Reviving the IC-735 — Part 3: The Real-World Impact of the Op-Amp Upgrade
In Part 1 and Part 2 of this op-amp upgrade series, we mapped the Icom IC-735’s aging analog signal path and began replacing critical ICs with modern, high-performance equivalents. Today, we wrap it up—not just by finishing the work, but by exploring what it actually means in daily operation. What changed? What didn’t? And why might this be one of the best-value mods you can make to your vintage HF rig?
Confirming the Functions of the Upgraded ICs
The original op-amps in the IC-735 were selected in the early 1980s, designed around the performance thresholds and pricing limits of that era. In 2025, we’re able to revisit those choices with better options—parts that are quieter, faster, more linear, and better suited for today’s demanding operating conditions.
Here’s what we actually upgraded, with functional roles confirmed directly from the service manual:
IC7: Original 4558 — Used in the FM modulator stage.
→ Replaced with NE5532, a low-noise bipolar op-amp ideal for baseband and audio applications.
Impact: Improved modulation clarity and stability.IC11: Original 072082D — Squelch/mute signal amplifier.
→ Replaced with OPA2134, a modern JFET-input op-amp.
Impact: Lower noise floor and cleaner squelch transitions.IC12: Original 4558 — Squelch comparator.
→ Replaced with a 5532, another solid bipolar op-amp.
Impact: Sharper squelch thresholds with reduced noise artifacts.IC15 & IC16: Original 5218L (SIP-8) — Handle ALC and AGC signal conditioning.
→ Both replaced with NE5532 using SIP8-to-DIP8 adapters.
Impact: Faster ALC recovery, improved AGC control on RX and TX, especially on CW/digital.IC17: Original 4558 — VOX and anti-VOX control.
→ Replaced with NE5532.
Impact: Improved VOX trigger reliability and cleaner switching.
Why These Op-Amps?
Every replacement was chosen with a purpose:
NE5532: A time-tested bipolar design that excels in low-noise, low-distortion linear circuits—perfect for audio and modulation stages.
OPA2134: A modern JFET op-amp with high input impedance and excellent phase linearity—well-suited to control functions.
This isn’t about installing trendy chips. It’s about tailoring the right part to the right job.
The Real-World Impact
Will these upgrades make your IC-735 behave like a modern SDR? Of course not.
But will they elevate your operating experience from “muffled and moody” to “clean and confident”? Without a doubt.
This IC-735 now includes:
A complete electrolytic recap, restoring power integrity throughout the signal chain.
A replaced and realigned VCO section, with new trimmers and full shielding.
A new 12.8 MHz TCXO, with C138 removed to reduce frequency drift.
Six carefully chosen op-amp upgrades, each improving noise, response time, or modulation fidelity.
On-air performance backs it up:
FT8 decode performance is now rock-solid. Where the stock rig missed decodes or drifted mid-transmission, the upgraded unit delivers stable, consistent signals.
SSB transmit and receive audio are both cleaner. On receive, you’ll notice more signal and less hiss. On transmit, you’ll hear tighter, punchier audio—especially when paired with a good mic and EQ.
On 60 meters, the rig previously struggled with noise and tone stability. Now it sounds like it belongs on the band.
I’ve gone from making the occasional SSB or FT8 contact to pulling in 5–7 solid QSOs during a CQ run on a vertical. I’m genuinely curious what this rig would do on a portable expedition from a park.
Lessons from the Bench
While DIP-8 swaps were straightforward, the SIP8 replacements were not. Both IC15 and IC16 had to be carefully desoldered, and in the process, two pads tore completely, pulling through their vias. Trace repairs were required before socketing the SIP8-to-DIP8 adapters.
That said, upgrading six op-amps—including three with modern 2025-grade components—is a solid step forward. And this is all on a rig that’s already seen a full recap, VCO stabilization, and improved thermal handling.
Final Upgrade Breakdown
Here’s what’s currently installed:
IC7, IC12, IC17: 4558 → NE5532
IC11: 072082D → OPA2134
IC15, IC16: 5218L (SIP8) → NE5532 on SIP8-to-DIP8 adapters
Every chip matters—and the sum total delivers real-world results.
Value-for-Value
If you’ve used this guide, learned from it, or it helped you bring an IC-735 back to life, please consider supporting the project: Buy me a Coffee
Your support helps keep this work independent, detailed, and freely available to the community.
Coming Up Next…
This isn’t the end—it’s the launchpad. We’re about to dive into the rabbit hole of mods from hams around the world—Japan, Germany, Eastern Europe—who’ve shared brilliant ways to reduce ripple, clean up the power rails, refine control circuits, and make the IC-735 a truly next-level classic.
Because this rig deserves more than “good enough.” It deserves greatness. Listen for yourself.
Tags: IC-735, ham radio, op-amp mod, NE5532, OPA2134, classic HF radio, radio restoration, DIY ham radio, SSB performance, FT8, IC-735 recap, TCXO upgrade, radio alignment, substack ham radio