Part 3: Main Unit Recap – Icom IC-735
The Main Unit of the Icom IC-735 is essentially the “brains and heart” of the radio’s circuitry. It takes the RF signals from the antenna and performs front-end attenuation and band-pass filtering, then mixes them down through its IF amplifiers and detectors. It also handles all audio processing (squelch, AF amplification, tone control, side-tone, etc.), transmit modulation (SSB/AM/FM mixers), automatic level control (ALC) circuitry, and the keying/vox switching. In practice, the Main Unit contains the attenuator, multiple RF/IF gain stages, audio amplifier (IC14, Q55, etc.), the band-switching relays, the ALC and send/receive timing circuits, and the microprocessor-based logic (scanning, memory, VFO/PLL control). In short, everything except the VCO/PLL board and the final RF/PA stage is on the Main Unit. (For example, RF from the filter unit enters J15 on the Main Unit, is attenuated by ~20dB, then passed through band-pass filters and mixers. The AF output is finally amplified by IC14 (16) and sent to speaker/headphones.)
Because the Main Unit houses the audio and control circuitry, bad capacitors here can seriously affect both receive and transmit performance. Failed electrolytics can cause distorted or “muffled” audio, malfunctioning squelch/VOX, spurious ALC behavior, and unstable bias or power rails. In particular, dried-out filter caps let DC rails sag and ripple, reducing gain and even RF output, while coupling caps with high ESR steal treble and add noise. For example, an old filter cap with high ESR acts like extra series resistance: it lets the supply ripple through and drags down the DC voltage. This often shows up as low transmitter power and audible hum. Likewise, aging electrolytics tend to “sound muddy” – users report loss of clarity and high-end in audio. Other symptoms include relays clicking irregularly (from noisy or sagging supply) and weak/unstable ALC (since the ALC timing cap has drifted). In summary, recap of the Main Unit fixes fading audio, squelch or VOX glitches, low TX/RX power, and relay chatter – problems all consistent with bad electrolytics in the IF/AF and power-supply circuits.
Why Electrolytics Fail. Why to Replace Them. (It’s not a question)
Electrolytic capacitors have limited lifetimes, especially under heat. Their liquid electrolyte slowly dries out or leaks, which drastically increases their internal resistance (ESR) and lowers their capacitance. High-ESR caps can no longer hold a steady voltage, so any filtered node (like a DC rail or bias point) will begin to “bounce” or drift. In radio circuits that means audio and RF stages get starved or biased incorrectly. In the IC-735 Main Unit, this shows up as distorted RX/TX audio (from bad AF coupling caps), flaky ALC/squelch (timing caps), and low linear power (weak drive when DC decouplers go bad). Bottom line: caps are not like resistors (which slowly drift a bit) – they can catastrophically go high-ESR or leaky. Any cap that’s old (over ~15–20 years) or bulging should be replaced pre-emptively. This ensures full RF output and clean audio, and prevents serious damage from a shorted cap.
Electrolytic Capacitors on the Main Unit
The IC-735 Main Unit board uses about a dozen electrolytic caps. Below is a complete list of electrolytic capacitors (not including tiny film/ceramic or tantalum caps) found on the Main Unit, with their reference designators, values, voltage ratings, and general function. (Values are per the official service manual parts list.)
C307 – 10 µF / 16 V – Small supply decoupling (logic section power filter).
C308 – 2.2 µF / 50 V – CPU/logic decoupling (fast rail bypass).
C309 – 1.0 µF / 50 V – Logic/CPU decoupling or ALC timing decoupler (short hold-time capacitor).
C316 – 47 µF / 16 V – AF coupling or bias decoupling (audio amplifier filter).
C317 – 22 µF / 25 V – AF coupling or AGC/ALC filter.
C318 – 100 µF / 10 V – Main +5 V regulator output filter (power-supply smoothing).
**C300 – [see note] – ALC hold capacitor (used in ALC timing network).
Note: C300’s exact value may vary (often a few µF) and is part of the ALC attack/hold circuit. Check the service manual or measure it. Because C300 is critical for ALC timing, it should also be replaced if out of spec.
(If you have a recap kit or parts list from a source like Klondike Mike, eBay seller or procured your own, the above values should match the included parts. In case of doubt, compare to the service manual “Parts List – Main Unit” for the exact values.)
Each of these electrolytics serves a known role: for example, C318 (100 µF/10 V) is the largest smoothing cap for the +5 V line; C316/C317 are typically used in the audio/AGC circuits; and the smaller C307–C309 decouple the digital/logic supplies. Replacing all of them ensures no weak links remain.
Finding Each Capacitor on the Board
On the physical Main Unit PCB, each cap is labeled by its Cxx number silkscreened on the board. Use the nearby component references to locate them:
Large electrolytic (C318): This 100 µF can is usually the tallest cap on the board. It’s found near the 5 V regulator IC or the power connector. Look for a circle of big caps – the 100 µF will be the largest.
