Hammarlund HQ-110 Series
Failure Prevention Kit — Component & Modification Design

Section 1 — Variant and Production Run Guide

The HQ-110 family is more complex internally than its modest appearance suggests. Three major production runs of the base HQ-110 are documented, each with circuit differences that affect both restoration procedure and the choice of correct schematic. Always identify your production run before beginning IF transformer work.

Section 2 — Root Cause Failure Analysis

The following ten failure modes account for the overwhelming majority of HQ-110 restoration casualties. They apply to all variants except where noted. They are in priority order.

• 1
  Filter Capacitor C73 — Audio Oscillation and Hum from Dried Electrolyte C73 is the main multi-section electrolytic capacitor providing B+ and cathode bypass filtering. It is typically mounted on the chassis with its sections shared between the B+ rail (C73A/B) and the 6AQ5 audio output stage cathode bypass (C73C). After 60–65 years, the electrolyte has dried and capacitance has dropped significantly. A dried C73A (the B+ section) has a documented and specific failure mode: audio oscillation at a volume-dependent threshold — the receiver produces a sustained screech or squeal that was traced to C73A in a documented restoration. Replacing C73A eliminated the oscillation completely. The C73C section (audio stage cathode bypass) also causes oscillation when it fails: the 6AQ5 loses its cathode bypass, its operating point shifts, and the feedback conditions in the audio stage become unstable. Replace the entire multi-section C73 as a unit. Hayseed Hamfest produces a universal HQ-170/HQ-180 electrolytic kit; verify the section values against your specific production run before ordering.
• 2
  Z1 Couplate (HQ-110A) — Positive Grid Voltage Destroys Audio Output Stage The HQ-110A uses the same printed RC couplate module architecture as the HQ-180’s Z2. The Z1 couplate (between the 12AX7 driver and the 6AQ5 audio output tube) contains a small 0.01 µF coupling capacitor that leaks DC with age and heat. A leaky Z1 allows positive voltage to appear on the 6AQ5 control grid, dramatically increasing plate current, overloading the cathode bias resistor, and potentially damaging or destroying the audio output transformer and power transformer components. The HQ-110A manual specifically instructs the restorer to measure the 6AQ5 grid voltage immediately after first power-up and again a few minutes later. Any positive reading is a red flag requiring immediate shutdown and Z1 replacement before further operation. Z1 couplates can no longer be sourced new and must be rebuilt from discrete components (see MOD-1 and Section 5).
• 3
  T1 IF Transformer Internal Silver Mica Capacitors — B+ Leakage Fault (Second Run and Later) In the second-production-run HQ-110 and all HQ-110A units, the T1 IF transformer carries the B+ supply rail through its winding. The T1 transformer contains internal silver mica capacitors in an exposed leaf sandwich construction — the same construction prone to failure in early HQ-180 and HQ-170 units. Contamination (dust, nicotine, humidity) causes carbonised conductive tracks to form from the exposed mica capacitor leaves to the grounded T1 can, creating a leakage path from the B+ line to ground. The symptom is varied: reduced sensitivity on the affected dual-conversion bands, IF misalignment that cannot be corrected, and in severe cases a direct partial short of B+ to chassis. The T1 design is specifically described as “a known problem spot” in the Radiomuseum.org production history. The fix is to remove the internal mica capacitors from T1 and mount replacement external dipped mica or C0G ceramic capacitors across the appropriate transformer terminals.
• 4
  6AQ5 Cathode Bypass Capacitor Remote Mounting — Long-Wire Oscillation Risk In many HQ-110 production units, the C73C cathode bypass capacitor (25 µF) for the 6AQ5 audio output tube is not mounted at or near the 6AQ5 tube socket. Instead, it is one section of the main filter can (C73), which may be located at a considerable physical distance from the 6AQ5 socket. The connecting wire from the remote capacitor to pin 2 of the 6AQ5 socket acts as an antenna and inductive element that can destabilise the audio stage feedback at radio frequencies. The correct restoration practice is to mount a dedicated 25 µF bypass capacitor directly at the 6AQ5 socket pin 2, regardless of whether the original C73C is intact. This removes the long-wire RF path and stabilises the audio stage, especially after the main C73 is replaced with a new can that may be mounted in a slightly different position.
• 5
