On Sun, 21 Sep 1997 18:56:21 -0500 "Gary Smith" <standard@pcs.mb.ca>
writes:
>I've been following the thread this week regarding the SB-220 saga,
>and it
>must have cursed mine.
>
>First of all, the amps been working 100% up to now. No mods have been
>done
>and the 3-500Z's were replaced a few years ago with a pair of Eimac's.
>
>I reached over and threw on the power switch, with the amp in the tune
>(low
>voltage) position as I usually do. The exciter was in receive mode,
>as I
>scanned around 20m to see who was around. I smelt the smoke at first,
>looked over at the plate current meter, and it was sitting at .3 amps.
> I
>didnt' get a chance to look at the grid current before I powered off
>the
>Heathkit, but there was a faint sizzle noise also.
Could very well be a tube. As has been mentioned here many times Eimac
had a number of bad tubes several years ago.
I sold quite a few to hams for spares and when they plugged them in 3-4
years later they were full of gas.
Other problems are poor welds of the plate seams.
That sizzle was probably the filament xfmr starting to boil. Hopefully
you saved it. Another thing that can give similar meter readings and
symptoms is a shorted C-4 in the bias supply. No bias will run the Ip up
in standby AND the shorted cap will cook the filament/bias xfmr.
>One of the tubes has a dark black line down the crease inside the
>tube, and
>a light brown mark on the inside of the glass accross from it.
Could be defective. The best test is try one tube at a time after any
other problems are fixed.
The
>high
>voltage wires that pass the filter caps inside a clear plastic tube,
>appear
>OK, but the plastic is burnt as it passes one of the shorting
>resistors.
Thats normal... BTW those are equalizing resistors. While you are at
it, measure the DC across each cap; they should all be within a 5% range.
Otherwise it is time to replace them.
>The rectifier metering board has a large burn mark under one of the
>diodes,
>plus it looks a little black at the other end where the high voltage
>lead
>departs.
>I'll replace the tube, and was thinking of new caps and metering /
>rect.
>board anyway.
If the Zener is still OK the original board is fine. Just replace the
wimpy diodes with 1N5408's. If the zener and the .82 Ohm resistor R-3 is
good it is doubtful you had a tube short.
>What got it? Inrush current? Tube failure? Filter cap?
>Just curious on the chain of events.
My guess goes with C-4 first and a tube second.
No magic, no little green men, no parasitics.
73 Carl KM1H
>Gary Smith VE4YH
>
>
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>
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