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RCA BTA 1R1 Audio Issues




 
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Author Topic: RCA BTA 1R1 Audio Issues  (Read 5399 times)
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Bob McGraw
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« on: October 14, 2006, 09:20:37 PM »

In going through the audio of the recently acquired TX, I discovered that the modulation lacked a lot to be desired. Namely, it seems to take a lot of audio drive to modulate 100%, and the modulation didn't seem too symetrical, and the frequency response was very unsatisfactory. Now I know a lot of these have been used in BC stations for years and obviously work well. Although the audio input is a balanced input, it is balanced to ground. Meaning that driving it from a unbalanced source creates problems. The reason being is that the center tap of the audio input transformer is grounded. Thus when driving it from an unbalanced source escentially 1/2 of the primary is shorted. The complex inductance of the winding, the winding capacity and wiring creates all sorts of ills. The solution is simple. Clip the jumper on the transformer that grounds the junction of the two windings. Be sure to leave the ESS terminal connected to ground. This still provides one with a balanced input but balanced above ground. If one wishes to drive it from a unbalanced source the the unbalance occurs at the source as it should.

Once done, I found that modulation symetery was greatly improved, frequency response greatly improved and it is much easier to drive from either a balanced or unbalanced source.

Also, I removed/disconnected the RLC network on the input of the audio transformer. Didn't determine exactly the reason for its existance, but perhaps that's another project for another day.

73
Bob, K4TAX
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