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Just about any mic should work well with the SP75. Are you suspecting RFI from the amp? One thing you can try is, despite what the manual says, try adding a ground strap from the SP75 to the TR7 chassis, and also a strap from the amp from the TR7. Also modify your mic plugs to connect the cable shield to the connector shell as described in Heil's tech note:

"Shielding of the Mic Connector

We, here in the Heil Sound lab, have discovered a very interesting fact. Most of the major transceivers
today do NOT ground their microphone shields! That’s correct – the mic connector shields FLOAT! Now
wasn’t this one of the FIRST things we learned about building RF transmitters with speech audio sections?
GROUND those shields!! So, we came up with a very simple fix that just about anyone can make to their
rig. You don’t have to get inside the radio, so you don’t void any warranties. The fix is simple and effective.
This applies to the 4 and 8 pin Foster (that’s the Japanese company that builds those dang little mic
connectors!!) microphone connectors so common on today’s rigs.

First, unplug your Heil (what – you don’t have one yet??) microphone cable from the front panel of the
transceiver. Do this first, because you don’t want the mic plug connected to the sensitive circuitry inside the
radio while you’re soldering.

Remove the two small #4 Phillips head screws and the cable clamp they hold. Then remove the tiny Phillips
head screw that holds the rest of the metal sleeve. Slide that back onto the mic cable. Now, cut off the end of
a resistor, or get a piece of #20 solid, tinned wire about 3″ long. Locate the mic pin that has the shield of
the mic cable soldered to it.

With a small iron, carefully solder this solid wire to that shield and pin. Bring the sleeve back up the cable
and attach to the connector with the small screw. This leaves the solid wire coming out the back of the
connector. Replace the cable clamp, and (as you do that), tightly wrap that solid wire around one of the #4
Phillips head screws and tighten the clamp assembly very well.

What you have accomplished is grounded the shield to the transceivers chassis ground through the ring on
the mic connector. (Make sure that ring is tight). This has been a big help to many stations with RFI
problems and should help you clean up your signal."



This is a variant of the "Pin 1 problem" as described by K9YC. You will want to set the levels of the SP75 exactly as described in the manual with the barefoot TR7. I was changing mics up so much that I came up with this solution to the inconvenience in adjusting the levels:

/g/DRAKE-RADIO/album?id=201880


On Wed, 26 Mar 2025 19:33:46 -0700
"K6OXN Bob via groups.io" <bobk6oxn@...> wrote:

My question is, is there a mod to the SP75 if you are using a Heil Mic on the TR7?? I am getting some problems with Audio and TX on my station that I installed an SP75 with a Heil Goldline mic.? The amp is an L7 that is very stable with the SP75 out of the circuit.? Any comments are most welcome.

Bob

--

73

-Jim
NU0C