If you recall, about a couple of months ago I requested input from the group
regarding my SB-220 that went snap,crackle and pop. Eureka! The patient has
survived. The amp is back in use and humming along very nicely. I've waited
a couple of weeks to make sure all is well before providing this info. The
amp has been used in some of the latest contests and regular qsoing without
a hitch..
Before I could start to attack the problem(s), I had to find a replacement
for the blown Plate Amperes meter. With luck, I managed to find both brand
new original meters (one for a spare just in case). The amp already had the
Harbach parasitic mods, rectifier/meter board and soft keying incorporated.
I replaced the blown .82 ohm resistor, replaced plate current meter with
glitch diodes added, removed Heath interlock and black liquid shoe polished
the insides of the finals compartment. I removed RFC2 and replaced it with a
20 ohms 25W Brown Devil. Upon very close inspection I found that RFC6 on RF
output lead to be of incorrect value and therefore replaced it with one of
correct value. I also added a diode across the relay coil. And finally, I
shorted out 1 bias diode on the Harbach rectifier board to give me 210 ma
plate idling current (in the SSB position).
With the original finals, the amp puts out about 1200 watts without really
pushing it. I can get about another 200-300 watts with new finals that I've
decided not to use until necessary. The amp is performing really well
without any RF arching even with some high SWR at times. I'm driving the amp
with an FT-1000D, limiting its output to 100-125W of course, and without ALC
connection to the amp.
I have been starting the amp in the CW/Tune position but tuning and
operating it from the SSB position. Plate voltage is about 2800 VDC under
load and I can easily load it to 800 ma. Grid bias is always within the
required limits. On the air reports are excellent and yours truly is now a
happy camper.
Back in the 50s and 60s I use to work on Wilcox transmitters (remnants of
USAF WWII) and the manual never gave a recommended plate current but simply
indicated to tune the finals for a cherry red glow. Some truth in that
method.
Thanks to all for your input with special thanks to Carl and Rich.
Walter - VE3EN
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