Touching Mic causes mod plate current to soar?

[AA8A]

Hi,
Just finished a new HB rig, 813 mod by pair of 811A's. Works, puts out 150w
and is fully 100% modulated. Got good signal reports; however, cann't touch mic
even though grounded to chassis? When I touch any part of the mic, mod plate current shoots up. Just talking into the mic everything seems normal? Could I have RF in the shack getting in mic and when I touch mic I'm acting like an antenna? Need some advice to solve this glich?
Thanks, Bob  AA8A

[wavebourn]

What kind of mic is it? Transformerless dynamic mic with asymmetric output?

[AA8A]

Hi,
Mic is a Shure Model 55S Unidyne Dynamic

[wavebourn]

Does it have symmetrical XLR output? Does it have a transformer inside (you can check if impedance switch works)? Does your mic amp have an input transformer? If yes, you need to add RF filter on both wires near the mic input connector (2 resistors, 2 capacitors, a ferrite bead)

[Carl WA1KPD]

Bob,
I have the tubes etc for that type of rig. Do you have a copy of the schematic you used?
73
Carl KPD

[k4kyv]

I'd bet you have rf feedback.  It's probably oscillating at some higher audio frequency beyond the response capability of your receiver, so you don't hear it.

Have you looked a the envelope pattern on an oscilloscope? 

Try listening on your receiver and tuning off from both sides of the carrier  frequency for spurious sidebands.  Don't keep the high modulator plate too long; it might damage tubes or mod transformer.

I have had that exact problem myself.

[AA8A]

Hi,
Thanks for advice...I replaced the mic with a Shure 444 and no longer have that problem!
Think the other mic was rf sensitive? But really don't understand?
Bob, AA8A

[Mike/W8BAC]

Take an ohm meter and check to see if you have continuity between the plug pin, ring or shell that the cable shield is tied to and the mic body. Check both microphones. If the cable shield is tied to the transmitter chassis and the metal mic body you could have a ground loop. If that is the case, lift the shield connection at the mic body and try again. You will still have proper shielding.

Mike

[wavebourn]

Take an ohm meter and check to see if you have continuity between the plug pin, ring or shell that the cable shield is tied to and the mic body. Check both microphones. If the cable shield is tied to the transmitter chassis and the metal mic body you could have a ground loop. If that is the case, lift the shield connection at the mic body and try again. You will still have proper shielding.

Mike

The mic itself has to be symmetrical, no one shielded wire from the secondary of transformer connected to ground. Shield of the cable has to be grounded.

[VE7 Kilohertz]

Yup,

Yup,

Three little words...RFI

I had the same problem a few years back and had to use ferrite beads to solve the problem. Even with balanced audio all the way thru the system, when you have 750W carrier in the same room as the mic, things can get funny.

Glad you got it fixed.

73

Paul