Hammarlund HQ-180 Series
Failure Prevention Kit — Component & Modification Design

Section 1 — Variant Guide

All HQ-180 variants share the same fundamental failure modes. The differences below are relevant to specific restoration steps; always identify your exact variant and obtain the correct schematic edition before beginning.

Section 2 — Root Cause Failure Analysis

The following ten failure modes account for the overwhelming majority of HQ-180 restoration casualties. They are in priority order.

• 1
  Main Filter Capacitor Can — Electrolyte Seepage, Hum, B+ Sag The HQ-180’s main power supply filter is a multi-section electrolytic in a twist-tab metal can. After 60–65 years, virtually every example encountered in unknown condition shows the characteristic sign of degradation: white or yellow crystalline deposits at the base of the mounting tabs, or visible brown electrolyte goo weeping from the positive terminal seal. The symptoms: 60 Hz hum on all received signals, B+ sag under normal tube current load, reduced sensitivity, and — on a cold power-up of a severely depleted unit — potential transformer damage from high leakage current. Inspect the filter can before applying any power to an unknown unit. Any visible deposit at the tabs is an unambiguous replacement indicator. Do not rely on a DMM resistance check to qualify the original capacitor — a modern digital capacitor tester that applies rated voltage is required for a meaningful leakage test. Hayseed Hamfest manufactures a twist-tab replacement that is electrically and dimensionally correct for the HQ-170/180 family.
• 2
  Couplate Z2 Leakage — 6AQ5 Grid Goes Positive, Burned Cathode Resistor The HQ-180 audio output stage uses a printed “couplate” module Z2 mounted on the chassis. The couplate is a small printed assembly containing two 500 kΩ resistors, one 0.01 µF capacitor, and two 250 pF capacitors — the resistive and capacitive network that sets the grid bias and frequency response of the 6AQ5 audio output tube. The 0.01 µF capacitor in Z2 becomes leaky with age, particularly when the receiver gets warm. A leaky Z2 allows DC to appear on the 6AQ5 control grid, driving it positive, which dramatically increases plate current in the output tube. The cathode bias resistor for V12 (6AQ5) is rated at 1 W (410 Ω) but will overheat and fail when the tube is driven into excessive conduction, and sustained high current can damage the output transformer and the power transformer. Z2 couplates are no longer manufactured and individual examples cannot be sourced new. The Z2 must be rebuilt from discrete components. See Section 5, MOD-2.
• 3
  IF Transformer Open-Leaf Silver Mica Capacitors — Early Production HQ-180 Only In early production HQ-180 receivers, the first and second IF transformer assemblies (at 3.035 MHz) used an open-construction silver mica capacitor design: exposed metal leaves and mica insulators assembled in a stack rather than the sealed dipped-mica type used in later production. This construction is highly vulnerable to contamination. In humid or smoky environments, dust and nicotine film accumulate on the exposed leaves and form carbonised conductive tracks to the grounded mounting structure, creating a leakage path from the capacitor terminal to ground rather than across the mica itself. The result: the 3.035 MHz IF becomes mistuned (the leakage acts as a shunt loading element), producing reduced sensitivity, poor image rejection, and alignment cores that cannot be peaked correctly. In later production HQ-180 and HQ-180A units the sealed dipped-mica type is used and this failure mode does not apply — but when purchasing a unit, verify the IF can construction before assuming the later type is present. Identify by removing one IF transformer cover: open-leaf construction is immediately visible as a stack of silver metal plates.
• 4
  6C4 HF Oscillator Filament Hum — FM Noise on Upper Bands The 6C4 triode is the HF first oscillator, and its filament heater current produces a magnetic field that modulates the oscillation frequency (FM-on-the-carrier). On the lower bands this FM is too small in absolute frequency to be audible. On the upper bands (10 m, 15 m, and to a lesser extent 20 m) the FM is large enough to be perceived as 60 Hz modulation superimposed on received signals — a form of hum that does not disappear when the audio gain is reduced. In the HQ-180A this effect is compounded by the keep-alive transformer that keeps the 6C4 filament energised continuously: the filament eventually settles to a temperature-stable state over hours but is always in some state of thermal drift. The fix is to operate the 6C4 filament on DC (see MOD-3). Hammarlund’s own interim recommendation was to select a low-hum 6C4 from a box of tubes by trial — a solution that is no longer practical with today’s tube supply.
• 5
