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Post by Tombstone (R.I.P.) on Aug 19, 2008 23:34:28 GMT -5
I can imagine that you're sick of messing with the limiter. If you can control the modulation from going over 100% with the mic gain then I say leave well enough alone but if you have to turn the mic gain way down to make the radio sound right then restore the limiter function.
Tombstone
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Deleted
Deleted Member
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Post by Deleted on Dec 29, 2008 18:50:11 GMT -5
OK. This radio is an enigma. It has a 9 out od 10 appearance after restore. Has great ears. Transmit audio downright sucks. Mod limiter is a 3.3uf cap.
1. I adjusted modulation on a scope for slightly above 100%, with the limiter intact. The reports are horrible. In the grass, etc.
2. I lift the limiter, and get much louder, but complaints about clarity (ironically, from the same people that said to clip it. Duh!). It does not like older power mics. It seems to need a newer D104 or Turner +3. I get decent reports with a stock mic, but sounds tinny.
Anyway, is there any advice on replacing the mod limiter with either a different cap or a resistor? This is my bench rig that I use to talk while I repair another unit. But I am sick of hearing how bad it sounds.
Thanks.
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Sandbagger
Administrator/The Boss
Posts: 6,250
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Post by Sandbagger on Dec 29, 2008 22:02:11 GMT -5
OK. This radio is an enigma. It has a 9 out od 10 appearance after restore. Has great ears. Transmit audio downright sucks. Mod limiter is a 3.3uf cap. 1. I adjusted modulation on a scope for slightly above 100%, with the limiter intact. The reports are horrible. In the grass, etc. 2. I lift the limiter, and get much louder, but complaints about clarity (ironically, from the same people that said to clip it. Duh!). It does not like older power mics. It seems to need a newer D104 or Turner +3. I get decent reports with a stock mic, but sounds tinny. Anyway, is there any advice on replacing the mod limiter with either a different cap or a resistor? This is my bench rig that I use to talk while I repair another unit. But I am sick of hearing how bad it sounds. Thanks. If there is sufficient mic gain such that your average modulation is over 50%, and you limit the peak to 100%, there is no reason that anyone should tell you that you're "in the grass". But tone issues do have some part to play in the overall quality of the audio. A thin, tinny sound will not convey as much information and will sound weaker. If the radio is sufficiently old, you may have electrolytic coupling cap value drift issues. Sometimes, certain brand/model radios just don't sound good (The old TRC-23 series Navaho comes to mind) and some circuit modifications to alter the tone quality of the audio can help improve an otherwise tinny sounding radio. I wouldn't worry too much about the limiter unless it is limiting low frequency audio more so than high, resulting in a high frequency emphasis. You can test this by putting in a variable frequency audio generator. Set the limiter for wide open. Set the generator for 1000 hz and adjust the audio level to just make 100% modulation. Now vary the frequency of the generator from 300 to 4000 hz and see if it maintains 100% or if it falls off and make a note of how much. Now repeat the test again only this time set the generator to exceed 100% and adjust the limiter to hold at 100%. Compare the before/after tests and see if the limiter causes the audio to fall off more so at the low frequencies than what it did without the limiter. If it does, you can change the value of the limiter's coupling cap to a lower value, which should shift it higher in frequency. The same sort of stuff can be done to the audio circuit. If the audio falls off without the limiter in the lower ranges, you can increase the coupling caps between audio circuits, which should shift the response lower in frequency. That RCA radio is a simple circuit, where the audio chip is pretty much the whole modulation circuit. So it's not out of the realm of possibilities that the chip could be bad (Just not catastrophically bad). There are also caps attached to the chip which set the audio response. You can play with them and see if it improves things. Look at the engineering design notes for the TA-7205 chip and use their standard values for best fidelity. The best thing to do is play around until you're happy. Your best tools are the audio generator and your ears.
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