KWM-2A Defect Symptom: Instantaneous 9 khz shift after 1-2 hours of operation – VK6ADA

Collins KWM-2 PTO Frequency Jump Troubleshooting Guide

By VK6ADA • March 25, 2026

This guide documents the troubleshooting process for an intermittent 9 kHz frequency jump in a Collins KWM-2/2A transceiver that occurs after 1-2 hours of operation. Based on community advice and restoration experience, this issue typically stems from temperature-sensitive component failure in the 70K-2 Permeability Tuned Oscillator (PTO).

Symptoms Observed

After extended warmup (1-2 hours), the KWM-2 VFO exhibits an instantaneous frequency jump of approximately 9 kHz. This is not gradual drift but a sudden shift that occurs without touching any controls. Verification steps include:

Initial frequency alignment with calibrator remains stable for first 10-15 minutes
After 1-2 hours of operation, frequency jumps ~9 kHz higher (confirmed by checking calibrator signal)
Jump is repeatable after cool-down and re-warmup
External VFO (312B-5) remains stable, confirming issue is internal to KWM-2 PTO

Initial Troubleshooting Steps (Community Advice)

Following suggestions from the Collins Collectors Association community, begin with these non-invasive tests:

Swap PTO tubes between KWM-2 and 312B-5 – The KWM-2 uses a 6AU6 (or 7543 military equivalent) in V301 position. If the frequency jump follows the tube to the other unit, it indicates tube-related issues like inter-electrode shift or emission changes with heat.
Check PTO voltages – Measure B+ and cathode voltages on the 6U8A before warmup and after the jump occurs. Significant changes can indicate tube emission changes or failing cathode bypass capacitors.

Likely Root Cause Analysis

If tube swapping and voltage checks show no issues, the 9 kHz jump after extended warmup strongly suggests a temperature-sensitive capacitor failure in the PTO tank circuit. Based on Collins 70K-2 PTO design and common failure modes:

Most Probable Culprits

C302 (999 pF mica) – Develops voltage-dependent capacitance causing instability correlated with RF level
C303 (3051 pF mica) – Standard mica capacitor that can drift with age and temperature
C304 (207 pF mica) – Often replaced with 50 pF mica to reduce warm-up drift (SB 9 Change 20B)
CR301 (1N34A germanium) – Junction capacitance changes with temperature; replacing with 1N4148 silicon reduces frequency jumps
C308 ceramic trimmer – Can develop intermittent contact as ceramic disc ages

Recommended Repair Procedure

If initial troubleshooting doesn’t resolve the issue, proceed with PTO service:

Document current settings – Note dial zero and 200 end-point frequencies before opening
Access PTO – Remove the 70K-2 assembly from the chassis (refer to Collins manual or DJ7HS guides)
Visual inspection – Look for evidence of previous service, disturbed components, or lacquer on trimmers
Capacitor replacement – Replace C302, C303, C304 and CR301 with new mica capacitors and 1N4148 diode
Preserve C301 – Only replace if frequency has shifted >20 kHz (it’s extremely fragile)
Clean and lubricate – Clean lead screw spindle and apply fresh lithium grease
Re-align span – After reassembly, set electrical span to 2.501-2.701 MHz using L302, then set mechanical end stops

Preventive Notes

As noted in VK6ADA’s restoration deep-dive: “Apply all SBs. Replace the filter can and all waxed-paper capacitors proactively. Service the relays before you do anything else. Then open the PTO only if you have a legitimate drift complaint — a 70K-2 that’s working well is best left alone.”

Related Resources

Final Thoughts

The Collins 70K-2 PTO is a marvel of 1950s precision engineering. When properly serviced, it provides exceptional stability better than 100 Hz/hour after warmup. Addressing this 9 kHz jump will restore your KWM-2 to its intended performance level, allowing reliable SSB operation for decades to come.

73 de VK6ADA