  Keep-Alive Transformer (HQ-180A) — Accelerated Tube Burnout The HQ-180A added filament transformer T22 to keep the 6C4 HF oscillator and the first converter 6BE6 filaments energised at all times (regardless of the front-panel power switch), with the claimed benefit of reduced warm-up drift. In practice this works but the continuous operation burns out 6C4 and 6BE6 tubes within one to two years of regular use. A unit that has been operated with the keep-alive feature intact will have exhausted 6C4 and 6BE6 tubes regardless of other indicator condition. Disable the keep-alive transformer by rewiring T22 so that it is switched along with the main power (see MOD-4). Use an outlet strip with its own power switch as the simplest field approach, or rewire T22 primary to track the main transformer. If the receiver has the optional 115/230 V or clock wiring, get the exact schematic for your unit before touching the AC primary side — the wiring in these units is described by experienced restorers as a “nightmare.”
• 6
  HQ-180A Coax Antenna Trimmer — Parasitic Oscillations on 10 m In the HQ-180A, the antenna trimmer capacitor was moved to the front panel and connected to the antenna coil assembly by a length of coax cable. This eliminated the dial cord and pulley mechanism of the original HQ-180 but introduced a new problem: the coax creates a resonant stub at 10 m frequencies (and more severely at 6 m in the HQ-170A equivalent), causing parasitic oscillations that appear as multiple spurious responses when the antenna trimmer is rotated. The symptoms are particularly obvious at 10 m — multiple apparent “peaks” on the antenna trimmer that are actually oscillation artefacts, not genuine signal peaks. The 6 m band in the HQ-170A equivalent is essentially unusable due to this problem. For the HQ-180A: 10 m reception is compromised; experienced restorers recommend treating 10 m with the antenna trimmer set to a compromise position rather than attempting to peak it conventionally. The addition of a low-value resistor (22–47 Ω) in the plate or grid lead of the 10 m oscillator coil can help reduce the parasitic tendency.
• 7
  Discrete Electrolytic Capacitors — Audio and IF Chain Aging Beyond the main filter can, the HQ-180 contains discrete electrolytic capacitors in the audio output stage, the noise limiter circuit, and the AGC chain. In most HQ-180 examples the discrete electrolytics have not failed catastrophically, but their capacitance has drifted and ESR has risen from 60 years of service. The HQ-170/180 Hayseed Hamfest re-cap kit covers both the filter can and all discrete electrolytics as a matched set for any HQ-170 or HQ-180 variant (some capacitors may be left over from the universal kit depending on variant). Note that some versions of the HQ-180 used Mylar (polyester film) capacitors in positions that may appear to be paper dielectrics — these do not require replacement and should be identified by testing value and leakage before removal.
• 8
  Tube Socket Contact Contamination — Intermittent or Non-Lighting Tubes Tube sockets in any receiver that has been in storage will accumulate oxidation and contamination on the contact springs, causing intermittent tube operation even when the tube itself is functional. In documented HQ-180 restorations, the 6BZ6 RF amplifier tube (V1) has been specifically noted as non-lighting due to socket contact contamination, giving the appearance of a dead tube when DeoxIT and flex-cycling the tube in the socket restored full operation. The HQ-180’s relatively dense chassis packing makes it impractical to spray all sockets externally with contact cleaner; each tube should be pulled individually, a small amount of DeoxIT D5 applied to the socket pins with a fine applicator, and the tube re-inserted and rotated several times before being seated firmly.
• 9
  Frequency Drift — HFO Tube and Variable Capacitor Rotor Wiper Contacts The HQ-180 exhibits more frequency drift than later-generation crystal-controlled receivers, and this is expected — it uses a free-running LC HF oscillator (the 6C4 circuit). However, drift that is excessive even after a 30-minute warm-up period, or drift that tracks with the tuning dial position, indicates one of two specific mechanical causes: (1) dirty rotor wiper contacts on the main tuning variable capacitor(s), which create variable contact resistance that modulates the oscillator frequency as the tuning changes; or (2) a weak or gassy 6C4 HF oscillator tube. Clean the rotor wipers with IPA on lint-free swabs followed by one or two drops of DeoxIT applied to each wiper with a precision applicator. Test the 6C4 for emission and gas. The HQ-180 is inherently more stable than the HQ-170 due to its better oscillator coil components, but neither will hold frequency to SSB standards without regular maintenance of these two items.
• 10
  60 kHz IF Alignment After Component Replacement — Complex Triple-Reference Procedure The HQ-180’s dual/triple conversion architecture requires a three-stage IF alignment procedure: the 60 kHz IF first (requires a 60 kHz signal source, not a standard signal generator — an audio oscillator capable of 60 kHz or a purpose-built 60 kHz source is required), then the 3.035 MHz IF, then the 455 kHz IF. The manual alignment procedure contains errors in its ordering that have been documented by experienced restorers: different editions of the manual contradict each other on whether the 3.035 MHz or 455 kHz IF is aligned after the 60 kHz stage. Additionally, the 100 kHz calibrator crystal is a single-purpose component; if a previous restorer replaced it with the wrong value (110 kHz has been documented), all HFO calibration will be in error by the multiplied offset. Verify the 100 kHz crystal is the correct value against an external frequency reference before using it for any alignment procedure.

