MicMix MasterRoom XL-305 Spring Reverb Clone

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mediatechnology
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by mediatechnology »

Chris sent me some demos he's working on and I'm blown away.
I'm really looking forward to receiving mine.
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mediatechnology
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by mediatechnology »

Chris now has a product page up at Audio-Scape: https://www.audio-scape.com/products/xl-305r

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mediatechnology
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by mediatechnology »

Looks like the first run of 20 units sold out in under 2 minutes. https://realgearonline.com/post/280190
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ajk519
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by ajk519 »

Hi Everyone,
This might not be the correct place post about troubleshooting issues with an XL-305, but I figured I might be able to get some help here since this thread has a discussion about the Xl-305 internal circuitry.

I recently received a crusted and busted XL-305 with a broken power switch, missing fuse holder, dirty pots, etc.
Before I dive deep into troubleshooting questions -- I would like to ask some basic questions about the XL-305, since I can't find good demos of the XL-305 in use online / good written documentation of its functionality.

I've read that the XL-305 Left and Right spring reverb 'chambers' have different timing characteristics to achieve a natural reverberant sonic profile, however does the XL-305 have completely identical input and DRY/WET output mixing signal circuitry/paths for both the Left and Right channels?

The right channel mix output potentiometer on this XL-305 is not properly controlling the right mix output, and I am trying to understand why.

There isn’t any dry signal coming through when I have the right channel 'MIX OUT' pot at full CCW. When the mix output pot is fully CCW, it sounds as though there is simply a less ‘reverberated sound' coming through the right channel output jack (both XLR and 1/4”), compared to the output if the right channel 'mix output' knob were turned partially or completely CW (again however no dry input signal).

The left channel behaves as I expect it to -- reverberant sound cuts out when the left channel mix output knob is rotated fully CCW leaving only the dry input signal on the left channel output jacks.

This situation of not having any dry signal coming through on the right channel has me thinking that there is an issue somewhere between the balanced input/buffer (U3C/U4C) to the (K/J) jumper points that feed into the right channel 'MIX OUTPUT' knob and 'Mixing Amp'/Line Output Buffer stage (U3D/U14B).

Unless this is by design in the XL-305 -- does anyone have any ideas or tips on how to narrow down where the dry signal disappears in the right channel circuitry?

I have an oscilloscope and a signal generator that I've been trying to use / work backwards from the output. However since there is an output on the right channel, it is hard to isolate when or where the dry signal component of the right channel MIX OUTPUT disappears.

I've already fully recapped, reworked the power supply, and replaced the front panel control potentiometers with contemporary replacements.

Thank you for reading any help would be appreciated!
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mediatechnology
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by mediatechnology »

Thanks for posting and joining us!

The left and right mix controls should behave identically.

Take a look here for schematics: viewtopic.php?f=12&t=197&p=4101

Specifically here: https://www.ka-electronics.com/MasterRo ... _Tile3.JPG

Is there dry signal on the left-hand side of R71 or whatever the designator is for the right channel?
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ajk519
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by ajk519 »

Late follow-up on this, however thanks for the suggestion -- I did end up tracing the issue down to an out of spec resistor at R171 in the right channel wet/dry mixing circuit.

Now the channels are more or less behaving identically except that the right channel reverb signal seems to be slightly quieter and less defined regardless of dry/wet mix knob position or attempts to level match using the XL-305 input level knob or at my mixer once the signal leaves the 305. The right channel reverb is just quieter all-around, It's as if maybe the spring signal recovery amplification stage ('ret. gain') is not calibrated to match the left channels return/recovery stage.

Return gain calibration can be made by adjusting the R43/R143 50kohm trimmers.

Anyone here have an idea how to calibrate these amplification stages to factory or what these variable resistors should ideally be trimmed to? In principle I could measure the left channel trimmer resistor value in its current position (since it sounds more 'true') and try to match the right channel trimmer to the left trimmer value? Guess I could always do it by ear too.

Return Gain at the 'Receive Input' and Meter calibration appear to be the only relevant calibrations -- I haven't found any official documentation for the 305 calibration online, but I'm hoping someone can provide insight on this.

My best,
A
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mediatechnology
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Re: MicMix MasterRoom XL-305 Spring Reverb Clone

Post by mediatechnology »

Is R171 the same value as R71? Were they 4K7?

WRT level I would always use white or pink noise as the source for level-setting since tone will produce gross errors due to the natural comb filtering.
Measure at the output of U10A/B and compare the two.
FWIW the channels will sound slightly different since their timings are different and you may see and hear a small level error between channels.
Make sure the drive levels are about the same and if they are then adjust the trims for equal level.

One of the EQ sections on the right channel could be inoperative.
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