[Heathkit] Electrovoice 638 Microphone

[W5RKL]

Lee,

Hum is not always caused by a bad microphone or microphone cable. You did not mention whether or not the hum is present in CW/TUNE modes. I recommend checking CW/TUNE modes for hum. If hum is present in ALL modes then more than likely the source of the hum is the power supply filter capacitors. There are 5, one is a dual capacitor can. 

The first are the two tall black cardboard covered 125ufd 450VDC on top of the chassis between the final cage and power transformer, a dual capacitor can under the chassis, a 100ufd 350VDC axial lead cap next to the filter choke, and two 20ufd 150VDC caps under the small board directly under the LMO.


73
Mike
W5RKL




73
Mike
W5RKL


--- On Wed, 1/14/09, Lee Bennett <leeb at utm.edu> wrote:

From: Lee Bennett <leeb at utm.edu>
Subject: [Heathkit] Electrovoice 638 Microphone
To: "heathkit" <heathkit at mailman.qth.net>
Date: Wednesday, January 14, 2009, 8:50 AM

I purchased an Electrovoice Microphone for my Heathkit SB-401 on ebay
about 2 years ago.  I have had nothing but good reports on the quality
of the audio except for a very slight hum in the background.  I
purchased the microphone for 45 dollars.  Here is a picture of my
microphone and heathkit station (see address below). I purchased the
SB-401 on EBAY. It was not working when I received it but was easy to
fix. I found the SB-301 half buried in the dirt under a lawn mower at a
friend's house.  I cleaned it up, plugged it in and it worked.  The
SB220 amp in the picture needs tubes. The SB200 works fine.  I picked up
the SB200 at an auction for 25 dollars in working condition. The station
takes up a lot of desk space but it makes a good heater in the winter
time.

 http://www.utm.edu/staff/leeb/station.jpg





      
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