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Sherlock: The Case of the Three Orange Pips

Created by Created by Sherlock on 2019-11-18
Sherlock: The Case of the Three Orange Pips

This case may seem similar to the reader as the "Case of the Five Orange Pips" that was chronicled by Dr. Watson in 1887, but in name only.

Just as in the Case of the Five Orange Pips, this story began on a dark and stormy night, but in 1997, over one hundred years later.

Many of the United Kingdom's military radio's (PRC319) had just been released by the Ministry of Defense (MoD) as surplus and unneeded. The Special Air Service (SAS), The British Army's most distinguished and renowned special forces unit had used them in the Middle East with great success in the late 1980's.

The one that peaked my interest just showed up on eBay, It was described as "Operational" but my experience has been that no radios from that source were ever completely "Operational".

The bid was placed and the auction won. The radio arrived 2 weeks later wrapped in an old thin piece card board that had originally been a cereal box. The radio is pretty robust so there was no obvious shipping damage other that scratches from field use.

One month later a small envelope arrived, postmarked Crowley, England, it contained Three Orange Pips, See picture 1. They are frequently called Orange Drop (Epoxy Dipped) Tantalum Capacitors and there are many in the PRC319. The part envelope was marked, "M2222-122-56688, C3, C6, 6N8, PSU". What had they to do with the PRC319? Was it a practical joke, a curse? The 319 manufacturing plant was in Crowley.

The PRC319 was designed to be difficult to repair in the field. If one was captured they didn't want it to be repaired or reverse engineered, so they disguised the main fuse and made it hard to access, and they had no reference designators (R1, C1, D1, etc.) or polarity markings on the Printed Electric Circuits (PEC). That made it difficult for the assemblers and the inspectors to check during the assembly process too.

There are many self protection circuits that shut the radio down if activated; Thermal Shutdown, Gamma Radiation, Current Limit, and Over Voltage sensors. Any failure will turn the radio off and erase the frequency memory. This makes trouble shooting difficult because it prevents any catastrophic evidence from being uncovered. There won't be any burned components after a short circuit, for example.

So here is a $21,000 (1989 USD) "Door Stop" radio that worked well until about 30 minutes of operation, then it failed and would recover so that the failure could not be duplicated. It probably was sent out on another mission to fail again.

The radio was taken out to the field on a warm sunny day and after about 30 minutes of 50W CW operation it shut down. It was taken back to the repair shop and the failure was attempted to be duplicated. After the unit was heated up it would fail. It is designed to operate from -31C to +55C. The internal Power Supply Unit (PSU) was replaced with a known good power supply and the unit did not fail again. The problem was in the PSU. Reheating just the PSU caused the failure to recur. The PSU contained many of those mysterious Orange Drop capacitors. See pictures 2 and 3.

Had someone in Crawley known what was wrong and directed me to the defective part?

The Capacitors were only faintly marked and the PEC was not marked. The wide positive polarity pin could not be seen after the anti-vibration mounting adhesive was applied. A small soldering iron was used to heat the PSU parts individually and C3 seemed to be very heat sensitive, and it was reversed! Backwards. C3 is a 6.8 MF, 25V, Orange Drop solid Tantalum capacitor, See picture 4. When the capacitor became leaky (resistive) it shut the radio off.

There are over 2200 components in the radio and it even has BITE (Built In Test Equipment) but that would not work with the radio off.

The failure rate of a reversed solid Tantalum capacitor is very high. See This was not the result of the "Self Healing" properties of solid Tantalum Capacitors.

The capacitor was replaced using one of the "Pips" sent from Crowley and the unit worked correctly at temperature. This single 50 cent part caused the radio to be scrapped and might have caused a mission failure.

'It is true that I have been generally successful' but some other failures have eluded me, not this one. "None of our cases have been more singular as this, except the Sign of the Four".

No radios were ever lost to the enemy but many failed because of human error during assembly.

Picture 5 shows the front view of the PRC319. Additional information about it can be found at:

If you want to inspect your 319 power supply for correct Ta Orange Pip polarity positioning I have made a little template showing which way they should be mounted. The arrows on the picture shows the side of the capacitor where the printing is. You have to remove the power supply to see them. While you are inspecting them you can also look down the connector slot and see if you TO-220 transistor is flush with the case. You will see the little red feed through washer. The transistor sits on a thin grey silicon pad. It must be flush.

Sherlock, W0RW