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Re: Topband: SB-1000

To: topband@contesting.com
Subject: Re: Topband: SB-1000
From: Herbert Schoenbohm <herbs@vitelcom.net>
Date: Mon, 21 Dec 2015 11:26:44 -0400
List-post: <topband@contesting.com">mailto:topband@contesting.com>
Tom is correct as usual but if you want to get the rig working for the Stew coming up a junk box substitute may get you on the air until the exact replacement arrives.

Herb, KV4FZ

On 12/21/2015 10:21 AM, Tom W8JI wrote:


If you can't find the exact value of 170pf the ceramic 5KV door knobs at 200pf will work. Better yet even a couple of 500pf door knobs in series will give you 250 pf which will just mean a little less on your variable will tune. Another common ceramic door knob value is 500pf and three in series will put you in the ball park.

NO.

170 pF was used because at 200 pF the capacitor breaks over into a different temperature coefficient. You will go from a N750 or so TC to up near or above N2000.

This will make the tuning drift with temperature.

Use the original part or parallel smaller values with the same or lower TC's. Also be ware the number on the case is often not met. Sometimes the caps cannot meet the stamped TC numbers. This is because the clay formula used is difficult to make temperture stable.

Just buy the correct part.

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