Music Tech DIY • Seeking Help/Mentor to Repair My Roland System 100-101 VCO (LFO Leakage, Distortion, Tracking Issues)

Hi all,

I’m hoping to see If there's any experienced Roland Repair Men lurking around here, as I'm seeking some guidance on repairing my Roland System 100-101 VCO, which is having some noticeable issues despite being in nearly mint condition.

I recently acquired this synth while on a trip to Japan, and it's so pristine that I even peeled some protective plastic off the potentiometers! However, as expected with vintage gear, there are a few problems I’m hoping to fix.
The Problem

Here is an audio clip : https://filebin.net/72npmgmir39pjtpi/Br ... llator.wav comparing the 101 VCO (Faulty) with the 102 VCO (Flawless). You’ll hear me switching between square and saw waves for each. Here are the key issues with the 101 VCO:

LFO Leakage:

• The LFO is bleeding into the pitch, causing wobbling or instability.

Distortion/Clipping/Aliasing:

• The sound is distorted, almost like it’s being overdriven or clipped.

• This gets worse as the VCF opens up, revealing more of the harmonics.

Pitch Tracking Issues:

• The oscillator seems to always play a variation of B flat, regardless of input pitch.

• It’s not tracking properly across the keyboard range.

What I Have Done So Far

• Verified that all capacitors look OK (no bulging or leaking).

• Calibrated the power rails to -14V and +14V, as per the service manual.

• Compared it to my flawless System 102 VCO, which confirms this is specific to the 101.

I’m seeking mentorship or advice on how to troubleshoot this issue systematically. I have:

• A Rigol DS1054Z oscilloscope.

• An HP 34401A multimeter.

However, I’m a programmer by trade, not an electronics engineer, so I’d love to learn how to approach the troubleshooting process and identify which components might be failing. Luckily the Service Manual seems pretty clear : https://www.synthxl.com/wp-content/uplo ... -Notes.pdf

If you’re an experienced tech, I’d even be even happier to compensate you for a mentoring session (video call or live guidance) so I can absorb as much knowledge as possible while working on this.
Questions

Has anyone ever heard this before and know immediately what part to check or replace? If not what specific parts of the VCO circuit should I test first?

Could this be an issue with op-amps, transistors, capcitors, resistors, or the VCO tracking trimmer (VR208)?

Any advice for diagnosing LFO leakage into pitch or tracking issues with the tools I have?

Thank you in advance to anyone who can offer insights or guidance! I’m excited to learn more about maintaining this beautiful piece of gear!

Statistics: Posted by OldFashionedSweets — Tue Dec 03, 2024 12:00 am — Replies 4 — Views 106

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