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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