[Heathkit] SB 200 with Harbach mods has problems

[k5jv]

Greetings to all (answer to John Kings SB-200 problem),

        The Harbach Soft Start mod is  a well made, great add-on to just 
about any amp.  The only problem, if it can be called that, is that  there 
are no fuses to protect the soft start resistors.  If, for what ever reason, 
the relays do not pull in, shorting the resistors out of the circuit, 100 % 
of start current goes through, and continues to go through, these resistors 
and they (perhaps just one of them) will fail within a few seconds.  They 
are only 20 watt resistors.  Just forget to open the shorting bar in an 
SB-220 one time and see what happens.  Some guys replace the original 20 
watt resistors with larger resistors, like Collins did in the 30S-1.   I 
have collected a box of these boards, that I have replaced over the past ten 
years, or so, with burned out resistors, foils completely burned off the 
board, and/or  holes burned completely through the circuit board.

        All this can be avoided by adding a 3/4 amp, slow blow, fuse in 
series with each of the soft start resistors.  Alpha took this approach. 
However, to save money, Earhorn  chose to fuse only one of the resistors 
(this does give a good bit of protection, but this is an AC circuit and the 
current is not the same is both sides, even though they are actually in 
series (the primary).

        Do not try to fix the problem by increasing the wattage of the soft 
start resistors, like Collins did.  All this will accomplish is more damage 
to your amp when a short occurs.  Ever wonder why so many Collins 30S-1's 
have the relay tray burned to a crisp?  A fuse in series with each resistor 
is all that is needed whether it is an SB-200, SB-220, TL-922, Collins, 
Alpha, or what ever.

        John, check these resistors with an ohm meter.  They can fail with 
no visible indication.  If just one of them opens, the amp will never start 
up.



 73 de Lon, K5JV