Repurposing Older Equipment
Oct 1, 2014 9:00 AM, By Doug Irwin, CPBE DRB AMD
A recurring theme throughout this column has been repurposing older gear that one often finds around a radio station. Many of us don''t like to relegate classic pieces of broadcast gear to the e-waste pile.
Figure 1: Orban 8100 with XT Chassis
I think the one device I have seen on storage shelves more than any other classic is the Orban 8100 (Figure 1). No one wants to get rid of those it seems, and for good reason. The 8100 is a dual-band AGC, HF limiter and stereo generator combination all in one box: analog audio in, composite baseband out. The 8100 had a production run from about 1981 through about 1992.
Like any other device that is 20+ years old, the 8100 has parts that wear out. If you plan to put one back into service, consider changing the major power supply caps at the very least. The attenuator pots also get noisy as they age; you''ll likely need to change those out. You might be able to get older ones to be quiet enough by applying a product such as Stabilant 22 from D.W. Electrochemical (Figure 2).
Figure 2: Stabilant 22
A small cottage industry has grown up around the Orban analog processor line. You could consider getting an older 8100 completely rebuilt prior to use. (See: http://www.optimod.fm)
So just what could you do with an older analog FM processor like the 8100? The most obvious use would be its original purpose�an on-air processor for FM. Since you found one on the shelf, I''ll infer that you have something else on the air as a �main� processor. Instead of letting the 8100 collect dust, put it in place as a ready-to-go spare. Don''t just wait until something happens to your �main� processor; have the backup installed and ready to go, with the ability to switch to it remotely. An easy-to-use composite switcher can be made using a Broadcast Tools SS 2.1 MLR/BNC. (See Figure 3)
Figure 3: Broadcast Tools SS 2.1 MLR/BNC
Some of you likely know that the Orban 8100 had an accessory known as the �XT chassis.� (Pictured below the 8100 in Figure 1) This was a 6-band AGC that was inserted between the output of the dual-band AGC section and the HF limiter. Many people like the sound of the XT but there really aren''t that many of them around. If you want to �hotrod� the 8100 (to some extent) you can make use of the same send and return audio points available on the motherboard that were used to make insert points for the XT. Instead of the XT chassis, you can make use of a pair of Texar Prisms, for example. Your system would then be dual band AGC >> multiband AGC >> HF limiter >> stereo generator. You would use the 8100''s AGC to ride gain into your multiband, and then you would use the output level of your multiband to drive the HF limiter inputs. Not a bad �backup� processing scheme by any means.
Another common use for the 8100 is a headphone processor for the air studio. �Back in the day� it was common for jocks to listen directly off-air to themselves. Today there are many reasons why that may not be possible: a profanity delay inserted in the program chain, delay due to HD Radio transmission, or delay due to a digital STL system or certain digital audio processing techniques. Jocks rarely listen to themselves in real time anymore. Universally, jocks seem to like to hear themselves with some sort of audio processing going on. The 8100 can fulfill this role pretty well, and hopefully give your jocks some feel for how the AGC affects the on-air mix. This is important because when someone talks over a music bed (for example) they really need to hear the effects of AGC so that they can tell whether or not their own voice is loud enough with respect to the music bed and not getting �drowned out� by it. If they listen to �flat� (unprocessed) audio it''s really hard to tell if the mix will be good from the standpoint of the listening audience. That''s where an AGC for headphones helps.
If you use the 8100 for this purpose, there are RCA connectors on the rear apron that have processed audio on them�with pre-emphasis. You can use these, but you''ll need to add de-emphasis ahead of the feed to your headphone amps. The better way is opening up the 8100 and stealing the output of the dual-band AGC from the correct points on the motherboard. Clearly you will need to have an 8100 manual for this.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at email@example.com.
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