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These Spurs Have No Boots

Frank Hertel troubleshoots spurious signals in an exciter

Frank Hertel’s repair shop, a division of Newman-Kees Consulting, was asked to resolve a problem of spurs on either side of the main carrier in an Energy-Onix 30W Stealth FM Exciter. 

As these exciters age, the electrolytic capacitors in the RF power output module will dry out. At this point, the capacitors begin to become inductive and lose value, as well as developing an elevated equivalent series resistance, or ESR.

The repair is easy, Frank says, but it can be time-consuming. To provide a tight RF cabinet shield, the manufacturer uses a lot of screws. You can expect to spend more time accessing the RF module than you will spend in replacing the two electrolytic capacitors that are inside the RF Module.

Fig. 1 shows an overview of Frank’s workbench. Note the spurious signals (spurs) on either side of the main FM carrier, in the center of the display.

Fig. 1: Frank’s setup for repairing the FM exciter.

Fig. 2 is a better view of the spectrum analyzer showing these unwanted emissions.

This exciter was managing to interfere with some slightly distant stations, as far away as 30 miles from the offending exciter’s transmitter site. This was a citation (fine) waiting to happen.

Fig. 2: Spurs are seen on the spectrum analyzer of the exciter RF output.

Figs. 3 and 4 show what the capacitor checker revealed, once the defective capacitors were removed.

Fig. 3: To confirm Frank’s diagnosis, he measured the capacitance and the ESR of each capacitor.
Fig. 4: Although the ESR is acceptable for this capacitor, the capacitance value is low.

Fig. 5 shows the clean spectrum after replacing the bad capacitors. 

Frank adds that if you are careful, you will not need to remove the circuit board from the heat sink (more screws). The 100 MFD capacitor is surface-soldered, on the pads of the circuit board. However, the 10 MFD capacitor has its positive lead soldered on a through-the-hole connection. If you apply your skills, you can still solder the positive lead without removing the circuit board from the heat sink. 

Fig. 5: The spurious emissions (spurs) are nonexistent after replacing these two electrolytics.

Using your ohmmeter you will be able to find that the leads of some smaller, nearby RF bypass capacitors are in a parallel, with the connection, for the positive lead, of the 10 MFD capacitor. Use a little ingenuity and view the schematic to see that soldering the positive lead of the 10 MFD capacitor to the appropriate lead of one of the small bypass capacitors is essentially the same physical connection as the through-the-hole connection provides. It will work quite well and save time without compromising the stability of the RF driver stage.

Fig. 6 shows a view of the two capacitors to be replaced. Each is marked with a red slash.

Fig. 6: The two capacitors to be replaced are marked with a red slash, one on the left, and one on the right of the RF module board.

Fig. 7 displays the spectral performance and RF output power after the repair is completed. This easy repair will save you both time and money, which will make the bean counters happy. Don’t forget to tell the GM you did so, too.

Fig. 7: The repaired exciter with no spurs and making licensed power.

[Read Another Workbench by John Bisset]

Make songs in seconds

While searching the web, Dan Slentz found a piece of equipment that could be useful for every production director. Artiphon offers the Orba 2, a $149 handheld device that permits the composition of music using synthesized instruments like drums, guitars or piano. It can also record and modify voice. 

The Orba website includes a video explaining the features.

Remember the Sonovox Talk Box, which dates to the 1940s and was popular in some of the 1960s PAMS jingles? The Orba offers similar effects. 

Here’s a fun early video of the Sonovox in action. 

Tips please! Workbench submissions are encouraged and qualify for SBE recertification credit. Email [email protected].