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Midland Alan 87 Modifications: Clarifier Unlock, Audio, Power, Band Upgrades, EPT3600 PCB

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User question

alan87 mod

Artificial Intelligence Response

Direct answer to the question

  • “Alan 87 mod” typically refers to common service and performance modifications for the Midland Alan 87 export CB transceiver (25.615–28.315 MHz, AM/FM/USB/LSB/CW, A–F band switch).
  • Typical, safe areas: refurbishment/alignment, RX audio/selectivity improvements, controlled TX audio/ALC setup, clarifier behavior, and usability features (e.g., talkback).
  • Please tell me exactly what you want to achieve (e.g., unlock clarifier, improve audio, fix low power, add +10 kHz, extend bands), and your test gear. I’ll provide step‑by‑step, board‑rev‑specific guidance.

Detailed problem analysis

  • Platform overview
    • Architecture: classic export chassis with PLL synthesizer (commonly MC145106 family), dual‑conversion IF, balanced SSB modulator, discrete driver + bipolar final (often 2SC2312 on Alan 87), audio PA (e.g., TA7222), and A–F band matrix.
    • PCB families: Many Alan 87 units share lineage with the “EPT3600” style main board used in multiple export models; trims and designators vary by revision. Do not assume trim‑pot numbers without confirming the exact PCB print.
  • What most owners call “mods”
    • Refurbish and align (recommended first): Replace aged electrolytics in PSU, audio, and IF paths; verify regulator rails; clean/reflow VCO can and PLL area; then do a full RX/TX alignment (VCO lock, IF filters, carrier balance, ALC, FM deviation). This restores most lost performance.
    • Clarifier behavior: Stock sets often bias the clarifier for RX only or limited TX range. The “unlock” concept is to feed the varactor with a constant regulated voltage so TX and RX slide together. Afterward, the synth must be realigned so center‑detent is on‑frequency. Implementation details vary by PCB; confirm schematic first.
    • TX power/ALC cleanup (peaking without abuse): Set AM carrier conservatively (leave headroom for modulation swing), verify SSB ALC for clean PEP (two‑tone), and set FM deviation to the standard ±2.5 kHz. More power without thermal headroom simply flattens audio and stresses finals.
    • RX selectivity/audio: Upgrading the 455 kHz ceramic filter to a tighter Murata part improves adjacent‑channel rejection. Recapping and verifying AF PA bias often yields cleaner, louder audio.
    • Usability: Non‑destructive talkback (small resistive mix into the audio path) helps adjust mic/echo boards; keep gain modest to avoid feedback. Many units already have a +10 kHz function; where absent, operators sometimes repurpose a front‑panel switch to toggle the relevant PLL code bit—again, board‑specific.
  • Risks to manage
    • VCO lock range: Any PLL/band changes must keep the VCO in lock over temperature and supply variations. An unlocked VCO produces spurs and “chirp.”
    • Thermal management: The PA is already near its safe operating area in a compact chassis. Raising AM dead‑key or SSB PEP without heatsinking and bias checks accelerates failure of driver/final.
    • Board‑rev variance: Component IDs (e.g., VR12, VR13, Dxx) differ across runs. Always map the schematic to your PCB before moving wires, lifting diodes, or changing links.

Current information and trends

  • Regulatory context (United States, as of 2026): CB FM is permitted, but CB operation remains limited to 26.965–27.405 MHz and 4 W (AM/FM carrier) and 12 W PEP (SSB). The Alan 87 is an “export” set covering 25.6–28.3 MHz; transmitting outside CB allocations or above limits is unlawful. Many owners refurbish/align and keep operation strictly within Part 95 limits.
  • Parts and serviceability: Original bipolar finals (e.g., 2SC2312) are scarcer; ensure bias is correct and SWR is well controlled. MOSFET conversions exist but require bias, neutralization, and LPF verification to maintain spectral purity.

Supporting explanations and details

  • Why “less is more” on AM carrier: Over‑high dead‑key leaves no headroom for modulation; audio peaks clip, causing flat‑topping and splatter. Setting AM carrier to a sane level with proper mic gain yields louder, cleaner comms.
  • Two‑tone SSB setup: Inject a standard two‑tone audio (e.g., 700/1900 Hz) and adjust ALC for clean, symmetrical, non‑flat‑topped RF envelope on a scope/wattmeter. This maximizes intelligibility without IMD.
  • Filter upgrade effect: A narrower, high‑Q 455 kHz filter reduces adjacent‑channel bleed but slightly increases group delay; acceptable tradeoff for crowded 27 MHz environments.

Ethical and legal aspects

  • United States: Operating outside 26.965–27.405 MHz, altering band plans for “freeband,” or exceeding power/deviation limits violates Part 95. Avoid any modification that enables unlawful operation, spurious emissions, or interference with licensed services.
  • Spectrum integrity: Any PLL or PA changes must be validated on a spectrum analyzer for harmonics and spurs; add/verify LPF where necessary.