  Silicon Rectifier Over-Voltage — Elevated B+, Transformer Stress, Audio Instability Many HQ-110 and HQ-110A units encountered in the field have had the original 5U4 tube rectifier replaced with silicon diodes at some previous service point. Silicon diodes have a lower forward voltage drop than the 5U4, producing a B+ approximately 10–15% higher than the specified 260 V. The elevated B+ stresses the power transformer and all electrolytics through excessive current draw, and also contributes to audio oscillation by raising all tube operating points above design specification. Additionally, silicon diode replacements without an inrush-limiting resistor subject the filter capacitors to much higher repetitive peak charging current than the 5U4 provides. If your unit already has silicon diodes fitted, add a dropping resistor of approximately 33–47 Ω / 5 W in series with the B+ output to restore the correct B+ voltage. Alternatively, re-fit a new 5U4GA or 5U4GB rectifier tube. See MOD-2 for the complete procedure.
• 6
  Mains Line Safety Capacitor Failure — Fire Hazard Across AC Input The HQ-110 includes a small capacitor directly across the mains AC input terminals (as a line-bypass / RF filter). These original capacitors are not rated to modern mains safety standards (X2/Y class) and fail in a fire-hazard mode: rather than failing open, they fail short or semi-conductive, running hot and potentially catching fire. In the related HQ-100 receiver, this capacitor was found literally burned to char in a documented restoration. The original capacitor across the AC input should be replaced with a modern X2-rated (or X1/Y2) safety capacitor at an appropriate value (typically 0.01 µF to 0.047 µF / 275 V AC X2). Never replace with a standard polypropylene or polyester film capacitor in this position — only a safety-rated class X or Y type is appropriate here.
• 7
  Absence of Mains Fuse — No Fault Protection in Original Design The original HQ-110 design did not include an in-line mains fuse. A short circuit anywhere in the power supply or primary wiring has no protection and will continue to draw current until the power transformer burns out, wiring melts, or a house circuit breaker trips. A documented restoration specifically recommends adding a mains fuse as a safety modification, noting that a convenient empty hole in the rear chassis was already present (presumably drilled for this purpose by a previous restorer). Install a 1.5 A fuse in-line with the mains hot conductor — either in a panel-mount fuse holder secured to the rear chassis, or in an in-line fuse holder on the mains lead. This modification is non-reversible only in the sense that it is always correct to have a fuse. See MOD-4.
• 8
  Rear-Panel T/R Relay Socket — Missing Dummy Plug Disables Receiver (HQ-110A) The HQ-110A has a T/R relay socket on the rear panel. In the factory default configuration, an AC-style dummy plug is fitted in this socket to complete the signal-path circuit. Without the dummy plug, the signal processing tubes cannot function — the receiver produces only static and noise. This failure mode is uniquely difficult to diagnose because the tubes all light, all voltages measure correctly, and there is no obvious fault symptom other than complete deafness. It is easily missed by restorers not familiar with this specific design feature. The previous owner of one documented example had removed the dummy plug to connect a T/R relay, leaving the socket empty. Verify the presence of the dummy plug before performing any diagnosis on a deaf HQ-110A. Plug configuration: AC outlet-style plug with the two contacts connected together (or to an appropriate relay).
• 9
  Band Switch Contact Contamination — Dead Bands or Reduced Sensitivity The HQ-110’s band switch is subject to the same contact oxidation and contamination from 60+ years of storage as all receivers of this era. Dirty band switch contacts produce dead or low-sensitivity reception on specific bands while other bands work normally, or cause intermittent operation that clears when the band switch is rotated repeatedly. The combination of ceramic-mounted contacts and light contact pressure typical of Hammarlund band switches makes them more sensitive to this failure than some competitors. Before performing any alignment, verify all bands produce a response by connecting a short antenna and stepping through each band. If specific bands are dead, clean the band switch contacts with a proper contact cleaner (DeoxIT, not WD-40 as a permanent solution) applied carefully to each contact wafer.
• 10
  Multiple Schematics for Different Production Runs — Wrong Schematic Causes Alignment Failure The HQ-110 was manufactured in at least three distinct circuit revisions with the most significant change between the first run (T1/T2 each handling one IF frequency) and the second run (T1/T2 each handling both IF frequencies via inductive coupling). The second mixer crystal also changed from 3.49 MHz to 2.58 MHz. A restorer attempting alignment using the wrong schematic version will: encounter T1/T2 terminal connections that do not match the diagram, find the 2.58 MHz crystal not listed in the parts list, and be unable to correctly align the IF transformers because the coupling topology they are tuning differs from the one described. Always identify your production run by serial number range before sourcing a schematic, and verify your chassis wiring against the schematic before attempting alignment.

