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FT-301 low mic sensitivity
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Does anyone have a 301 that does not require all mic gain controls to be maxed out and close talk the mic to even get close to full output?
I have been struggling with this 301 since I got it trying to get enough modulation to get full output on SSB. On CW, full output is no problem. I thought maybe I had a defective LD3141, but every test I do tells me otherwise. The only thing that works is an outboard op-amp at 300x voltage gain driving the balanced modulator directly with 700-800 mv at full audio. I have built duplicates of the LD3141 on the breadboard with exactly the same gain as the original component. With about 2-3 mv into the LD3141 from a dynamic mic, I get between 150-200 mv out to the balanced modulator but I cannot find gain specs for the LD3141 to know if this is good. I do not want to condemn this component until I know better. All the modulator diodes junction test ok and exhibit the correct Vf for a germanium diode. I do not see any indication that they were changed before I got it. Carrier points have been verified to be correct, as well. |
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You probably already know this but just in case and a reminder for others;? Are you using an active circuit true peak reading (pep) wattmeter to monitor the output of the 301?? On a regular wattmeter like a Bird 43 that doesn't read the peaks you will only measure average power which is 10-30%, depending on the voice, of the actual power the transceiver is developing on voice peaks.
The 301 makes full power nicely using the hand mic that was originally supplied with the transceiver.? I have found other mics generally lacking and in need of a preamp.? A D104 crystal mic works great when followed by a FET used as an impedance converter to go from high to low impedance.?? https://www.w8ji.com/d104_to_low_impedance.htm |
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Rex,
Yes, I have tried a peak reading wattmeter and verified that I do not get more than about 15-20 watts peak when speaking about 3 inches from the mic (mic gain at max). With mic gain controls at maximum, I need to whistle sharply into the microphone with it directly in front of my mouth to get full power out. I have checked using the Heath HM-2140 in PEP and average modes and the W-4. All will show 100+ out if I whistle directly into it with no clearance at the mic. I would expect a sharp whistle at a few inches with microphone gain at a mid setting to produce full power (or close to it). It acts as though I have a bad mic, but I have tried a half dozen known good microphones and they are all the same. The original Yaesu mic is actually a little better than the rest. The radio has no problem producing the power in CW. In fact, It does not require more than 50-60% drive to achieve full power. I am fairly confident that I do not have a gain issue in the remainder of the TX chain. |
开云体育Give full output in CW is now gets complex.Yeasu never supplies levels, anywhere. My guesses: 1. Audio board capacitors, level out to IF board low.. 2. IF filter cocked up, were you able to set the CAR OFFSETS for USB and LSB? 3. Is there any difference with speech amp on of off? That is another stage before the driver board. Good luck, Randy W7CPA On 8/29/2021 12:01 PM, n3kcb via
groups.io wrote:
Rex, |
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Randy:
All audio board caps new and no problems setting carrier points. Even if the filter was jacked, I was certainly able to get the carrier in a good spot for testing. I did do the carrier point adjustment by injecting audio into the mic jack. When I do this I can get full power out and set the 50w/12.5w points as specified. I just cannot do it with a microphone. Any microphone. With the speech compressor on, it has more gain. However, I have to crank those settings to max to do that. If I leave the settings for the compressor at midpoint where? I found them, it is about the same as compressor off with mic gain at max. If I talk with the mic touching my mouth, it would work, but that cannot be normal. |
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Well,
It was the filter. I did not seem to have any problems with the passband, but it must have been lossy. I put in a spare XF-92A that I had laying around and it seems ok now. I can get good modulation without having to crank the mic gain. It is ashamed, I was going to use that filter for a project. |
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