astatic d104 feedback/hum

[camaro1]

ok, so a friend had a bunch of mics and wanted to get them all wired for his radio's and re-new some of them to working condition. 4 astatic d104 desk mics, and some turner mics and some old radioshack power mics.

so I have been working on a couple at a time, I got all of them wired and working and he even gave me a very nice all chrome d104 powered desk mic for my time, but the one he really wants for his set up is one of the d104's that has a black base and head, all the d104's have the amplifier in the base

I have been replacing the worn out old elements in the d104's with the radioshack elements and adding a resistor in the base to power up the element, got 3 of them working perfect and sounding great on my radio (rci 2950dx) and his rci 2995dx.

the one he really wants working is giving me a hard time, it works perfect on my radio, tested for a full weekend on my radio and talked to him with it on my set up and he even said it sounds great, but then I give it to him and on his radio it gets a hum/feedback/clicking noise that I can here on my end when he talks. the only thing I noticed when I wired the mic plug on this one is that the wires were very,very small/thin compared to the other d104's that we did the same exact mods to, now im thinking it has to be some type of RF noise or something gettiin into the mic but only on his radio setup

I went through the mic 2 times now, and on my radio it works perfect every time.

is it possible that this mic has a lower quality mic cord and is picking up RF on his setup,, I suggested that he pick up some ferrite chokes and put them on the mic cord and give that a try?? does this sound like it might be whats going on

thanks, jay

 

[1FatBoy]

If the other mics arent doing the hum/click then it is quite likely that the cord is receiving some stray RF. He does not hear these clicks in his receive does he?
Yeah, some Ferrites may work but you are always better off eliminating the problem then putting a band-aid on it.

 

[camaro1]

he picked up a clamp on ferrite choke and it didn't help at all,, im going to replace the mic cord and try that later this week

 

[Shockwave]

The first thing you must determine is if it's a hum, feedback, or clicking. Each have their own cause and are not usually related. Hums are usually low frequency 60 cycle and are caused by poor shielding or an open ground. Feedback is high frequency often around the pitch of a whistle and caused by RF feedback into the mic line. Reducing the mic gain or adding chokes or bypass caps help here. The last symptom is confusing. Clicking in the audio could be dirty contacts if the clicks sound scratchy and have no regular intervals or repetition. If the clicks are repetitive and keep a consistent frequency, it is likely the audio pre amp is defective and is oscillating.

 

[camaro1]

its strange that the mic works perfect on my 2950 dx, but not on his 2995dx

and all the other d104's that I did the same element mod to all work fine on his radio, the only thing I noticed on this one was that the wires in the mic cord were very,very small compared to the other d104's, so ill change that out probly tomorrow evening for him and try again

 

[Shockwave]

Describe the sound it makes in more detail? The thin 6 wire mic cord is the better of the two so that's not the problem unless one of the wires is open.

 

[camaro1]

its a higher pitch hum along with a repetitive clicking noise that covers his voice only on his radio, his radio is an untouched 2995dx

 

[Shockwave]

Since it only does it on his radio, I'd look at the radio a little more. I've seen these radios snap wires off on the inside of the radio mic jack. Sometimes it's just a wire off a RF bypass cap so the radio works fine with a stock mic but when you put an amplified mic on them, they squeal. Another issue is that some radios squeal with a D-104 when the modulation guard is left in the stock setting.