Weirdness in a microphone tester...... (the continued adventures...)

[guitar_199]

A long while back... I bought one of those popular microphone testers on the usual site......

Battery clip wires were broken off of the PCB (pulled off is likely more the case!!!). But I dutifully pulled the board, repaired the battery clip wires and... as long as I was in there.... recapped it (only about 5 caps or so...). Then, for whatever reason, I put it away....and just kind of forgot about it.

Until today!!! Put a 9v battery on it, hooked up my audio generator directly to the input cap to the pot (this is where ALL of the mic audio comes in from the mic audio buss on the pc board!).

Half-yay!!!!! scratchy audio comes out.

Cleaned the pot with some control cleaner.

Full-yay!!!!! clean clear tone comes out.

Put it back together.... including soldering all the mic connector pins.... tried it with a microphone and....
RCV/XMT switch leds work fine. NO MIC AUDIO.

Pulled the bottom off to expose the pc board foil side, clipped the audio generator back on...but THIS time on one of the mic pins way down on the mic audio buss. CLEAR AS A BELL. not really LOUD....but clear.
I guess the thing just doesn't have enough gain to push the dynamic mic cartridge audio out.

I never would have figured it.... but I guess I need to go look up the LM386 and figure out how to get some more gain... then.... pull that board again. Is it worth it? Who knows.....

 

[nomadradio]

pull that board again. Is it worth it?

I have one that sounded really scratchy and distorted when I first got it. Made sure the speaker wasn't the cause. Put off pulling that board. I have LM386 spares, but the next time I tried it the thing sounded clear as a bell. But only for a while. A week later it's scratchy again.

Really oughta crack it open and find out why.

73

 

[guitar_199]

From the datasheet, with pins 1 & 8 OPEN the voltage gain should be 20.
With a 10uF cap across pins 1 & 8 the voltage gain should be 200.
My circuit has the 10 uF cap.... so it should be PLENTY loud.

This is easy enough to measure...... it's got to be something basic......

 

[Woody-202]

A long while back... I bought one of those popular microphone testers on the usual site......

Battery clip wires were broken off of the PCB (pulled off is likely more the case!!!). But I dutifully pulled the board, repaired the battery clip wires and... as long as I was in there.... recapped it (only about 5 caps or so...). Then, for whatever reason, I put it away....and just kind of forgot about it.

Until today!!! Put a 9v battery on it, hooked up my audio generator directly to the input cap to the pot (this is where ALL of the mic audio comes in from the mic audio buss on the pc board!).

Half-yay!!!!! scratchy audio comes out.

Cleaned the pot with some control cleaner.

Full-yay!!!!! clean clear tone comes out.

Put it back together.... including soldering all the mic connector pins.... tried it with a microphone and....
RCV/XMT switch leds work fine. NO MIC AUDIO.

Pulled the bottom off to expose the pc board foil side, clipped the audio generator back on...but THIS time on one of the mic pins way down on the mic audio buss. CLEAR AS A BELL. not really LOUD....but clear.
I guess the thing just doesn't have enough gain to push the dynamic mic cartridge audio out.

I never would have figured it.... but I guess I need to go look up the LM386 and figure out how to get some more gain... then.... pull that board again. Is it worth it? Who knows.....

That's weird that yours does that. I've had one for the last 10 years or so and Dynamic stock mics sound very good (and usually loud). The only mic I've put on it that doesn't have a lot of audio through the tester is the Astatic 636L.

 

[loosecannon]

i think they must have just used a very cheaply made pot in these things because i had the same scratchy audio from mine until i pulled the board (pain in the butt!) and cleaned the pot.

its worked well ever since, but i am starting to get some scratchyness again when i turn the knob so i think i just need to replace that pot.

Guitar199 you might check yours and make sure its not the pot itself that is bad.
LC

 

[Dopple]

Yeah, mine needs the pot cleaned every year or so. I try to use the tuner cleaner with the silicone in it, seems to last longer.

 

[guitar_199]

Guitar199 you might check yours and make sure its not the pot itself that is bad.
LC

I cleaned mine while I was in there... and it is as quiet as can be now.
Unfortunately STILL.... the scratchiness is all I hear... no mic audio....

and I have tested a cobra, a uniden and a 5 pin Cobra type... no joy.

It is weird.....

 

[loosecannon]

well i know im kind reaching here but just because you don't hear scratchyness from the pot doesnt mean its actually working.

it could be quiet because its not passing anything through it.
LC

 

[guitar_199]

it could be quiet because its not passing anything through it.
LC

When I tap an audio generator onto the mic jack pins..... the tone comes through like crazy.....

 

[Enterprise312ok]

Another dead mic tester for the pile.

 

[guitar_199]

Another dead mic tester for the pile.

Nah!!! Another rainy day project! We have plenty of rainy days here on the Tx Gulf Coast!

 

[loosecannon]

ok guitar199 what did you do to my mic tester!?

just kidding.

So, i went to test a known good 4 pin cobra coffin style mic yesterday (just to hear the audio from it) and experienced exactly what you did.

scratchyness when turning the pot but no audio out.

just for kicks i plugged that same mic into the midland 4 pin jack and actually heard some audio out when keying the mic.
however it was about half the volume i would have expected.

i know that's not good data, but i do wonder if we are in one of those situations where a certain run of a certain part is now reaching its end of life in a bunch of mic testers of a certain vintage.

hmmm...
LC

 

[loosecannon]

the problem on my mic tester ended up being that echo/mic switch in the upper right hand corner.
started playing with it and could get audio when held in a certain position but it disappeared when i let go.
switch replaced and she's working good as new.

might be something to check.
LC

 

[guitar_199]

the problem on my mic tester ended up being that echo/mic switch in the upper right hand corner.
started playing with it and could get audio when held in a certain position but it disappeared when i let go.
switch replaced and she's working good as new.

might be something to check.
LC

I've noticed that some have that switch... mine does not. This one is really simple.
I have mapped out the PC Board traces .... the audio line goes from the audio pin on each mic jack... to an electrolytic cap...to the top of the pot. The wiper of the pot goes straight in to the input of the LM386. The electrolytic cap is new......and when I tap an audio generator to the audio line at the mic pin... the tone comes out loud.

It's weird!!!