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Re: [Amps] Crackling noise, SB-220 repair notes

To: <amps@contesting.com>
Subject: Re: [Amps] Crackling noise, SB-220 repair notes
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Fri, 5 Jan 2007 19:46:01 -0500
List-post: <mailto:amps@contesting.com>
> Why guess?  Why try this, try that?

I think the suggestion was to look for likely things, not 
replace things at random. Better and faster to look for 
likely things first.

> The logical way to troubleshoot it is to isolate it, 
> easily.
> Don't assume anything, other than possibly that it is high 
> voltage related.

I disagee. Like listening to an engine...you can tell an 
awful lot by the sound.

> But to go head first into checking every component just 
> because someone else
> had this problem once is a big waste of time.  Use your 
> head.

Considering a well designed amp if it is an arc that doesn't 
take the HV to ground it is almost certainly not a component 
that goes from HV to ground. Any arc there would dump a lot 
of current until the HV went to a very low voltage.

For example?

A leaky blocky cap will normally only cause a repeated tic 
if the tank has no safety choke path. The leakage will 
charge the tank caps until they arc, but the high leakage 
resistance limits current. As a matter of fact a **normally 
working** blocking cap can cause a periodic tic of the 
safety choke is open or (worse yet not installed). A bad cap 
makes a bang and faults the HV to ground.

A repeated metallic or hollow sounding tic or thump is 
normally caused by an open equalizing resistor on the filter 
caps, or one that arcs internally.

A higher pitch buzz under load is usually something like an 
open diode (again a series path that doesn't dump high 
current ) but a HV transformer, once it arcs, almost never 
recovers. The exception is an open lead like a bad solder 
joint on the HV, but it will buzz at a 120 Hz rate....not 
pop and stop.

One trick I use is a long hollow Teflon tube that can be 
placed on or pointed towards components and I listen for the 
noise. Of course it takes common sense and care in use.

I've seen a lot of GOOD components get changed to correct 
other problems. For example I've seen blockers replaced 
because the safety choke was open. I've very commonly seen 
electrolytics replaced because the equalizing resistors were 
off value, and the new electrolytic temporarily cured the 
symptom.

I think it is actually better to listen, think, and check 
the likely troublesome components first. They type of arc or 
noise tells us quite a bit if we use the information 
logically. That way we don't pull the oil pan, rod caps, and 
valve covers only to find out is a bad alternator bushing.

73 Tom 


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