SB-104A audio problem?

[uoba]

Sandy,

You wrote: Radio works very well, 
> but in order to get proper TX audio out, I need to run the mic gain 
> almost
full on.

What are you looking at to determine this? Scope, Wattmeter, or ALC
indication on the radio's meter?

Benton



-----Original Message-----
From: Heathkit Owners and Collectors List [mailto:HEATH at LISTS.TEMPE.GOV] On
Behalf Of JF
Sent: Monday, September 27, 2010 3:40 PM
To: HEATH at LISTS.TEMPE.GOV
Subject: Re: SB-104A audio problem?

Alexander N. Gerli wrote:
> I've noticed something odd about my SB-104A. Radio works very well, 
> but in order to get proper TX audio out, I need to run the mic gain 
> almost full
on.
> The audio appears fine on the scope with the mic gain up that far, but 
> something's not right. I would expect the audio gain to be 50% or less 
> for typical full output. Using a standard EV-638 (HDP-121) Heath/EV 
> mic. And, that mic works FB with my SB-102.
> On tuneup, the radio puts out full power with the mic gain less than 50%.
> Now, that's correct.
> Anyone have any idea which board to start with?
> 73,  Sandy AC1Y


First, check the mic output with a scope to see if it's putting out enough.
As has been noted before, the SB-104 series was designed for use with high
output unamplified HI-Z mics of around 50K. For example, the older Shure 444
works very well with the 104A with its audio output of up to about 200 mv
peak-to-peak; it drives my 104A to full RF output with the mic gain set at
about 65-70% of full clockwise rotation. In tune mode full 104A power output
is reached at about 55%. Mics with less output are not suitable for the 104A
unless they have a preamp boost.

If the mic is good, then circuit board B (TX Audio/Regulator) gets the mic
input and gain control connections. From there TX audio goes to board E
(Carrier Generator).

You should perhaps go through the Tests and Adjustments section of the
Operation Manual.

73
John   KE5ZB

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the parties
involved.  This list and the City of Tempe are not responsible for losses or
misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTS.TEMPE.GOV
Archives - http://lists.tempe.gov/archives/HEATH.html
No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 9.0.856 / Virus Database: 271.1.1/3164 - Release Date: 09/28/10
01:34:00

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the parties
involved.  This list and the City of Tempe are not responsible for losses or
misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTS.TEMPE.GOV
Archives - http://lists.tempe.gov/archives/HEATH.html

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTS.TEMPE.GOV
Archives - http://lists.tempe.gov/archives/HEATH.html