Consider ESR When Troubleshooting Audio Circuits

Be aware of the internal resistance between component leads, plates and electrolyte
By John Bisset ,

Broadcast consultant Tom Osenkowsky recently constructed a panel with two vu meters and a DPDT switch. The switch allows him to configure the meters to display conventional Left and Right channels or Left-Minus-Right and Left-Plus-Right signals. Tom used a Gentner routing distribution amplifier to serve as the electronics to feed the meters.

Upon connecting the meters and attempting to calibrate them, Tom realized that what he was hearing and what he was seeing were inconsistent. Upon examination, he discovered defective coupling and other capacitors in the RDA. The coupling capacitors are non-polarized electrolytic, with values of 1 uF, 10 uF and 100 uF. Capacitors in the level indicator circuits were common polarized electrolytic types. Tom decided the amplifier was worth repair, from both technical and financial perspectives.

When one tests capacitors, two common measurements are value and leakage. A third and important test is for equivalent series resistance or ESR. This refers to the internal resistance between the component leads, plates and the electrolyte.

Some circuits, particularly those used in high frequencies, can be grossly affected by high ESR. Many of today’s switching power supplies operate at high frequencies, and are subject to self-heating when high ESR develops in filter capacitors.

Fig. 1: The Sencore LC75 can be used to test for high equivalent series resistance.

Fig. 1 shows a Sencore LC75 Z Meter 2, which can test for ESR as well as capacitance value and leakage. In the RDA, Tom found some capacitors had deteriorated to values in the pF (picoFarad) range, and one had a measured ESR of 841 ohms! Replacing all the capacitors restored normal operation with wide frequency response and proper level indication.

ESR can be measured in several ways. A small amount of current is passed through the capacitor under test. The capacitor will not be fully charged. The ESR will be the voltage across the capacitor divided by the current (Ohm’s Law).

Another method is referred to as “waveform analysis.” This test employs a trapezoidal waveform, composed of a square wave and a saw tooth wave used as a ratio of resistance and capacitance. This method is employed in the Sencore LC75.

Fig. 2: A listing of divisions of Marlin P. Jones and Associates comes on a small card …

Fig. 3: … and the reverse side displays the resistive color code.

As mentioned, it has been Tom’s experience that some circuits are sensitive to capacitor ESR. Tom says he’s also found it critical to use low ESR capacitors in the Harris MW-1 transmitter audio board. There are a number of test instruments capable of measuring ESR. As a troubleshooting tool, the Sencore Z Meter is a wise investment, in Tom’s opinion.

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Jim Otte, a volunteer engineer for Radio Esperanza in Edinburg, Texas, noted our mention of the Marlin P. Jones and Associates website and wanted readers to know about its seven divisions. Displayed in Fig. 2, the divisions provide a selection of parts useful to the broadcast engineer.

On the flip side of the laminated card is a visual image for determining resistor color code values. Seasoned engineers probably have the color-code memorized, but those new to the business, or from the IT field, may find this little card useful. It’s available free from the Marlin P. Jones and Associates Co. (www.mpja.com).

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In response to our Workbench column “Is Your Transmitter Starving,” Roberta X writes that we seem to have omitted the old-fashioned filtered outside air intake.

The simplest version uses a very large opening with a good filter, keeping air infiltration via other paths down to a minimum, by reducing pressure differential to whatever is normal for the transmitter itself. More advanced versions add an intake fan, moving enough air to maintain slight positive air pressure. Adding cooling coils will lower humidity, as well as providing temperature control in warm weather.

Transmitter exhaust can be recycled in colder months to “temper” incoming air. While it is never as clean as a closed system, an open cooling system can be cheaper to operate. Sophisticated open cooling can cost as much as a sealed system to build but can be implemented gradually with careful planning.

In the heyday of tube transmitters, some collocated, small-market stations incorporated the transmitter room and airflow through the transmitter into their HVAC return-air loop.

The important takeaway is that air has to come from somewhere, and go somewhere — either moved in a closed loop through the HVAC, or into the building though an air intake and back out via the transmitter exhaust. Too often, we try to cool equipment by blowing cold air on it and ignore airflow.

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For stations looking to add a text-in request line, Iowa’s Coloff Media Engineering and IT Manager Lewis Callaway says look no further than Google Voice. For the monthly cost of $0, you can get a phone line for listeners to text to. You get a nice web interface that the DJs can log into and respond to listeners’ text requests.

To set it up, you need to sign up for Google Voice. In Lewis’ case, he created a new Gmail account for each station, which kept it simple. For Google Voice to give you a new number, you have to connect your cell phone to the account to “verify” that you aren’t just some spammer. (You will want to delete this phone number later, so you, personally, don’t get every text coming in to the station on your smartphone!)

Then you can choose a new number. For Lewis’ stations, he was able to get a vanity number with their call sign or station slogan.

Now you can go to hangouts.google.com for the account you just set up and go to the settings. On the settings page of Hangouts, turn on the option to send SMS with Hangouts. Any time you get a text to the number you setup, it will show up on Google Hangouts, and your jocks can interact with your listeners.

You can also use Google Voice as a virtual voicemail, where you can download MP3 records of any voicemail left when people call your number. Another station Lewis works for has used this service as a birthday and anniversary line, and more recently as a news hotline. It works great and sounds as good as a phone call can sound.

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I received a note from Jim Gorman of Gorman-Redlich Manufacturing regarding Dayton Industrial receivers: Jim bought the last of Dayton Industrial’s inventory when they were closing and can supply engineers with most anything you need made by Dayton Industrial.

Head over to www.gorman-redlich.com or contact Jim Gorman in Athens, Ohio, at (740) 593-3150.

Contribute to Workbench. You’ll help your fellow engineers and qualify for SBE recertification credit. Send Workbench tips to johnpbisset@gmail.com. Fax to (603) 472-4944.

Author John Bisset has spent 48 years in the broadcasting industry and is still learning. He handles West Coast sales for the Telos Alliance. John is SBE certified and is a past recipient of the SBE’s Educator of the Year Award.