Practical guidelines

  • Minimum test bench
    • 50 Ω dummy load (≥50 W), in‑line wattmeter (true PEP reading preferred), frequency counter, oscilloscope, RF voltmeter or SDR/spectrum analyzer, AF generator (for two‑tone), and a deviation meter or service monitor for FM.
  • Stepwise workflow
    1. Baseline: Verify supply, regulators, and VCO lock; measure stock TX power, modulation, deviation, and spectral cleanliness.
    2. Refurbish: Replace electrolytics in PSU/audio/IF, reflow suspect joints (VCO, PLL, PA), clean band/mode switches.
    3. Align: Follow the service manual for RX front‑end/IF, carrier balance, ALC, and deviation. Set AM carrier conservatively; set SSB PEP for clean peaks; set FM deviation to standard.
    4. Options: If desired, implement clarifier TX/RX tracking per your PCB’s approved method; then re‑zero the clarifier at center.
    5. Validate: Confirm thermal stability (long TX at reduced duty), SWR tolerance, and full‑band VCO lock. Check spurs/harmonics against masks.
  • What I need from you to go specific
    • PCB photos (top/bottom), board code silk (e.g., EPTxxx), goal (e.g., “low power on SSB,” “unlock clarifier,” “+10 kHz on a spare switch”), available instruments.

Possible disclaimers or additional notes

  • Trim IDs and diode/jumper references differ across production; incorrect assumptions can disable TX/RX or cause unlock.
  • Many online “quick mod” notes omit re‑alignment steps; skipping these is the main reason for drift, chirp, and dirty spectra.

Suggestions for further research

  • Obtain the Alan 87 service manual/schematics for your exact revision.
  • Reputable communities and archives discussing Alan 87/EPT3600 family practices (service procedures, IF filter part numbers, alignment notes).
  • Study CB Part 95 rules to ensure any configuration you choose remains lawful where you operate.

Brief summary

  • The best first “mod” for an Alan 87 is a careful refurbish plus full alignment; it restores output, audio clarity, and stability.
  • Clarifier behavior, TX audio/ALC, and RX selectivity upgrades are popular and safe when done methodically.
  • Avoid power/band hacks that push the PA or PLL beyond design limits and violate regulations.
  • Share your exact goal and board revision, and I’ll provide precise, step‑by‑step instructions tailored to your unit.

User question

nie liniowa regulacja clarifiera

Artificial Intelligence Response

Direct answer to the question

  • “Non‑linear clarifier regulation” means the clarifier (fine‑tune/RIT) does not shift frequency proportionally to knob rotation. The usual causes are the potentiometer taper and the varactor/VCO control network.
  • Key fixes:
    • Use a true linear‑taper (type B) pot of the correct value.
    • Feed the pot from a clean, regulated rail and center it with a symmetric divider so the wiper sits at mid‑voltage when the knob is centered.
    • Limit the tuning span so the varactor stays in its most linear region.
    • Realign the VCO/PLL after hardware changes.
    • If needed, add simple “pre‑distortion” (resistor shaping) or an op‑amp/DAC driver to linearize the V→f transfer.

Detailed problem analysis

  • What “clarifier” does
    • In CB/SSB radios (e.g., Midland/Alan 87, EPT‑3600 family) the clarifier varies the reverse bias on a varactor in the VCO or BFO chain to shift RX (and, if “unlocked,” TX) frequency a few kHz.
    • Varactor physics: C(V) ≈ C0/(1+V/Vj)^m (m≈0.3…0.8). Oscillator frequency f ∝ 1/√C, so V→f is inherently non‑linear even if V vs knob is perfectly linear.
  • Where non‑linearity comes from
    • Potentiometer taper: Using an audio/log pot (type A) bunches most action into one end of rotation.
    • Asymmetric bias network: Series/parallel resistors around the pot can compress one side of the swing.
    • Varactor region: Driving the varactor too close to its low‑V or high‑V extremes exaggerates the C(V) curvature.
    • TX/RX switching diodes or “unlock” mods: Voltage drops and leakages change the effective curve between RX/TX.
    • Ageing parts: Worn pot tracks, leaky coupling caps, or tired varactors cause jumps and dead zones.
  • What “good” looks like
    • Center detent = on‑channel with clarifier wiper at ~½ of the regulated supply.
    • Equal shift per degree either side of center across the useful range (typically ±1.5…±3 kHz on SSB).
    • TX tracks RX by the same amount (if unlocked).

Current information and trends

  • Practical field fixes favor:
    • Keeping the clarifier span modest to stay in the quasi‑linear section of C(V).
    • Using linear‑taper pots and symmetric dividers to linearize the mechanical‑to‑voltage mapping.
    • For premium results, replacing the analog pot with a rotary encoder and a small MCU + DAC (12‑bit) that outputs a shaped control voltage (filtered PWM or DAC through an op‑amp) so the overall V→f curve is near‑linear and repeatable.

