Could be quite a few possibilities. I would check these points first:
- Check your AC ripple voltage. (Measure the amount of AC present at the DC testpoint). If you are reading more that .2-VAC (which is border line), change out the DC power supply. If you have a bad filter capacitor in the DC supply, it will cause the AGC pilot carrier to flux up and down (similar to a sine-wave) as this is what the DC detector voltage references to in the AGC circuit. If you have low AC into the amplifier, this too could cause the DC supply to loose regulation and give the same effect.
- Confirm that the AGC is the CORRECT version for your specific model SAII unit. Different AGC's were mated with specific modules. There is a part number cross reference chart available. I have but not where I am at present. There were grounding problems with the earlier units and some AGC's were more specific to the unit it was working with. If you have the SAI (Suitcase Amplifier) this would not be applicable.
- Confirm that the unit was properly balanced. Input, inter-stage and AGC pilot padding are all equally important when finding the "sweet point" of the AGC. The 450/550-MHz SAI units were notoriously over-equalized on the front-end due to technicians not installing the interstage equalizer (P3 Filter) that typically gave 9db of interstage tilt. Thus, LARGE equalizers were often used (28.5dB/30dB) on longer trunk runs when a smaller (19.5dB/21dB) would be more applicable. This would in some instances, if over equalized, effect the AGC capture range of the reference carrier. If over-equalized, this would also cause poor carrier-to-noise in the lower frequency spectrum.
- Confirm that the the system sweep signature is within an acceptable flatness specification. If the AGC pilot in within a "peak" or a "valley" that is outside the AGC pilot capture range, this too will cause improper AGC tracking.
Good luck!
Greg Tobin
314-422-4423 cellular
Re: old sa agc's
Member #: 7190
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Greg Tobin
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