35 microvolts for a readable signal is definitely a red flag for a Super Skyrider. A healthy Hallicrafters SX-28 should boast a sensitivity of around 2 to 8 microvolts across most of its ranges. The fact that the antenna trimmer isn't doing anything is the smoking gun here—it means your front-end RF stage is completely disconnected, dead, or severely detuned.
Because you have correctly aligned the IF chain and local oscillator to the physical frequency of the crystal (462 kHz instead of the stamped 455 kHz), you did the right thing regarding the IF. However, migrating the entire radio's IF up by 7 kHz has severe ripple effects on the RF front-end tracking.
Here is the systematic breakdown of why your trimmer is dead and why the radio is deaf.
1. The Tracking Shift (The 7 kHz IF Penalty)
Because you aligned the IF to 462 kHz to match the aging crystal, the Local Oscillator (LO) must run exactly 462 kHz above the dial frequency to produce that IF.
When you did the front-end RF alignment, if you aligned the RF coils strictly to the panel's dial markings, the RF front-end tuning peaks are now completely mismatched from where the LO is looking. * Example: If the dial says 7.0 MHz, the LO is parked at 7.462 MHz. But the RF front-end circuits (which are tuned by the main tuning capacitor gang) are physically peaking at 7.455 MHz if you followed standard alignment points.
The Result: The front end is looking at a different frequency than the radio is actually translating down to the IF. This causes a massive drop in sensitivity, and it will render the antenna trimmer powerless because it cannot swing far enough to correct that 7 kHz discrepancy on higher bands.
2. Why the Antenna Trimmer is "Dead"
If the antenna trimmer knob results in absolutely zero change in background noise or signal strength, the problem is localized to the Antenna Coil assembly or the 6SK7 RF amplifier tube stage.
Check for these classic SX-28 failures:
The Antenna Coil is Fried: The antenna trimmer (C1) is directly in parallel with the secondary of the active antenna transformer coil. It is incredibly common for vintage sets to have a open/burned primary or secondary antenna coil because a previous owner hooked up a transmitter nearby or the antenna took a static lightning hit.
Band Switch Contacts: The SX-28 has a notoriously complex, buried rotary band switch. If the contacts for the antenna trimmer or the RF coils are oxidized or bent, the trimmer is physically disconnected from the circuit.
AVC Short or Open Grid: If the A1/A2/GND terminal links on the rear terminal strip are missing or incorrectly strapped, the grid of the first 6SK7 RF tube might be floating or hard-shorted to ground, bypassing the tuned circuit entirely.
Systematic Diagnostic Steps
Step A: Verify the First RF Stage
Inject your 35 µV signal generator directly into the grid (Pin 4) of the first 6SK7 RF tube, rather than the antenna terminals.
If the signal suddenly gets massively louder, the 6SK7 tube and the rest of the receiver chain are working perfectly. Your sensitivity loss is entirely inside the antenna coils, the band switch, or the antenna trimmer circuit itself.
If the signal remains faint, the 6SK7 RF stage is dead, missing its plate voltage, or severely biased down by an AVC malfunction.
Step B: Re-align the RF Front End to the New IF
When aligning the RF trimmers and padders on the gearbox for each band, do not trust the dial markings. 1. Set your signal generator to exactly what the dial says.
2. Peak the LO (Local Oscillator) adjustment until you hear the signal.
3. Crucially, adjust the corresponding RF and Mixer trimmers for maximum volume/AVC voltage based on that peak, forcing the front end to track with your new 462 kHz offset rather than its factory 455 kHz specification.
If the RF coils cannot be peaked to match the new 462 kHz IF gap, you may have to accept defeat on the crystal and shift the entire radio back to a true 455 kHz IF chain—even if it means operating with the crystal filter switched off.
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Last modified: Sunday, 12 Jul 2026