This video documents the initial troubleshooting and repair of a vintage 1970s Cossor CRM.555, a precision signal generator for testing aircraft Instrument Landing Systems (ILS) and VOR navigation equipment. The unit was non-functional, immediately blowing its input fuse and drawing excessive current upon the first power-on attempt.
The initial diagnosis focuses on the power supply. Through reverse-engineering the schematic and using a thermal camera, the fault is traced to a misadjusted 20V power rail. The elevated voltage was triggering a Zener diode-based crowbar protection circuit, causing the short circuit condition. After correctly adjusting the power supply trimmer, the unit powers on with a normal current draw.
Subsequent functional tests reveal several remaining issues. While the carrier frequency generation for the Marker, Localizer, and Glide Path functions is largely operational, a faulty crystal prevents output at 109 MHz for both VOR and Localizer modes. Modulation testing confirms the Marker and Localizer functions are working correctly, but a critical fault is found in the VOR mode, which is missing its essential 30 Hz modulation. Additionally, the Glide Path modulation signal appears to be too high, causing saturation. The video concludes by summarizing these faults for a future repair session.
Troubleshooting a Vintage Cossor CRM.555 Aviation Test Set
0:04 Introduction to the Device: The video introduces the Cossor CRM.555, a test set from the 1970s designed to generate signals for four key aircraft navigation systems: VOR (radio navigation), Localizer (runway alignment), Glide Path (descent angle), and Marker Beacons (distance indication). The unit is known to be faulty.
5:26 Initial Power-On Failure: When first powered on with a 28V DC supply, the unit immediately blows its 250mA input fuse and draws over 1A of current, indicating a short circuit or major fault.
5:56 Power Supply Diagnosis: After reverse-engineering the power supply schematic, the problem is traced to a 20V rail that was outputting an incorrect voltage of 22V. This was caused by a misadjusted trimmer potentiometer.
7:25 Repair #1 - Crowbar Protection Circuit: The excessive voltage was triggering a 22V Zener diode acting as a "crowbar" protection circuit, intentionally creating a short to protect the downstream electronics. This was confirmed with a thermal camera which showed the Zener diode heating up significantly (7:48). The issue was resolved by readjusting the trimmer to produce the correct 20V.
9:48 Carrier Frequency Testing: With the power supply fixed, tests show most carrier frequencies are generated correctly. However, a faulty crystal for the 109 MHz range is identified, affecting both the VOR and Localizer functions at that frequency.
14:41 Marker Function Works: The Marker Beacon function is tested and found to be working correctly, generating the proper 75 MHz carrier with the specified 400 Hz, 1.3 kHz, and 3 kHz amplitude modulation tones.
15:34 VOR Modulation Failure: A major fault is discovered in the VOR function. While the 9.96 kHz sub-carrier is present, the essential 30 Hz modulation signal is completely absent. This makes the VOR signal unusable and points to a problem with the 30 Hz generator.
16:20 Localizer and Glide Path Modulation Issues: The Localizer modulation, which combines 90 Hz and 150 Hz signals, works perfectly. However, the Glide Path modulation signal, which uses the same tones, appears to be too high, causing saturation or clipping in the output (17:14).
17:35 Summary of Remaining Faults: The video concludes with a list of identified issues to be addressed in a future part:
A faulty 109 MHz crystal.
No 30 Hz modulation in VOR mode.
An excessively high modulation signal in Glide Path mode.
A minor issue with a step attenuator.
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