[Amps] Crackling noise, SB-220 repair notes

[Marc Wullaert ON4MA]

i got this with a drake L7 some years ago.The couldprit whas a broken diode
in the ps.
amp wkd ok ,later on it before fixing it started also to smell !
on4ma

----- Original Message -----
From: "Christopher J Galbraith" <cgalbrai at umich.edu>
To: <amps at contesting.com>
Sent: Thursday, January 04, 2007 4:10 PM
Subject: [Amps] Crackling noise, SB-220 repair notes


> Hi gang,
>
> Problem:
>
> I am hearing a soft, but definite "crackling" noise coming from an SB-220.
> It is loud enough to notice over the fan noise, but not much, and
confirmed
> by another person.  There are no visible arcs or other operating
> problems--HV and plate current (load/no load) is perfectly stable and the
> amp is working perfectly otherwise.  The noise is there while the amp is
on,
> in standby or in transmit, and does not change with the load.   It is
> difficult to locate exactly (physically) where the noise is coming from.
>
> I'm just wondering if anyone has had this noise and found its source.
Maybe
> the HV transformer arcing internally, a bad bypass capacitor, or something
> else?  Any comments welcome!
>
> History:
>
> The amp sat unused for 20+ years, covered in an indoor environment.  I
just
> installed the Harbach kits for the power supply, diode/metering board,
step
> start, keying interface, and new fan/motor.  I also replaced the plate
> suppressors with ~ 120 nH in parallel with 100 Ohm (5W Xicon metal oxide)
> resistors (using AWG16 Cu wire, not Nichrome), removed the RFCs and caps
on
> the 3-500Z grids and replaced with 1/4 in copper straps to ground (on all
> grid pins).  I did not notice the crackling noise when first powering up
the
> amplifier after finishing the work, but noticed it the second or third
time
> during testing.
>
> Again, the amp otherwise works very well.  Even on a 120 VAC circuit (its
> own), it puts out an "easy" 650 W (CW/TUNE) and 1200 (SSB) on 80-15m and a
> bit less on 10m, with very smooth tuning and a power added efficiency of
> 60-65% (55% on 10m).  It was interesting that after removing the grid RFCs
> and caps and replacing with Cu strap, input VSWR and output power improved
> dramatically on 15m (and a bit on 20m, too).  The only other problem is
that
> the input VSWR on 10m is a bit high (2.5), but that may respond to future
> adjustment.
>
> Thanks for any comments, 73,
>
> Chris KA8WFC
>
>
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