At 03:25 PM 10/12/97 -0800, you wrote:
>...
>>I have been following with great interest the thread about the
>>B+ glitch resistors and guess what happened yesterday?
>
>alas
Welcome: you have now been initiated, and only have years and years of loud
BANGS to look forward to (in between reports of "loudest signal on the
band", of course...).
When you've burned up your first bandswitch, let us know and you'll be
registered a full-fledge member in good standing with all voting rights!
>>My glitch
>>resistor opened up with a very loud bang spreading chunks of
>>ceramic all over the bottom plate of my amplifier chassis.
>In ancient times, we used 10 ohm, 10w ceramic wirewound resistors, but
>they had a habit of breaking down and throwing schrapnel about during a
>glitch.
A preventative measure to the inevitable cleanup task is to design and build
an insulated compartment within which to house the resistor to confine that
scrapnel. This is especially important if you have a habit of operating the
power supply in positions such that the resistor is able to explode into the
shack/lab, possibly throwing shards of glass or ceramic into your face and EYES.
A decent way to do this is obtain some clear plastic sheet (such as "page
protectors" from the stationary store) or thin plexiglass, heat it in a pan
of hot water, and form it into a shield within which the resistor is
mounted. Something to keep in mind is the heat dissipated from the resistor:
it can (and has) melt the plastic/plexiglass, creating a real stink in the
shack for a few hours. If possible, form the shield such that it is
essentially closed so that particles of the exploded resistor remain inside
when the PS is turned over for repair. You can hold the plastic in place by
mounting the ends under nearby chassis screws or other terminals (if
terminals, make all the electrical connections by themselves first, then lay
on the plastic, with another nut or whatever is needed). You can also use
nylon screws/nuts through holes in the plastic to hold it together and in
place permanently. Using clear plastic allows you to inspect the resistor
for signs of abuse or damage, of course. As an alternative, I've used teflon
sheet.
A side benefit is that the plastic cover provides some protection against
accidental direct contact with the resistor carrying HV.
As far as I'm concerned, since one nearly always needs to peer inside a new
PS to at least look for corona and other such undesirables, one should
ALWAYS have SOME sort of shield over those kinds of components that can
violently disintegrate; including, if you use them, discrete diode packages
such as 1N4007 or the like (yes, I've had them explode, too). And if you use
electrolytics in your PS, orient them in such a manner that when you need to
peer inside the chassis, the bakelite bottom with its bleeder hole does not
peer back at YOU. There are few things more disgusting (nor dangerous) than
to have to wipe a gooey mess off of your face when your HV PS has suddenly
hiccupped and you can't see to turn it off!
>Ok, here's one, Mike: Measure the anode/screen leakage current at
>6000v with a high-pot. before you burn in the replacement tube. Then
>measure the leakage after burn in. I'm betting that the leakage current
>does not decrease more than 2% during burn in. If the leakage current is
>above 10uA cold, I would toss the tube in the trash bin.
Rich, why do you say do it before burn-in? Are you thinking that running the
filament during burn-in can reduce some of the leakage gas, thus
"artificially" increasing the breakdown within the tube; and that later,
under the heat stress of full-power operation, the leakage could increase,
resulting in a possible internal breakdown?
73, Steve Ko0U/1
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