HT ceramic Mic

[Jamie]

I don't do a lot of repair work these days how ever,
I have an HT the user/owner complains about low audio
on TX.
I don't have a real service manual but I do have the
original print that comes with the HT. This is a
Standard dual band, HeathKit also made one of the same thing.

Now, I can turn up the drive levels via pots that are
on the board for the VHF and UHF how ever, I don't see why
I should have to do this. Changing it from the original set
point to correct a failure somewhere isn't my cup of tea.

Since I don't have the real service print to go by, I don't
know exactly what is suppose to be at the varicap diode section
of the exciter oscillators for the UHF and VHF.

For an average level of audio into the mic of the HT, I'm
getting ~ 150..200 mv PP into the varicap diode section of the
oscillators..

To me, the audio level appears to be fine, the service
monitor appears to show it's spread is with in normal limits.

The only think I can think of is a bad ceramic element in the
2 wired condenser mic.

Do those things go bad by producing poor/low quality audio ?

I'm thinking this may be more of a clarity issue, than
anything else.

Thanks..
--

http://webpages.charter.net/jamie_5"

 

[Jeff Liebermann]

On Mon, 05 Jan 2009 21:06:00 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

I have an HT the user/owner complains about low audio
on TX.

Low mic audio or low deviation? There's a difference.

Finding an FM deviation meter. Scream into the microphone.
Can you get to +/-5Khz deviation? If not, you have low mic output or
low mic amp gain.

Does it look like the audio limiter is working and keeping the
deviation below 5KHz? If so, the modulator section is working, and
it's just low mic or audio gain. If it limits, but below 5KHz, all it
needs is to adjust the deviation pot(s).

I don't have a real service manual but I do have the
original print that comes with the HT. This is a
Standard dual band, HeathKit also made one of the same thing.

Do you have a model number? I used to work for Standard and still
have a pile of old manuals.

Now, I can turn up the drive levels via pots that are
on the board for the VHF and UHF how ever, I don't see why
I should have to do this. Changing it from the original set
point to correct a failure somewhere isn't my cup of tea.

Those unspecified pots probably adjust the deviation, not the mic
gain. They should be set for 5KHz *Maximum* deviation on each band,
and left alone. Few HT's have mic gain pots. You're probably going
to have to fix the lack of mic gain by either parts replacement, or
modifying the design.

Since I don't have the real service print to go by, I don't
know exactly what is suppose to be at the varicap diode section
of the exciter oscillators for the UHF and VHF.

Since I don't have a model number, I check for you.

For an average level of audio into the mic of the HT, I'm
getting ~ 150..200 mv PP into the varicap diode section of the
oscillators..

To me, the audio level appears to be fine, the service
monitor appears to show it's spread is with in normal limits.

Since you have a service monitor, set the pots to 5KHz maximum
deviation and leave it.

The only think I can think of is a bad ceramic element in the
2 wired condenser mic.

It's unlikely to be a ceramic mic. Those were big cans, with conical
aluminum foil diaphragms across the mouth of the can. Diameters
varied from 0.5" to 2" diameter. Few internal microphones in HT's
used ceramic elements. It's probably an electret mic, which is easy
to find and replace.

Do those things go bad by producing poor/low quality audio ?

Ceramic mics seem to last forever. Some early electret microphone
elements seem to have leakage problems, which eventually results in
loss of output. Find a voltsguesser and measure the DC voltage across
the microphone element. If it's about 5VDC, it's an electret mic. If
it's zero, it's ceramic, dynamic, or something else.

I'm thinking this may be more of a clarity issue, than
anything else.

Maybe. Many HT's come stock with minimal microphone gain. There are
various dumb reasons for this, but without a gain adjustment, it's
difficult to compensate for component value changes. We have a local
ham, who absolutely insists that the correct way to talk into his HT
is from about a 12 inches away. Typical deviation is about 1KHz and
nobody can hear or understand him. He also has a rather quiet voice,
and tends to wave the HT around while talking. After perhaps 10 years
of this nonsense, I've given up suggesting that he eat the microphone.
If the HT owner sounds like low audio on every radio he uses, then
it's very likely an operator problem.

Thanks..

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Dave Plowman (News)]

In article <7Zy8l.31421$Ac2.29720@newsfe01.iad>,
Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

The only think I can think of is a bad ceramic element in the
2 wired condenser mic.

Do those things go bad by producing poor/low quality audio ?

If it's an electret, yes. They loose their charge over time. With early
pro personal mics of this type I used to change the part after perhaps 5
years or so.

--
*The more people I meet, the more I like my dog.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.