[Amps] SB220 - 10 Metre input coil adjustment

[Sam Ferris]

Wonder if anyone can tell me how sensitive/sharp the 10 metre input tuning
coil tuning is on a SB220.  I am repairing an SB220 for a fellow ham and
when I received the amp the central tuning slug for the 10 meter input coil
(part# 40-964) was in a bag of parts 
 seems the tuning slug had fallen out
sometime in the past.  It was noted that the 10 and 15 metre input coils
have the same part number so I measured the depth to the outer face of the
slug to the outer edge of the 15 metre coil to get an idea of just where the
10 metre coil should be set initially.  On initial tune up I was not able to
get more that about 450 Watts at 0.5A plate current and 110 mA grid current
on 10 Metres into a 1:1 SWR at 28.600 MHZ (Sweet spot for my antenna).  I
have tried one adjustment moving the coil slug in two turns which proved to
be the wrong way to go.  Have taken another look and decided to return to
the first setting plus about ½ turn out – then try again.  Can anyone advise
just how sharp the tuning on these input coils is?  Would it be best to
devote the time to making a temporary face plate that I can drill an added
access hole through to access the tuning coil from the outside?

 

The amp functions fine on other bands with reasonable output on a set of
EIMAC tubes.  Other repairs to this amp have included replacement rectifier
board, new parasitic suppressors – old ones showed significant signs of
heating, replaced broken bandswitch, soldered loose wires, general cleaning
and check of components.

 

Thanks

Sam

VE5SF