Section 3 — Kit Component Reference

The table below lists all components in the HQ-110 failure prevention kit. Tier 1 items are mandatory pre-power replacements. Tier 2 items address secondary aging. Modification components are listed separately.

Section 4 — Pre-Power Safety Protocol

Visual Inspection Checklist

• Identify the production run using the serial number on the rear chassis. Match the serial number to the correct schematic edition before proceeding.
• HQ-110A only: Verify the T/R relay socket dummy plug is present on the rear panel. If missing, fabricate one before any reception testing.
• Inspect the C73 multi-section filter capacitor for physical swelling, electrolyte deposits, or visible damage. Any evidence of leakage: replace before power-up.
• Inspect the mains AC input capacitor (across the AC line terminals) for discolouration, cracking, or burn marks. Replace with X2-rated type regardless of apparent condition.
• Check for presence of an inline mains fuse. If absent: install K-006 before first power-up.
• Check whether the 5U4 rectifier has been replaced with silicon diodes. If so: plan to measure B+ voltage at first power-up and compare to specification (260 V DC at V8 plate).
• Inspect the T1 IF transformer (second-run and HQ-110A units): look for any discolouration of the transformer can or surrounding chassis area indicating B+ leakage.

Section 5 — Circuit Modifications

  Z1 COUPLATE CIRCUIT TOPOLOGY (HQ-110A discrete rebuild)
  — same architecture as HQ-180 Z2

  12AX7 driver plate ──┬──[500kΩ R1]──┬──[0.01µF/630V]──┬──[500kΩ R2]──┬── 6AQ5 control grid (pin 5)
                       │              │                   │              │
                    [250pF C1]      GND                 GND          [250pF C2]
                       │                                               │
                      GND                                            GND

  R1, R2  = 500kΩ (or 470kΩ) ½W carbon film or metal film
  C1, C2  = 250pF / 500V silver mica or C0G
  C_link  = 0.01µF / 630V polypropylene (ZERO leakage mandatory)

  CHECK AT FIRST POWER-UP: DC voltage at 6AQ5 pin 5 (grid) must be zero.
  Any positive reading → immediate shutdown → Z1 still has a leakage path.

Figure 1. Z1 couplate discrete component topology for the HQ-110A.

Section 6 — Installation Sequence

• 1
  Identify production run and obtain correct schematic Read the serial number from the rear chassis apron. Cross-reference with the production run guide in Section 1. Locate both the first-run and second-run HQ-110 schematics from BAMA archives plus the HQ-110A schematic if applicable. Verify which schematic matches your chassis wiring at T1 and T2 before any work begins.
• 2
  Install mains fuse (K-006, MOD-4) Install the inline 1.5 A slow-blow fuse before any other work. This protects both the receiver and the workspace from any fault discovered during restoration.
• 3
  Replace mains line capacitor (K-004) Remove the original capacitor across the mains AC input terminals and replace with an X2-rated safety type at the same nominal value. Do this before any power is applied to the chassis.
• 4
  Replace C73 filter capacitor (K-001) and mount 6AQ5 cathode bypass (K-003) Replace the entire C73 multi-section can with a new unit. In the same session: mount a dedicated 25 µF / 100 V capacitor directly at pin 2 of the 6AQ5 socket (the cathode bypass for the audio output stage).
• 5
  HQ-110A only: rebuild Z1 couplate (K-002, MOD-1) Build the discrete Z1 replacement per the Figure 1 topology. Test the 0.01 µF coupling capacitor at 500 V DC for zero leakage. Install the built assembly before any power-up.
• 6
  HQ-110A only: verify T/R socket dummy plug (K-010) Confirm the rear-panel T/R relay socket dummy plug is present and correctly fitted before any reception testing.
• 7
  Clean all tube sockets and band switch (K-005, K-008) Pull each tube, apply DeoxIT to socket contacts, re-insert and flex. Then apply DeoxIT to band switch contacts and step through each band position several times.
• 8
  First Variac power-up and 6AQ5 grid voltage check Raise mains voltage from 0 to full over 20–30 minutes. At full voltage: immediately measure the 6AQ5 control grid voltage (pin 5). Must read zero or slightly negative. Any positive reading: shut down immediately and investigate Z1 leakage. Also measure B+ and compare to the voltage chart in the manual.
• 9
  B+ voltage verification and silicon diode dropping resistor (MOD-2) With the receiver at full operating voltage, measure B+ at the 6AQ5 plate. Target: 260 V ±10 V. If B+ is high (silicon diode replacement already fitted): add dropping resistor per MOD-2 procedure. Re-measure and confirm.
• 10
  T1/T2 IF transformer leakage test (K-007) On second-run and HQ-110A units: with receiver powered down, test internal IF transformer mica capacitors at 500 V DC via 100 kΩ series resistor. Any leakage: implement MOD-3 (external bypass capacitors) before alignment.
• 11
  Band-by-band reception check and full alignment Verify signal breakthrough on all bands. Perform IF alignment (455 kHz first, then 3.035 MHz, then front-end) using the correct production-run manual and a calibrated signal generator. Record a post-restoration performance baseline.

Section 7 — Verification Tests

Z1 / 6AQ5 Grid Voltage Verification (HQ-110A)

Filter Capacitor / Audio Stability Verification

B+ Voltage Verification

Band Coverage Verification