Section 3 — Kit Component Reference

The table below lists all components in the HQ-180 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

• Inspect the filter can (mounted on the chassis floor) for white/yellow deposits at its tabs and brown seepage from the positive terminal. Note or photograph before touching.
• Inspect Z2 (a small printed module near V12) for any sign of heat discolouration or physical damage. Also inspect the V12 (6AQ5) cathode resistor for heat damage — a cooked resistor is the primary external indicator of Z2 leakage.
• Inspect the output transformer and power transformer for dripping or blistering of potting compound. Any sign of thermal stress in either transformer is a serious pre-purchase concern — both are expensive to replace.
• Check that all tube shields are in place. Missing shields on audio, detector, or noise limiter stages cause ground loop hum and can degrade sensitivity significantly.
• For HQ-180A: verify that the keep-alive transformer issue is understood and that you have a plan (switched outlet strip) to prevent continuous tube burnout before operating the receiver.
• On early production units: remove one first-stage IF transformer cover and verify whether it uses open-leaf or sealed dipped-mica construction. Document this before proceeding.

Section 5 — Circuit Modifications

  Z2 COUPLATE — CIRCUIT TOPOLOGY (replace with discrete components)

  6AV6 plate ──┬──[500kΩ R1]──┬──[0.01µF / 630V film]──┬──[500kΩ R2]──┬── 6AQ5 grid
               │              │                          │              │
             [250pF C1]     GND                        GND           [250pF C2]
               │                                                        │
              GND                                                      GND

  R1 = 500kΩ (or 470kΩ) ½W  |  R2 = 500kΩ (or 470kΩ) ½W
  C1 = 250pF / 500V silver mica or C0G   |   C2 = 250pF / 500V silver mica or C0G
  C_coupling = 0.01µF / 630V polypropylene

  CRITICAL: C_coupling must pass 500V DC leakage test with zero measurable leakage.
  A leaky coupling cap drives 6AQ5 grid positive → excessive plate current → R burnout.

Figure 1. Z2 couplate circuit topology for discrete component rebuild.

Section 6 — Installation Sequence

• 1
  Documentation and visual inspection Photograph the chassis underside and top before touching anything. Identify the exact variant from the serial number and front panel configuration. Source the correct schematic edition for your specific variant — the HQ-180, HQ-180A, and HQ-180AX each have their own schematic. Check filter can, Z2, and V12 cathode resistor as described in Section 4.
• 2
  Replace main filter can (K-001) and all discrete electrolytics (K-004) Replace the filter can before any power is applied. When soldering the replacement tabs, mark the original wiring with the geometric symbol system (square, triangle, etc.) used on the can — multiple red wires connect to the can and colour alone does not distinguish them. Replace all discrete electrolytic capacitors at the same session.
• 3
  Rebuild Z2 couplate (K-002, MOD-2) and upgrade V12 cathode resistor (K-003) Build the discrete Z2 replacement per the Figure 1 schematic. Test the 0.01 µF coupling capacitor before installation. Replace the V12 cathode resistor with the 2 W rated version. Install at the same time.
• 4
  Clean all tube sockets and re-seat all tubes (K-006) Pull every tube individually. Apply a small amount of DeoxIT D5 to the socket contacts with a fine applicator. Re-insert and rotate each tube several times before final seating. Verify every tube is fully seated with its retainer or shield properly secured.
• 5
  HQ-180A only: plan keep-alive transformer management (MOD-4) Decide whether to use the switched power strip approach or internal rewiring. If internal rewiring: have the correct schematic in hand before touching any AC wiring. Do not proceed with internal AC work without the correct schematic for your specific variant.
• 6
  Verify calibrator crystal (K-010) Before first power-up, confirm the 100 kHz calibrator crystal is the correct value. Remove it and measure resonant frequency on a crystal tester or oscillator circuit against a known reference. A 110 kHz crystal in the calibrator position will make all HFO alignment impossible to complete correctly.
• 7
  First Variac power-up and B+ verification Connect via Variac and raise mains voltage from 0 to full over 30 minutes. Monitor B+ at the power supply filter (a scope or AC voltmeter will show ripple; target <100 mV at main B+ rail). Verify all HV rails are within specifications in the manual voltage table. Check that no tubes are dark after full voltage.
• 8
  Early-production IF inspection (K-007) Remove one IF transformer can cover and identify the mica capacitor construction type. If open-leaf construction: clean and test for leakage as described in Tier 2. Record the result.
• 9
  Rotor wiper contact cleaning (K-009) Clean all rotor wiper contacts on the main tuning variable capacitor(s) with IPA then DeoxIT. Monitor the HFO frequency on a counter while rotating the tuning through its full range — any jump or bump in frequency indicates a wiper contact that needs further cleaning.
• 10
  Audio modifications (MOD-1) With the receiver working and Z2 verified: implement the C151 removal and Schade feedback modification. Verify audio bandwidth improvement with a tone source at the antenna input across multiple audio frequencies.
• 11
  Full alignment Perform full alignment in the correct order: 60 kHz IF first (requires 60 kHz signal source), then 3.035 MHz IF, then 455 kHz IF, then front-end oscillator tracking and antenna trimmer optimisation. Use Ray Osterwald N0DMS’s Electric Radio series on the HQ-180 as the authoritative alignment reference. Record a post-restoration performance baseline.

Section 7 — Verification Tests

Z2 / V12 Cathode Resistor Verification

Filter Capacitor Ripple Verification

60 Hz Hum on Upper Bands (6C4 FM Modulation)

Sensitivity Verification