Supporting explanations and details

  • Fast diagnostic checklist (15–30 minutes)
    • Clean/verify the pot: Measure end‑to‑end resistance and smooth wiper movement; confirm it’s “B” (linear) taper.
    • Verify supply: The pot’s “top” must be on a constant regulated rail (often 8 V). Check it doesn’t sag on TX.
    • Map the curve: Measure varactor control voltage at full CCW, center, full CW, and at a few midpoints. Plot voltage vs angle and, separately, frequency shift vs angle (use a counter or service monitor).
    • If voltage is non‑linear: suspect taper/shaping resistors. If voltage is linear but frequency is not: you are seeing the varactor’s C(V) curvature.
  • Simple hardware linearization that usually works
    • Pot and divider
    • Use a 10 kΩ linear‑taper pot (type B).
    • Drive the pot from a regulated rail via Rtop and to ground via Rbot chosen equal (e.g., Rtop = Rbot = 3.3 kΩ) so center rotation gives Vref/2 at the wiper.
    • Feed the wiper to the varactor through a series resistor (e.g., 4.7–10 kΩ) to isolate the PLL loop.
    • Range setting
    • Target a varactor control span that yields ~±2…±3 kHz total. In many sets, keeping Vvar between ~2.5 V and ~6.0 V avoids the worst C(V) curvature.
    • Adjust the series resistor to the varactor or the pot value to set the span; avoid rail‑to‑rail excursions.
    • “Pre‑distortion” without op‑amps
    • If the frequency gain is higher near center than at ends, add a small shaping resistor from the wiper to the top or bottom end of the pot to bend the V(angle) curve opposite to the varactor’s curvature. Start with 47–150 kΩ and tune empirically while plotting frequency vs angle.
  • Precision approaches
    • Op‑amp shaper: Generate Vwiper = a·θ + b (θ = normalized knob angle) with a rail‑to‑rail op‑amp and trim a, b so that the measured f(θ) is linear. A simple non‑inverting stage with gain and offset referenced to the regulator works well.
    • Digital clarifier: Rotary encoder → MCU → DAC (e.g., 12‑bit) → op‑amp to 0–8 V. Implement a lookup table of Vset vs desired Δf; you can store calibrated points so linearity is near‑perfect and repeatable with temperature.
  • Realignment is mandatory
    • After hardware changes, center the clarifier mechanically, then adjust the appropriate VCO/BFO coil so the radio is exactly on‑frequency at the center. Verify PLL lock across bands and that TX follows RX if unlocked.
  • Practical numbers (example)
    • Vref = 8.0 V, pot = 10 kΩ (B taper), Rtop = Rbot = 3.3 kΩ → center ≈ 4.0 V at wiper.
    • Series to varactor: 6.8 kΩ. Measured span 2.8…5.8 V → typical SSB shift ~±2.5 kHz with good linearity near center.

Ethical and legal aspects

  • United States (FCC Part 95E): Transmitter frequency and controls are part of type certification. “Unlocking” the clarifier so TX can be shifted and/or extending range beyond the authorized channels can void certification and may be unlawful. Operate only within permitted CB channels and limits. If you intend to use amateur bands, you must be licensed and the radio must be suitable for that service.

Practical guidelines

  • Implementation
    • Replace the pot with a quality linear‑taper unit (Bourns/ALPS), keep leads short, and twist/screen the control wires.
    • Replace leaky electrolytics in the clarifier/VCO path; consider replacing an ageing varactor.
    • Re‑route the pot feed to a regulated rail; remove/adjust legacy shaping resistors that cause asymmetry.
    • Set span by series resistance and confirm the varactor never approaches the supply rails.
  • Best practices
    • Use 1% metal‑film resistors; ESD‑protect the varactor; decouple the control node (100 nF close to the VCO).
    • Document original values; change one thing at a time; plot before/after curves.
  • Challenges and remedies
    • Temperature drift: Keep the control range narrow; ensure VCO components are thermally stable.
    • TX/RX mismatch: If unlocked, ensure both RX and TX are fed from the same control node and re‑align the VCO.

Possible disclaimers or additional notes

  • Board designations and values vary between revisions; verify on your PCB before swapping parts.
  • Two varactors in series (back‑to‑back) help RF linearity but do not by themselves linearize DC tuning; span control and shaping are still required.

Suggestions for further research

  • Obtain the service manual for your exact PCB revision and follow the VCO/clarifier alignment procedure.
  • Characterize your specific varactor: measure Δf vs V to identify the most linear operating region; build a simple LUT if you go the digital route.
  • Explore encoder‑based digital clarifier projects using small MCUs and I2C DACs; they offer perfect repeatability and easy span/center settings.

Brief summary

  • Non‑linear clarifier action is normal when the pot taper, divider network, and varactor curve are not compensated. Start with a correct linear pot, symmetric regulated bias, and a modest tuning span, then realign. If you need better than “good,” add simple shaping or drive the varactor with an op‑amp/DAC that implements the inverse curve.

If you share your radio model/board revision, the measured varactor voltage at CCW/center/CW, and the total frequency span you observe, I can compute specific resistor values and a shaping network to linearize your clarifier.

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