Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Scratchy audio  (Read 2074 times)
AB2MH
Member

Posts: 263


WWW

Ignore
« on: April 24, 2006, 01:34:16 PM »

I'm currently repairing an old GE C541 AM/FM tube type receiver I picked up at a hamfest (for 3 bucks!).  

I got the radio to play (replaced shorted filter caps) but now the audio is really scratchy.

It's not the volume pot because I cleaned it up with deoxit and it still gives the same symptoms and I disconnected the input wire and the audio is smooth from a signal generator.  

The bandswitch was cleaned out too.  

What else could be the problem?
Logged
K3JVB
Member

Posts: 837


WWW

Ignore
« Reply #1 on: April 24, 2006, 01:47:51 PM »

Check the tubes first..as with any old radio.

Audio amp first...and how is the speaker? They can get dry rotted, and cracked

good luck
Logged
AB2MH
Member

Posts: 263


WWW

Ignore
« Reply #2 on: April 25, 2006, 04:47:08 AM »

Thanks for the suggestion.  Changing tubes don't really make a difference.  I tried changing them all one by one but no difference.

The AM section isn't sensitive at all, in fact I can only pick up WABC 770 here which is pretty close and it's at low volume with a slight hum (no scratching).

FM has loud scratching, but stations tune in.  

This leads me to believe that the speaker is OK as well.  If I inject an audio source at the volume control pot's wiper, it sounds good.

I'm thinking leaky caps as well?  This radio has all ceramic caps (no paper/wax caps).  I replaced the electrolytics as well.
Logged
AA4PB
Member

Posts: 14959




Ignore
« Reply #3 on: April 26, 2006, 05:51:03 AM »

The 455KHz IF transformers used to be a relatively common cause of "scratchy" audio. I'm not sure of the cause because we just replaced them. It might have been the capacitors across the pins inside the can.
Logged

Bob  AA4PB
Garrisonville, VA
AB2MH
Member

Posts: 263


WWW

Ignore
« Reply #4 on: April 26, 2006, 12:03:00 PM »

That's what I'm looking at next.  When I tap one of them it causes the scratching to change.
Logged
KA5N
Member

Posts: 4380




Ignore
« Reply #5 on: April 26, 2006, 04:12:13 PM »

Some IF transformers had the capacitors sort of held in place with clips.  These would get loose with age and vibration.  A simple cure was to remove them and replace with capacitors with leads soldered in.  No more problems.  The difficulty might be in figuring out what value the caps are.
Allen
Logged
KC8VWM
Member

Posts: 3189




Ignore
« Reply #6 on: April 28, 2006, 08:27:28 PM »

You might look at the audio output stage and insoect the interstage transformer.

Sometimes the transformer dries up and experience insulation breakdown between the windings inside them which results in static noises, pops and crackling sounds.  

My two cents.

73 Charles - KC8VWM
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!