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  Introductory Information on
Astron Power Supply
Regulator Replacements

Compiled from a number of different sources
by Mike Morris WA6ILQ
Maintained by Robert Meister WA1MIK
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One of the overlooked items is the compensation capacitor value. The average Astron linear power supply has NO compensation cap at all (leaving out that cap is one of the cost-cutting methods the Astron designer used). The 723 data sheet (which you can click on below) has a number of sample circuits, and some show no capacitor, some show 15pF, some show 100pF, the largest is 500pF. Some design notes rate the capacitors in nanoFarads (1,000pF=1nF). If your supply has a compensation cap you will find it connected from pin 4 to pin 13 on the DIP package (common), or from pin 9 to pin 2 on the TO-5 round metal can (rare). If your supply does NOT have a cap from pin 4 to pin 13, ADD ONE - a 470pF or 500pF tacked across the back of the IC socket is all that is needed.

The common Astron linear supply is not a finished and reliable design, as this email from Skipp May WV6F indicates:

Many early Astron supply regulator boards are problematic.

The 723 regulator chip is often placed in under engineered circuits. There are on-chip amplifiers with an incredible amount of gain. Said amps with a lot of open (and closed) loop gain makes a nice RF sniffer when the designer forgets to properly bypass various sections of the regulator circuit. Plus there is poor response to high frequency glitches and noise. Proper 723 pre-regulation is another common circuit shortfall. The end result is often seen as erratic operation, false crowbar circuit firing and regulator chip failure. The crowbar circuit itself works well when triggered. Most of the crowbar problems I've seen (once you get past the Astron regulator board) were traced to older filter capacitors under relatively heavy loads.

Many Astron power supplies work well for decades, a lot of the problems surface when feeding higher impedance and reactive loads. The load impedance and current demand presented to the supply can be a big factor in the performace and reliability of the power supply.

The 723 regulator chip is an excellent building block, but making one play well with multiple nearby 50kw (or higher) broadcast transmitters can be a test of ones engineering skills. Fortunately, the data sheet has all the required information. Much of the mentioned data sheet information is often overlooked. Problems resulting from poor 723 regulation circuit design shortcuts often rear their ugly head at much later dates.

he Astron circuit design engineer is certainly not the first, nor the last person to misunderstand or overlook certain characteristics of the LM‑723 regulator chip. I've found very few LM‑723 regulator circuit designs done really well.

More from the Skipp May WV6F email that started this article:

I have for sale an exact drop-in replacement for the Astron regulator board. This is a much improved circuit design... it addresses all the known problems, i.e. it has additional RFI and noise bypassing, overshoot control, improved regulation, fixes the dreaded crowbar circuit.... I test each board for proper operation, I've never had one fail, nor the crowbar circuit fire, even at high-level RF sites. There is an option available for a front panel variable dc voltage control. It's a complete redesign, much better than the original board supplied with your supply. It comes fully assembled and tested.

Upon initial install, the user with the new regulator board retrofit tests the crowbar circuit. Indeed no crowbar protection (function) has ever fired inappropriately in units with the new board installed. This classic gremlin has been properly killed.

I have applications where power supplies simply cannot fail. I came up with the retrofit regulator board project to keep the sanity of some very high-end customers and myself. Most all of the 30 plus boards I have "out there" have been retrofit by me for customers as a part of a complete supply upgrade package.

Yes, they are pricey at near US$50 each, but well worth a retrofit into the 75, 50 and 30 amp supplies. Commercial customers with life safety power supply failsafe requirements pay a considerably higher price for the same circuit board.

Installation is simple: You simply unscrew and unsolder your original regulator board after noting (and writing down) the original wire connection points. The replacement board drops right in and you solder the corresponding original wires to the same locations. The board connection points appear almost exact (but the circuit definitely is not) because I made an effort to lay out the board that way. If your power supply was working before the retrofit, you simply power up, test and go. Each regulator board is hand tested before they are sent out.

If your power supply had previously failed, you should first test the pass and driver transistors, emitter ballast resistors and a few other small items before you re-apply power to the supply.

Note that the regulator board must be ordered per the size / type of Astron supply that you have. They do not interchange from one supply size (amps) to a different supply size. There are / were a number of different Astron Regulator Board versions made and configured. What is placed on the specific board is related to its capacity and type of operation. Depending on the year of production, the size and the series the boards can and do change a bit.

If you are interested contact Skipp at Skipp025 -at- yahoo -dot- com   And that's skipp(zero)(two)(five), not skipp(oh)(two)(five). And note there are two "p"s in Skipp.

An email from someone who bought Astron's revised (newer) regulator board as a replacement part:

From Mike Perryman K5JMP
Subject Re: Astron's own update package
Date Mon, 2 May 2005

The package with Astron's replacement regulator board showed up this morning...

What a mess! I did as instructed, and snail-mailed an order including a check... like "pre-paid"... ya know... Package arrived $44.63 due COD? Of course UPS wouldn't release it until I stroked another check. So I called Astron, and the sales guy blamed the mix-up on the shipping guy (not surprising!!!). Says they will return the last check, as the first one has most likely already been deposited. I should have seen the "flake-factor" when they wouldn't accept a credit card.

The Astron sales guy said there is no documentation as it isn't required to change the regulator board. I asked him to fax over the info, as I have one of the really old units, and the TIP-29 and SCR are mounted to the chassis... this board bears zero resemblance to the one I have, and also must be modded for use with a variable voltage supply. The Astron sales guy allowed that he would fax over the detailed information for the mod.

I never received any faxed documentation from Astron.

Following further harassment, the sales rep said I could call back and talk to the tech when I got home, that the tech would be there until 5:00PM PST. With his assistance I managed to muddle through the modification to the board for a variable supply.

If you are familiar with the Astron linear supply and can do without documentation... the Astron "fix" worked just fine. But, if you need docs to get through the re-fit... Well, Skipp includes full documentation with his kit.

Next time I will buy Skipp's board, and avoid the flake-factor.

Mike K5JMP


Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.



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This page originally created in August 2000 by Kevin Custer W3KKC.
Totally rewritten 14-Oct-2004 by Mike Morris WA6ILQ.
Split into several pieces 13-Jan-2018 by Robert W. Meister WA1MIK.
Copyright © 2000 and and date of last update by Repeater-Builder.com

This web page, the hand-coded HTML on it, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.