[Amps] SB220 - no power output problem

[Bart Ritchie (VE5CPU)]

Good day to everyone on the Amps list.

As the subject line indicates, I am having some problems with my Heathkit 
SB220 amp.  It currently is very intermittent in providing power out on 10, 
15 & 20 meters.  It frequently fails on 40 meters as well, but 80 meters is 
working properly.

The amp loads up in it normal plate and loading positions for my 
antennas,  but then completely drops out, in terms of no power being 
displayed on a Bird 43 watt meter.  Although the power drops out, there is 
no change in the grid meter or plate meter reading when the power drop 
out... at least not to a noticeable degree.  The amp stays in this 
condition on that band, but often recovers on a band change, especially to 
80 meters.  After several attempts on band changes, it will provide power 
out and then, in the middle of a transmission, drop out again.

The accompanying drop out in power is sometimes preceded by some arching 
type sound in the amp.  I have not been able to see where in the amp this 
is taking place, but the occasional time, it does appear to be in the area 
of the top of the tubs or perhaps to the side of the tub area.  It is very 
difficult to tell as the amp is elevated on a shelf above my line of sight.

This condition has developed over time, about 14-18 months and has 
progressively deteriorated, with deterioration accelerating in the last few 
months.

Although very much a novice in the area of dealing with amplifiers, I have 
a reasonable background in base electronics and building/repairing 
equipment at the hobby level for 20 or so years, mostly in the computer 
field, but 14 years in the Amateur radio service dealing with some of my 
equipment (except my amp).

I have taken the case off and inspected the amp.  There are no obvious burn 
marks on any of the controls or key components that are visible.  The Amp 
has the Harbach parasitic suppressor modification and the soft key  kits 
installed about a year ago.  The amp exhibited this problem to a lessor 
degree prior to installing these updates, but I don't think there is any 
relationship to the deterioration and the updates.  Part of the reason for 
the updates was I was advised they may help with the problem.

Since there is no one in my immediate area to call upon, I would appreciate 
any comments or suggestions as to where the fault may be.  As well, if you 
feel there is a certain part at fault, I would appreciate any 
recommendation as to where to acquire replacement parts.

Thanks in advance (TIA in Internet speak).
  Bart Ritchie, VE5CPU
  Saskatchewan, Canada