Roland JX-3P Filter Resonance Boost Mod DIY: Detailed Walkthrough and Sound Demonstration

video by thesrabbit

follow-up to this post.

"Disclaimer: I cannot be held responsible if you try this out and break your gear or yourself. Please be careful! This mod requires the synth to be powered on with the panel open as you make adjustments to the trim pots. Please do not touch anything inside the synth other than the trim pots, and only touch the trim pots with your trim pot tool. I used a special tool for this, but I think it may be possible to use other tools as well.

This video is meant to show you how I accomplished my particular goal. It does not mean that this is THE correct way, if there is such a thing. I'm a total DIY noob and have never used an oscilloscope before. Please leave a comment if you know something important about safety or want to call me out for anything that I'm doing which is clearly wrong.

Contents:
0:00: Intro
1:35: Preparation and disassembly
3:00: Opening the top panel
4:07: Trim pots
6:54: "TP7" oscilloscope probe point
7:37: Connecting the scope
10:53: Test mode
12:54: Tuning the VCF
18:25: Boosting the VCF resonance
21:55: 800 mV
25:55: 850 mV
26:46: Soundcheck and short demo
31:00: Oscilloscope epiphany
32:53: 1500 mV (for shiggles)
34:50: 900 mV
36:40: 1000 mV
41:35: 1200 mV
46:38: Thorough soundcheck with sweeps
56:02: Factory presets check
1:09:30: Custom patches

Background:
I recently got a new to me Roland JX-3P. I immediately loved the raw sound, but was a bit let down by the resonance. It seemed to barely self oscillate, and polyphonic sweeps at high resonance were not very smooth, as if certain harmonics were disappearing. After a bit of researching the internets, I discovered that many people have boosted the filter's resonance by adjusting the trim pots for each voice inside the synth. Roland probably provided these pots as an easy way to service it, perhaps before they made components that were more stable? Whatever the case, it makes it very easy to boost the res and do other stuff if need be.

I decided to boost the res and tune the VCF as best as I could. To do this, I followed the instructions on Florian Anwander's site. I decided not to do this by ear, as I wanted to be a little more precise and try it out at different levels. I bought a super cheap digital oscilloscope and read the service manual to figure out where to attach the probe for making filter adjustments.

The service manual states that for resonance, the scope reading at TP7 for Test Mode A2 should be 600 mV for each voice. I had the initial goal of taking this to 800 mV. I ended up settling on 1200 mV, assuming I setup my scope correctly, which I think I did on day 2 of this experiment. Please tell me if I did it wrong!

I also used the scope to tune all 6 VCF's to each other with the "VCF Tune" trim pots. I double checked the results with a tuner. In the end, I wasn't able to get them all perfectly in tune, but it is certainly much better than before, and perhaps this explains the dropouts in harmonics that I was hearing previously with polyphonic sweeps.

So does it sound better or worse after boosting? To me, it's more versatile, which makes it better. It takes the 3P into squelchy territory with rough edges that it couldn't quite get to before. I don't have a classic Juno (6/60/106) to compare it to, but I think where I ended up is probably pretty close. Let me know if not so I can go back in and pump it up some more. :) Some of the sweeps after the mod remind me of the System 500 - 521 filter module. Although the res on that filter is more prominent, the way certain frequencies break-up and the quirks seem to be similar. You can check that filter out (and 7 others, including the JX-3P pre-mod) in my #FFS​ Extravaganza vid. There's no question that some of the presets are ruined as a result. These are highlighted in the video. You can fix them by dialing the res back with the parameter control on the front panel or hardware programmer. Unfortunately, the A and B banks cannot be overwritten, so you'll need to make the adjustment each time the preset is recalled. In the end, I'd rather have too much and have to back off than not have enough.
I'm happy with the outcome so far."