On Fri, 25 Apr 97 15:56:55 -0800 Rich Measures <measures@vc.net> writes:
>Typically, a shorted1000pF feedthrough capacitor in the grid circuit is
>indicative of such an occurence. The capacitor shorts when an anode to
>grid arc occurs as a result of gold-sputtering from the grid during
>the parasitic.
Here are a few SB-230 hints based upon experience:
Many SB-230 grid bypass capacitors failed from the time of installation.
The ones supplied were so small and flimsy that the ceramic insulation
cracked. Use a real 1000pf 500V feedthru and no more problems; of course
the mounting holes need to be enlarged.
Other 230 problems are arcing from the grid ring to the socket rivets and
arcing from the grid ring to the heat transfer block. The cures:
1. Cover the rivet heads with a bit of Teflon or Mylar and secure to the
chassis with some Scotch #27 HV tape.
2. Make sure the Berriylium block is positioned up as high as it can go.
Also clean off any excess goop. If the block is allowed to slide down in
the cut-out it can actually touch the grid ring clamp.
> The 230 uses no vhf suppressor in the vhf-resonant anode circuit,
>even though the hv-rfc is isolated from the anode circuit with a vhf
>attenuator bead. Some owners have reportedly been able to improve the
>vhf stability of the 230 by slipping a pair of type-73 vhf attenuator
>beads over the anode lead.
Heath changed the position of the 2 ferrite anode beads after the first
production run. They are still far from ideal and a real suppressor
should be used along with a series surge resistor at the base of the
plate choke. Once stabilized the 230 will make a nice little amp and quit
blowing bandswitches. Keep the drive down to a point where you get about
600-650W out if you value the life of the 8873. A fan on the heatsink
will also help.
Other problem areas are the wires to the thermal switch on the heatsink.
These should be "at least" taped to the back wall; a few ferrite beads
where the leads go into the PS compartment will further decouple RF.
The relay control lead is right on top of the relay and coupled to all
the RF further increasing feedback problems; it is not even bypassed! I
use a 150 Ohm 1/2W resistor, with very short leads, in series with the
control lead right at the RCA jack. Slip a FB73-801 bead over the lead
wire, right at the resistor and bypass that point with a .01 500V disc
capacitor. It can now be keyed by those pesky ICOM xcvrs with the
undersized relays.
I have a SB-230 on the bench right now that I rebuilt as a 6M
monobander. With the input resistor load removed and a tuned circuit
added this little amp will put out 350W with 10W of drive and 600W with
25W of drive. Nice and stable; shipping it to Europe on Monday.
73....Carl KM1H
>Note: a gold-sputtered 8873 can often be made usuable by moving the
>loose gold melt-balls into the base. Interested parties are invited to
>contact me.
>...snip...
>Rich---
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