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Author Topic: Johnson Valiant Microphone Related Problem  (Read 996 times)
KA4KOE
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« on: March 28, 2013, 12:12:30 PM »

I am working on bringing my Valiant up to snuff but I have a strange problem. I hooked up my Electrovoice microphone EV638 for the first time. No audio, so something must not have been correct in its wiring. However, noted some significant overheating in the area of components R22/R62, V21-6BY5, and the bias transformer T2, as well as two power resistors hooked up to R22/R62. Not able to locate the direct location of this issue as I killed power immediately as to not cause a conflagration. This problem does not happen when the rig is keyed up in Am mode without the microphone hooked up. I don't think anything was fried, but it got really hot.

I've checked the schematic and cannot determine how a mic wiring issue would cause this. Pin two goes to ground for PTT. IF pin one audio goes to ground via a short then the dc path to the grid of the first audio amp would be blocked by a capacitor. Plus I don't see how this would impact the bias area previously mentioned.

I recently changed out the 866s for solid state Taylor 866AS-1's.

I would've already posted in Amfone, but I'm having login issues (self-inflicted) at the moment.

You can also direct email me ka4koe@arrl.net

Tnx.

Philip
KA4KOE
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W6OU
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« Reply #1 on: March 28, 2013, 01:01:26 PM »

Your audio stages could be oscillating when the mic is connected or your mic may be picking up rf. Does the problem exist when the audio gain control is turned all the way down?
« Last Edit: March 28, 2013, 01:03:57 PM by W6OU » Logged
KA4KOE
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« Reply #2 on: March 29, 2013, 04:14:53 PM »

Don't recall.  I thought of this briefly, but didn't give it much thought.  Makes sense, though, as I had the mic about a foot from the dummy load at full carrier. A mic wiring issue may have turned it into a great antenna for the audio input stage.
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