Audio stage caps (C316, C317): These mid-size caps (47 µF and 22 µF) are often grouped near the audio amplifier IC (IC14 on the board). They are likely adjacent to the audio AF amp or squelch sections. For example, you might see them near the speaker/headphone output filter or close to the AF gain pot.
Logic/CPU caps (C307–C309): These small electrolytics (1–10 µF) live near the logic chips. Check around the microprocessor or near any tall IC sockets. For instance, look by IC3 or IC13 on the board; the small cans (C307/308/309) may be clustered near each other by the CPU reset or clock circuits.
ALC timing cap (C300): This cap is typically near the ALC amplifier (IC16) and keying circuits. If you find IC16 (the ALC comparator/amp), look just around it for C300.
In general, follow these tips: read the PCB legends (e.g. “C318” printed next to the part), and note nearby landmarks. For example, the Main Unit’s band-switching relays (RY1, RY2, etc.) are on one side of the board; caps close to those relays are often the power-supply decouplers. Nearby IC numbers (like “IC14”, “IC16”, etc.) also guide you to the audio and ALC areas. If you have a magnifier, trace the circuit lines: large electrolytics are usually at rail inputs or outputs (near regulators or connectors).
Safe Removal & Replacement (Through-Hole Soldering Tips)
Safety first: Unplug the radio and short the power supply leads or discharge large caps before starting. Wear eye protection. Work in a well-ventilated area (solder fumes).
Document and test: Before unsoldering, note each cap’s value and polarity (negatives are marked “–” on the can). If you have a parts checklist or kit form, check them off. You may optionally measure old caps’ ESR with a meter.
Desolder carefully: Use a quality soldering iron (25–40 W, clean tip) and desoldering braid or pump. Heat the pad, wick or pump away the solder. It helps to heat from the component lead side and apply braid on the solder side. Take care not to overheat any one pad for too long – 3–4 seconds of heating is usually enough if the iron is hot and well-tinned.
Avoid lifted pads: To prevent lifting the copper pad/traces, always heat the component lead first and then the pad. Don’t pry on the part. If solder is stubborn, alternate heating both sides or use more flux. A good tip is to add fresh solder or flux to help wick old solder out.
Remove the old cap: Once both leads are free, gently pull the capacitor out. If one lead is stuck, trim it flush, reheat, and remove. Inspect the holes to ensure solder is gone (use a small drill bit or the iron to clear it if needed).
Prepare the hole (optional): Some repairers thread the new cap leads through a bit of soaking wick on the pad to absorb last solder, then remove it before inserting.
Insert the new cap: Observe polarity! The negative lead (usually the shorter leg and the stripe on the body) goes to the pad marked with a “–” or towards more negative side. Insert so the can sits close to the board (flush), and bend the leads slightly to hold it if needed.
Soldering: Solder on the bottom side. Heat the pad and lead, and apply solder so it flows nicely around the joint. Use only enough solder to form a good fillet. Avoid cold joints – the solder should be shiny. Clip the excess lead flush.
Inspect: Look for good solder joints and no stray strands. Check for bridges or missed joints.
Throughout, go slowly. Let each joint cool before moving to the next. If a board pad seems gummy or lifting, let it cool, reapply flux, and reheat gently. Do not blast high heat or use a large soldering iron – that’s how pads lift. Use desolder braid and a temperature-controlled iron if possible.
Capacitor Replacement Checklist
Work unplugged and discharged. Verify power is off and caps are safe.
Parts & Polarity logged: Mark each replaced cap on the parts list with date. For example: “C316 – 47 µF/16 V – replaced 2025-05-01.”
New cap specs: Double-check you installed the correct value and voltage (higher voltage is OK as replacement).
Solder quality: Every new cap’s solder joint is smooth and shiny, with no excess blobs.
No damage: No lifted pads, broken traces, or scorched board areas. Verify all pads stayed intact.
Alignment: All caps are seated upright (unless specified) and clear of adjacent leads/components.
Visual check: Caps marked minus lead aligned to “–” on silkscreen.
After replacing caps, do one more visual check before reassembling. Wiggle each cap lightly to ensure the joint is solid.
Final Inspection before Powering Up
Ensure no solder bridges or solder wick fibers remain on the board.
Verify all connectors and screws are reinstalled (if removed).
Confirm nothing else was disturbed – e.g. check flat cables, connectors, heat sink contacts.
Optionally, use a multimeter to check power supply rails (with the new caps installed) for correct voltage before full testing.
Once everything is double-checked, reassemble the radio, power up on a variac or with a current-limited supply for the first time (if available). Listen and watch for any abnormal behavior. The audio should be clean, relays quiet, and RF output normal.
By carefully replacing all Main Unit electrolytics and following good soldering practice, your IC-735 will again have solid audio, reliable relay switching, and full power – ready for decades more use.