[R-390] What to do with a Garage sale find R390a.

[David Wise]

I kept the original C553 and inserted a current limiter in series.
Microchip (nee Supertex) makes a 500V N-channel depletion-mode MOSFET, the LND150, that's perfect for this.
Just connect gate to source, and current cannot rise above Idss, a couple mA.
The rest of the time it's invisible and under no stress - it should last forever.
Drain toward B+, source toward filter.

It's cheap and abundant at the major distributors.

Dave Wise
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From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Roger Ruszkowski <flowertime01 at wmconnect.com>
Sent: Wednesday, July 15, 2020 4:21 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] What to do with a Garage sale find R390a.

Fellows,
If you see an R390 or R390A setting for under $300.00 for the receiver then buy it to flip at $300 for sale at a new location. You have to consider the rest of the bundle (case) and the price difference over $300.00 to see if a sale makes sense on the offered bundle.
What condition and type of meters are in the front panel. $50.00 each
Until you know what condition the Plate Blocking Capacitor (for the Mechanical Filters) - When replacing the Plate Blocking capacitor C-553, remember you are replacing a 300wvdc Vitamin-Q type capacitor that, in the 1950s, was one of the best capacitors available. Don't replace this vintage quality capacitor with a Malaysian-made, less-than-one cent total-cost capacitor. If I was really worrying (like loosing sleep over it,) I'd use two .022uf caps in series for the .01uf C-553. This redundancy is pretty much fool proof for any future problems. Normally though, I use a .01uf 600wvdc 715P type SBE Orange Drop. The "double the working voltage of original" is usually enough precaution on this controversial component.

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