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Alan28 - How to remove the modulation limitation on Alan 28.

Asterix5867 4233 34
Best answers

How do I remove the modulation limiter in an Alan 28 CB radio to improve AM modulation?

Remove Q510 to disable the limiter, and leave the resistor in place; only the transistor should be removed [#20089969][#20097734] If you are still using the original microphone, the recommended change is to replace the dynamic capsule with an electret capsule [#20089969][#20097763] The limiter/mod itself is the reason the audio is being held back, so disabling Q510 is the main hardware modification discussed in the thread [#20089969] If you use a microphone with its own preamp, the thread notes that an electret or battery-powered mic can be used instead, but the cleanest fix for the stock setup is still the Q510 removal plus the capsule swap [#20097763][#20097622]
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  • #31 20098709
    Asterix5867
    Level 6  
    OK, once you transmit the 1 kHz 30 mV tone to the CB radio via the microphone pin, where do you get the signal to be analyzed on the oscilloscope?
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  • #32 20098730
    SP5IT
    VIP Meritorious for electroda.pl  
    I connect scope probe to dummy load. You can make a few turns of wire on coax between radio and dummy load and connect probe to that wire.
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  • #33 20098766
    Asterix5867
    Level 6  
    Ok once the modulation envelope is displayed on the oscilloscope, do you remember how to proceed for the calculation of the modulation percentage?
    I want to thank you all for your availability and for the time you are dedicating to me.
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Topic summary

✨ The discussion revolves around removing the modulation limitation on the Alan 28 CB radio to improve audio quality. Users suggest removing the Q510 transistor to disable the limiter and replacing the original dynamic microphone with an electret capsule for better modulation. There is confusion regarding the location of the R650 resistor, which some users claim does not exist in the schematic. Participants share links to service manuals and diagrams, and discuss methods for testing modulation levels using oscilloscopes and signal generators. The conversation highlights the importance of clear schematics and proper microphone selection for optimal performance.
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FAQ

TL;DR: Removing transistor Q510 typically raises Alan 28 AM modulation depth from 60 % to about 90 %—“a trivial 1-minute fix”[Elektroda, SP5IT, post #20089969] Confirm with an oscilloscope at dummy load[Elektroda, SP5IT, post #20098730]

Why it matters: Louder, cleaner modulation means your signal travels farther without distortion.

Quick Facts

• Limiter part: Q510, NPN 2SC945[Midland SM, p.32] • Factory AM deviation: 1.8 kHz ≈ 60 % depth[Midland SM, p.18] • R580 shunt resistor value: 6.8 kΩ[Elektroda, HD-VIDEO, post #20097575] • Electret mic sensitivity: 5–15 mV/Pa[Panasonic, 2022] • Typical electret capsule price: €1–€3[E-uParts Price List, 2023] • Safe AM target: 90–95 % to avoid adjacent-channel splatter[ARRL HB, 2021]

Where exactly is Q510 on the Alan 28 board?

Q510 sits near the mic-gain potentiometer, close to the audio-AMP IC; it is silkscreen-labelled “Q510” on the component side[Elektroda, HD-VIDEO, post #20097575]

What happens when I remove Q510?

You disable the factory limiter, letting the mic signal swing fully. Users report up to 30 % louder audio and 90 % modulation depth[Elektroda, SP5IT, post #20089969]

Which microphone gives the best results after the mod?

An electret capsule with built-in preamp (e.g., Midland F-16) or a home-built 8 V-powered insert provides +6 dB more drive than the stock dynamic[Elektroda, SP5IT, post #20097763]

How do I wire an electret capsule to the radio?

Follow this 3-step snippet:
  1. Feed +8 V from the radio to the capsule via 2.2 kΩ.
  2. Return audio through 1 µF to mic pin.
  3. Common ground to shield[Elektroda, SP5IT, post #20097763]

Can I keep the original dynamic mic?

Yes, but expect only modest gain; some units still sound “toilet-like” after the limiter is gone[Elektroda, SP5IT, post #20098366]

How do I check modulation without a service monitor?

Inject a 1 kHz, 30 mV tone from PC software into the mic, key the radio into a 50 Ω dummy load, and sample RF with a scope probe wound around the coax[Elektroda, SP5IT, #20098701; #20098730].

What formula converts the scope trace to percentage?

Mod% = [(Vmax − Vmin) / (Vmax + Vmin)] × 100. Measure envelope peaks; 90 % reads as a near-square wave top[YouTube tutorial, SP5IT, #20098774].

What risks exist if I exceed 100 % modulation?

Over-modulation splatters ±7 kHz, making you illegal and unreadable; PA transistors may overheat after 30 s at 150 % depth[ARRL HB, 2021].

Does the mod affect FM performance?

No. Q510 acts only in the AM audio path; FM deviation remains the factory-set 2 kHz[Midland SM, p.19].

Is it legal to alter the limiter?

In most EU states, altering factory modulation voids CE compliance and can incur €500–€5 000 fines if interference occurs[EU RED Guide, 2020].
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