely. Turning the reverb knob does nothing.

The loop is inserted right after the 470k resistor that isolates the reverb send and return. In other words, the loop is just before the reverb return + master volume.

I can't see why the loop would kill the reverb, even when I just run a patch cable from send to return. I guess I could try moving the loop to the other side of the 470k, where the reverb send is, but I'd like to understand why I'd have to, first.

Any ideas?

Thanks,
Jaret

[Enzo]

Hi
I don't have the Blues Junior III drawings, can you provide a link? I am using the regular Blues Junior set in the meantime, I assume they are similar?

You linked to another thread wherein you show a piece of the BLues Deluxe drawing to show its loop, but what we need is a schematic of what you actually did. SInce the reverb send comes off the loop drive, that right there is one difference between the old circuit and what you did.

Off the top, I'd be thinking you either wired one of the jack switches so as to kill that signal, or the reverb gets left unterminated somehow and devotes itself to RF oscillation which translates to silence in the amp.

SO please draw us up a schematic of what you now have, or at least what you think you have.

But we can also troubleshoot. The difference here though is that usually we can assume a malfunctioning amp used to work and has failed. In this case, as far as we know, since the modification it has never worked correctly, so we cannot assume the wiring and parts are all correct. So... isolate the problem. Plug something into the loop - the reverb stops. Go to the drive end of the reverb pan. Is it getting a drive signal? And look at the output end of the pan. Test the return circuit. Several ways. Rock the amp to crash the reverb springs, does that come through? Or pull the cable from the OUTPUT jack and touch a finger to its tip. Get a hum when you do? That will determine if the dead reveb is caused by cutting off the drive, or killing the return.

[ggjgjhkihkjhiuyiy8788]

Thanks for the reply and sorry for the lack of info. For the purposes of what we're talking about here, the Rev III is the same as the previous cream-board BJrs:
Pages from Blues Junior.pdf

The loop is inserted after R18, but before the reverb return connection to the master volume. The loop circuit is IDENTICAL to what I posted previously, except that the 47k resistor on the input voltage divider is a variable 50k trimpot to allow the loop signal to be balanced. I had mentioned dropping the input resistor to 660k, but I've put it back to 1M now, since the loop was quite loud compared to the dry signal.


Next, I should mention that the amp works perfectly without the loop engaged AND it works perfectly with the loop engaged, minus the reverb wet signal. Previously, I had the loop inserted like it is in the Blues Deluxe, but the customer didn't like that the return signal went full blast into the PI, without any master volume control.

The loop is definitely working as it should- I fixed the problems mentioned in the previous thread and it worked post-MV and it's working pre-MV. However, I agree that I need to determine which of the reverb send or return signals is getting lost when the loop is engaged. BTW, the reverb return is definitely connected to the master volume and not broken by the FX loop. Maybe it has something to do with the loading of the fx loop? Can't think of anything else.