[AMRadio] D-104 modifications

Todd, KA1KAQ ka1kaq at gmail.com
Fri Aug 27 11:26:40 EDT 2010

On Thu, Aug 26, 2010 at 5:51 PM, John <johndtate at post.com> wrote:
> What's there to modify?  Just bypass the dumb preamp and use the Hi-Z
> crystal directly into your Hi-Z input speech amp and you have a GREAT
> AM mic!

Yep, that's pretty much it. The other piece to consider is matching
the mic to your transmitter better. In my case, it meant changing the
mic input grid resistor from 1M to 4.7M which gave a much more natural

Two things to keep in mind: first is that *any* element can and will
sound bad if it's damaged. It's seldom visual physical damage that
causes trouble, more being exposed to wide variations in temp and
humidity. Being knocked over or dropped can do it to, along with
general aging. So the type of tag on the head is little help. Only
testing will tell the truth. The D-104 I currently use came in a box
of stuff with a transmitter I got back in the late 80s and hadn't been
used for some years. It has a stick-on metal label. Works fantastic
according to every recording I've heard.

Second, the preamp in the base of later T-UG8 stands is just that - a
preamp, to make the mic sound louder, not an equalizer to make a bad
element magically sound better. True, many D-104 mics were sold with
these stands when Citizens Band took off (think "Power Mic"), but the
stand was available with preamp from Astatic without any head. And
Astatic offered a number of different heads that easily interchanged
with these stands. Had they made this preamp just to make a crappy
D-104 sound better, it's doubtful it would be offered separately.

So, bypass the preamp, check that the mic is a good match with the
transmitter, and test. It helps to have a known-good head for
comparison in the event your mic sounds bad.

~ Todd,  KA1KAQ/4

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