"Sherlock Investigates High Noon Radio Failure" 

Sherlock Holmes investigates a PRC319 Mission Failure.

It was a dark and cloudy day when a PRC319 was returned to SAS HQ G4 Stores for analysis from Iraq.

The Sigs Platoon Technician took a look at it but there was not much on the failure ticket.

Once again, my Blogger John Watson was unavailable to accompany me, so these writings are only from my sketchy notes.

I hope they prove helpful to others faced with such a singular failure.

I took the Tube over to Charing X Station and then over to Whitehall to pick up the radio from my brother Mycroft.

It was wrapped in a plastic bag and the operator had attached some white mine-tape to the radio stating the following:

"The operator was on HP (High Power) when the receiver audio first started to become distorted”.  The Iraq environment was Hot.

The first thing that was done was to try and duplicate the failure. The radio was set up on the bench and nothing was wrong with it.  The radio was then moved to an outdoor test area in the hot sunshine.

After about 30 minutes while listening to CHR (Trenton Military SSB Weather Station) on 15,034 kHz. There was a slight ‘Donald Duck’ voice that was exactly mimicking the main speech, but the interference was upside down. It got worse and worse until after an hour the main speech could not be understood. (Radio case temperature was about +90F).

To understand this, you have to realize that the PRC319 uses a Direct Conversion receiver and the audio is injected into a Balanced Modulator such that the upper audio band 1.7 to 3.1 kHz is folded over to 1.4 kHz to zero.

This unwanted ‘Donald Duck’ sound became the primary suspect for the cause of the issue. There was no “Audio” distortion heard in the receiver audio. Any unbalance of the Balanced Modular could cause this.

The problem was a variable level of distortion, The hotter the radio was the greater was the interference and the interference went away at room temperature.

The trouble shooting continues by opening the unit and inspecting the Printed Circuit Boards for any visual defects, i.e.: Reversed capacitors or damaged parts. Nothing was found. After the circuit boards were placed on extender boards the suspect boards were heated with a heat gun.

The main Phasing Board was temperature sensitive, a quick board inspection resulted in a slight board twisting and the failure became a mechanical defect instead of a thermal failure. The distortion switched on and off with probing and pushing on components.

The failure was quickly located to C3, a Tantalum Chip Capacitor. (It looks like a Sprague, 2.2MF Orange Dog Bone, See picture).  The polarity was correct, and it was not shorted. The positive riser lead in the capacitor was not connecting all the time, this is typically called a "cold" solder joint.

For expediency, the capacitor positive lead was resoldered, and the defect was eliminated. The unit was reassembled and retested at +130 degrees F for 3 hours and never failed. The unit was returned to service the next day.

Sherlock