revbobuk Posted January 27, 2019 Share Posted January 27, 2019 Guys - discovered an elderly audio-technica ATW-T51 beltpack, and associated ATW-R14 fixed frequency (864.420 MHz) beltpack. Even without a mic attached, and even on standby, there is a substantial level of hum being received. A number of questions spring to mind! 1. Is this a known thing?2. Is it possible that some part of the transmitter circuitry has drifted out of alignment? - if so,3. Are schematics available anywhere?4. I've opened up the TX, and there's nothing obviously damaged. Nothing makes the hum obviously worse when prodded. Is the beltpack the likely culprit? I presumed it was, as the receiver is otherwise silent. Good ideas, anyone? Link to comment Share on other sites More sharing options...
richardash1981 Posted January 30, 2019 Share Posted January 30, 2019 Guys - discovered an elderly audio-technica ATW-T51 beltpack, and associated ATW-R14 fixed frequency (864.420 MHz) beltpack. Even without a mic attached, and even on standby, there is a substantial level of hum being received. A number of questions spring to mind! 1. Is this a known thing?2. Is it possible that some part of the transmitter circuitry has drifted out of alignment? - if so,3. Are schematics available anywhere?4. I've opened up the TX, and there's nothing obviously damaged. Nothing makes the hum obviously worse when prodded. Is the beltpack the likely culprit? I presumed it was, as the receiver is otherwise silent. Good ideas, anyone? Are you using the original mains power supply for the receiver or not? I found that the fixed VHF Auio-technical I had to replace the PSU for was very sensitive to ripple on the DC input (even from a switch mode PSU!) which made it hum.Is it possible that the lapel mic has a wiring fault, e.g. the screen is disconnected at the beltpack end? Depending on the capsule wiring you may get signal, but with an unscreened mic lead! Link to comment Share on other sites More sharing options...
revbobuk Posted January 31, 2019 Author Share Posted January 31, 2019 Original PSU. I've re-soldered the mic cable, as that was my first thought too, but the hum is present even without the mic cable, both when the transmitter is on and when it is on standby/mute. Goes when the transmitter is off, but of course that doesn't rule out the problem being with the receiver. The him does seem to change slightly as you move the transmitter around. Link to comment Share on other sites More sharing options...
revbobuk Posted January 31, 2019 Author Share Posted January 31, 2019 Or not. Just goes to show that you can't take anything for granted. I went back and checked the PSU again - and it wasn't the original Audio Technica. More than that, neither the voltage nor the current rating were correct. In fact, it wasn't even a DC output! It was an AC output PSU. I'm pleasantly surprised the receiver was still working at all, and swapping to a better suited PSU has improved the situation enormously. But I'm still not quite sorted. The tieclip mic is horribly sensitive to handling, and I'm not at all sure it has been soldered correctly. Pinouts I've been able to find online don't correspond to what I see on the mic. And the existing cable has screen and 4 conductors, 2 red and 2 blue, which are simply twisted as a 'red' pair and a 'blue' pair. Most pinouts have a resistor shown: Anyone know the correct pinout? Link to comment Share on other sites More sharing options...
timsabre Posted January 31, 2019 Share Posted January 31, 2019 The resistor is to provide a dc bias to the capsule if it is an electret capsule. Is this the original mic and do you know what type of mic it is? Link to comment Share on other sites More sharing options...
revbobuk Posted January 31, 2019 Author Share Posted January 31, 2019 It's the original AT829CW which certainly should be an electret needing bias. I'm going to try rewiring it according to the diagram, and see if that helps. Link to comment Share on other sites More sharing options...
revbobuk Posted January 31, 2019 Author Share Posted January 31, 2019 Good - resoldering with the correct pinout has fixed the low signal level, and there's now no hum. All I need to do know is source a replacement clip and windshield and we're all sorted. Thanks for the help, appreciated. Link to comment Share on other sites More sharing options...
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