>I guess I spoke too soon or its just the luck of the Irish. I was sitting in
>the shack with the SB-220 turned on but in standby when all of a sudden a
>very loud snap. No crackle, no pop, just one hell of a loud snap.
>Here's the setup; the amp was on in standby on 10m, multi-meter in the
>Relative Power setting and in SSB mode. Immediately after the loud snap I
>did some very quick checks. Plate voltage was OK but the Grid meter was
>pegged FSD. Immediately shut the amp off and went for a Guinness Stout. With
>the blood pressure still high, left ear ringing, and not in the best of
>moods, I decided to leave further investigation until tomorrow. I suspect
>the .82 ohm resistor and the glitch diodes across the plate meter blew.
>Confirmation of this will have to wait until frustration level subsides.
>So here we go again. Do I chuck Harbach's grid parasitic suppression
A parasitic can not be suppressed very much at the grid. Better to make
a change in the vhf Q of the anode circuit's. resonance.
>and return to Heath original, ground the grids directly or apply someone
>grid parasitic suppression kit?? or.......nah, that would be to easy.
If you want to try the other brand, let me know. I will send you a
suppressor retrofit kit. If it does not make the amplifier stable, you
don't pay. // One caveat. If a tube shorts from fil. to grid in a
stock SB-220 (of TL-922), it shorts out the +110v power supply during
receive. A shorted tube will destroy the fil. transformer quickly if the
amp. is not switched off. To prevent destruction of the fil. trans.,
disconnect the +110v ps from the bias contacts on the relay. The +110v
ps should connect only to the relay coil. The tube will then self bias
itself off during Rx. If the above mod. is made and a tube shorts, you
will see idling current during Rx and the grid current meter will
indicate neg. current. If this happens, the tubes can be inserted one at
a time to find out which one is shorting when the fil. is hot. .
>At least this time it didn't appear to blow the hell out the amp so some of
>the mods have done their job. But, the cure must be found as I'm getting
>tired of replacing that same .82 ohm resistor.
>I notice that Harbach has redesigned his rectifier/metering board,
>apparently adding heavier duty meter protection diodes, however, I don't
>think they have anything to do with the .82 ohm resistor popping.
After you install one of my kits, the 0.82 ohm resistor is protected with
200 peak A glitch diodes.
>Where do I go from here? Rather frustrating when the darn thing worked so
>well for about 2 weeks.
Parasites can be intermittent, seemingly without rhyme or reason.
- later, Walter
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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