Hi Ben,
> I bought an SB220 amp that wasn't putting any power out on 10 meters, but
> 1200 to 1300 on 20 and 15 mtrs. It turned out to be a fried bandswitch on
> 10 thru 20, but was hanging on with a hair on 15 and 20. I replaced the
> bandswitch and while I was at it, i installed Harbach mods. (parasitic
> suppressor, soft key, soft start, and rectifier-metering board kits) I
> also installed new resistors across HV caps from Rich Measures yesterday.
> Now on 20 it will arc a bluish color on bandswitch when driving it up to
> abt 300 watts out. I tried it on another band(I think it was on 15) and
> the exact same thing. I noticed that the tune control on 20 mtr tuneup
> was peaking out at the 10 mtr position(knobs are installed correctly).
> This tells me that there is too much inductance in the circuit.
You may have ruined the bandswitch from too little loading. Despite
what you hear, very few problems like yours (if any) are "parasitics".
What happens is this.
If the loading control is too far closed, or if you have a fault that
causes the load impedance presented to the tubes to go very high,
the peak tank voltage could easily become many times the normal
value. What happens is the tubes build up all that energy in the
tank, and it has nowhere to go.
Eventually something has to give.
What I would do is this. Look carefully at the switch and make sure
all the connection you made are smooth and rounded. Do this even
if you have to ball a little extra solder on the connection. Do NOT
leave sharp point off the connections, because they help start
corona.
Next look at where you have the load control set. Try to open the
load control as far as possible with FULL drive applied, until power
just drops off. You might have to "walk" the controls up as you
advance power, always keeping the load OPEN just beyond the
point of maximum output.
The single most common cause of switch failures is running too
much loading capacitance or having a momentary fault in the
antenna system that unloads the PA.
It has nothing to do with "tube gain". Also, check to be sure the
grid bypass caps are short leads and well soldered. The best thing
is to tie the grids directly to the chassis with short leads and a lug
mounted near the grid pins, although the 220 will work OK as is.
The resistors across the caps in the PS were fine as they were,
but the new ones probably won't hurt. Did you do anything to the
antenna relay??
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|