On Wed, 17 Mar 1999 22:41:24 -0500 "Walter Fairclough"
<wfairclo@netcom.ca> writes:
>
>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.
So we can first eliminate voodoo parasitics.
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
>and
>return to Heath original,
I opt for the Heath original.
ground the grids directly or apply someone
>else's
>grid parasitic suppression kit?? or.......nah, that would be to easy.
>
>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.
The bis diodes on the Harbach board have always been a weak link. I
always rebuild with 1N5408's since I too have had similar failures.
>Where do I go from here? Rather frustrating when the darn thing worked
>so
>well for about 2 weeks. I'll open her up tomorrow and report if I
>find
>anything blown other than what I already mentioned as to what I
>believe has
>popped.
A loud bang makes me wonder if there is not some voltage breakdown
problem external to the tubes. Something as simple as plate cap/parasitic
suppressor spacing to the top cover could be involved. Check everywhere
for signs of an arc path, carbon tracks, etc.
How about the safety choke, is that still OK?
Heath had a bulletin about defective plate blocking caps.
>
>Still open for suggestions (constructive of course).
No more Guiness until after you are thru checking the amp.
73 Carl KM1H
>
>Happy Saint Pat's Day!
>
>Walter - VE3EN
>
>
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>
>
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