The AM Forum

I bought a "new" EV 719 ceramic mike that is connected to a Heath Apache mike input that has no audio mods.  My D-104's all work fine, but the 719 has a weird problem.

Looking at the modulator current as an indication of actual audio, after 5 to 10 minutes of speaking into the mike, the audio gradually drops off to almost nothing, requiring me to shout into the mike to get the mod current to flicker.  If I disconnect the mike from the Apache and reconnect it, all is well for another 5 to 10 minutes.

The mike connector looks fine to me (Amphenol single center pin with a solder blob)  I have scraped, sprayed and re-soldered both sides, to no avail.

Any ideas, other than scrapping the mike and sticking to the D-104's?


TNX

Maybe the input grid resistor is open letting the grid of the first tube build up a negative charge. A shot in the dark. If all is as it appears your only breaking and making one connection, well actually two counting the shield. Could you push a small wire or pin in the back of the connector while it was connected momentarily shorting it?
   Enjoyed our qso last week, Jay-

If the grid resistor in the Apache was open, how come all my D-104's still work?

When I got the mike originally, there was a Triad audio coupling transformer in the mile line, but it looked to be a user modification, because the soldered connections were insulated with masking tape.

I may try putting a 1 meg resistor across the mike element directly.

TNX

Try to RE-install the transformer.
It will isolate the mic from the Apache circuitry and stop any DC coming back to the mic element. Possibly polarizing the element in the microphone and it stops moving. Voice=mechanical movement in mic=electrical audio voltage to the Apache.

Fred

Or add a coupling cap.

I tried the coupling cap (4.7nf) with no effect.  Still fails.  I'll re-install the transformer tonight and try again