Test Equipment

Don’t try this with your $35 Baofeng. :) The news here is that NIST has not only tested units at high temperatures, but has developed a methodology and a chamber for testing radios at high temperatures. This should help radio manufacturers develop more reliable radios for firefighters. This item is from the December 16, 2014 issue of NIST Tech Beat…Dan

New test results* from the National Institute of Standards and Technology (NIST) confirm that portable radios used by firefighters can fail to operate properly within 15 minutes when exposed to temperatures that may be encountered during firefighting activities.

A portable firefighter radio is instrumented for testing in a NIST-designed apparatus that consistently creates thermal conditions representative of typical fire environments. Data and performance measurements recorded with the equipment are furnished to the National Fire Protection Association, which is developing a performance standard for portable radios used by emergency personnel. Credit: NIST View hi-resolution image

Firefighters rely on the radios to report their location and to communicate with other first responders as well as the incident command post or communications center. Performance problems with portable radios have been identified by the National Institute for Occupational Safety and Health as contributing factors in some firefighter fatalities.

All seven of the firefighter portable radios tested by NIST failed to perform properly within 15 minutes when exposed to temperature levels encountered in “fully involved” fires, as when all the contents in a room or structure are burning. Four of the handheld radios stopped transmitting, and three experienced significant “signal drift,” rendering the radios unreliable for communication.

The failures occurred while the radios were subjected to a temperature of 160 degrees Celsius (320 degrees Fahrenheit), termed Thermal Class II conditions.** The temperature is representative of a fully involved fire or conditions outside a room when its contents burst into flames simultaneously, a phenomenon known as flashover.

During the post-test cool-down period, three of the radios did not recover normal function.

Funded by the U.S. Department of Homeland Security, the NIST tests further ongoing work to develop performance standards for firefighter portable radio equipment, which includes radios, wearable combinations of speakers and microphones, and related items. The existing standard provides only general guidance—that portable radios “be manufactured for the environment in which they are to be used.”

NIST researchers are furnishing their test data and performance measurements to the National Fire Protection Association, which is developing a performance standard for portable radios used by emergency personnel.

As important, the NIST team designed a prototype apparatus to electronically control testing equipment intended to consistently create thermal conditions representative of typical fire environments.

All radios tested by NIST performed reliably when exposed to a temperature of 100 degrees Celsius (212 degrees Fahrenheit) for 25 minutes, or Thermal Class I conditions, akin to a small fire in a room or fighting a fire from a distance.

No tests were conducted under more extreme fire conditions (Thermal Classes III and IV).

“Realistic and reliable performance tests provide clear design targets for portable radio manufacturers,” explains NIST fire protection engineer Michelle Donnelly. “Standards incorporating these tests provide firefighters with the assurance that their equipment will perform as expected under specified thermal conditions.”

*M.K. Donnelly, W.F. Young, and D. Camell, Performance of Portable Radios Exposed to Elevated Temperatures (NIST Technical Note 1850); September 2014.

**See NIST Technical Note 1474 at http://fire.nist.gov/bfrlpubs/fire06/PDF/f06001.pdf.

 

A couple of days ago I reported that I’d let the magic smoke out of my Tek 2215 oscilloscope. I was playing around with it last Friday evening, heard some arcing, and then saw a puff of smoke exit the back of the instrument. Oddly enough, the scope still seemed to work, though.

To get some help, I joined the TekScopes Yahoo Group and described my problem. Almost immediately, I got a couple of responses. One guy suggested:

I would take a look at surge suppressor VR901 and line to neutral capacitor C901 at the AC input before the preregulator. Either could have failed without significantly affecting operation of the oscilloscope.

Another said:

I’ll wager that your Tek 2215 has a failing AC-inlet filter. Visit the following site: http://www.vintage-radio.net/forum/showthread.php?t=88137, which discusses failures of Schaffner AC-inlet EMI filters due to a certain type of capacitor manufactured by Rifa…http://www.vintage-radio.net/forum/showthread.php?t=69128

One commentator mentions incompatible rates of expansion between the epoxy encapsulant and the capacitor innards. Another explanation (which I like better) is that microcracks in the encapsulant allow moisture to get into the metallized paper that forms the capacitor’s innards. The innards expand, which applies stress and makes more cracks, and the deposited metallization corrodes and forms conductive compounds with the moisture. Before long…pffffft!

Neither was completely correct, but they both were close. See the photo and schematic below:

It looks like the problem is indeed a capacitor in the filter section, but my scope does not have a Schaffner AC filter, like they used on some of the earlier (and more expensive) scopes. Instead, this filter is right on the main board. I’m guessing that the 0.068 uF cap shorted out,which led to the resistor burning out.

One odd thing about troubleshooting this problem is that while it looks pretty obvious from the photo, it wasn’t that obvious right off the bat. As you can see from the schematic L925 is not on the board. Instead, it’s mounted in a little housing that sits right above these two components. I had to remove that housing before I could see them.

At any rate, now I just have to find a suitable replacement capacitor and solder it all back together.