Back to Home   Technical Information on Astron™ Power Supplies
Compiled from a number of different sources
by Mike Morris WA6ILQ

Contact information:
    Astron Corporation
    9 Autry
    Irvine, CA 92618
    Voice: 949-458-7277
    Fax: 949-458-0826
Click here to visit the Astron Corporation web site
Click here or on the logo above to go to Astron's web site

You probably came to this web page because you have an Astron supply that has problems.   Note that the Astron supplies are advertised with peak duty amperage numbers and in continuous duty can supply only about half of the rated current (that is, a twenty amp Astron can in reality supply only about ten amps continuously). Astron marketing gets around that by saying that in normal operation the user listens (i.e. low current) a lot more than he talks (high current). This gives time for the heat sink (and pass transistors) to cool down. In other words, the digits in the model number represent the peak intermittent current capacity.

For example, an RS35 is rated by Astron for 35 amps peak, 25 amps continuous, but I have seen a number of them fail while driving continuous duty loads of 20 to 25 amps. My personal opinion, which a number of my repeater-building friends share, is that you never load any Astron at any more than 50%. In almost all cases you will want to add a fan blowing across the heatsink and a and a second one through the box (across the transformer and bridge rectifier) is definitely recommended (you will need to punch holes and add screening for that - and don't forget to ground the screening). More notes are below...

The linear Astron supplies are based on the common "723" series regulator chip (originally designed and sold by Fairchild Semiconductor in the 1970s as the µa723).   Most of the chip reference sites have the data sheet and applications notes, and the theory and design information you will find there is as true today as it was when the chip was introduced... Starting with the design notes is a good idea.

Note that the LM723C used by Astron is identical to the LM723 except that the LM723C is temperature derated - the performance is only guaranteed over a 0C to +70C temperature range, instead of -55C to +125C. If your Astron is going to a mountaintop repeater site that gets cold in the winter I'd replace the chip with one that is guaranteed below 32F...

Before you start debugging your Astron, I suggest that you read the first three articles in the "Linear power Supply Design and Theory" section below.

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 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 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 its reliability.

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.

Astrons 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.

One of the overlooked items is the compensation capacitor value - as this web page by Dr. Paul Webster VK2BZC elucidates: Power supply design with the LM723. The average Astron has NO compensation cap at all. 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 (also spec'd as 1 nF), and the largest is 500pF (5nF). Do not go any larger than 500pF. If your supply has a compensation cap you will find it connected from pin 4 to pin 13 on the DIP package, or from pin 9 to pin 2 on the TO-5 round metal can. By the way, the "this country" that Paul refers to on his web page is Australia.

That said, if your Astron is going into current limit at random times, this comment from an email sent by Ron Rogers WW8RR is relevant:

A VERY common cause of random current limit shutdown in linear Astron supplies is due to the manufacturing process: a bad solder joint on the collector tab of one of the 2N3771 pass transistors.   During manufacture they solder the buss wire from collector to collector.   If you take a pair of pliers and grab the buss wire next to the solder joint at each transistor and pull on it, you will most likely find one that will simply pull off.

All it takes is for one of those transistors all wired in parallel to have a bad collector solder joint and all of the source current from the supply tries to flow through the Emitter-Base junction of that one transistor that has the bad solder joint. Bingo !!! Immediate current limit mode !!

From yet another email to repeater-builder:

Another common failure (these supplies are full of them) is due to the use of TWO diode bridge rectifiers wired in parallel. You can see this on some of the RS35 supply schematics. They use two 35A bridge diodes that are wired in parallel with three pieces of #10 busbar with no attempt at current equalization. To use a Martha Stewart term, this is A Bad Thing, engineering-wise; at least the designer used current balancing resistors on the pass transistors. Only the positive half of the bridge rectifiers are used; the negative end is left floating. Unless the two bridges are closely balanced, one will take most/all of the current and the other will just sit there and watch. Eventually one diode will short or open, and that often takes the other one along with it. This causes the supply's primary fuse to blow up. If your supply instantly blows the primary fuse, unsolder the two heavy transformer wires (frequently colored yellow) from the bridge rectifier terminals and then see if the fuse blows. If not, check for a shorted main filter capacitor and shorted pass transistors. If all checks out OK, chances are high that one or more diodes have shorted. They have to be completely unwired to be properly checked. I bought several 1,000V 50A diode bridges for about $5 each as replacements and just use one of those in RS35 supplies rather than two parts in parallel. The ones I have here are labeled KEST KBPC 5010 and some are still being sold on eBay as of October 2007.

From an email to repeater-builder from an amateur that works in the power supply design field:

If you have to replace all of the pass transistors in a supply, and the old ones are 2N3055s, 2N3771s (also known as the ECG181) do yourself a favor and buy at least 2N3773s and preferrably 2N5686s instead. They are drop-in replacement transistors with better performance for a very reasonable price. Why the different original numbers? Astron used different transistors at different times.
For those that want details, here you are:
Part IC
(see note 1)
min / max
at IC
Power Dissipation
(watts) at 25°C
(see note 2)
2N3055 10 60 20-70 @ 4a 75 $2
30 40 15-60 @ 15a 150 $2.50
2N3773 16 140 50-60 @ 8a 150 $2.25
2N5301 30 40 15-60 @ 15a 200 $4
2N5302 30 60 15-60 @ 15a 200 $4
2N5686 50 80 15-60 @ 25a 300 $8
Note 1: The maximum collector current specified above is only valid as long as the internal transistor temperature is less than the rated maximum.
Note 2: 25 degrees C = 77 degrees F. As the temperature rises, the power disippation goes down. The internal chip temperature of the transistor is a lot hotter than the case, or the heat sink, especially during long key-down sessions. The characteristic that causes a time lag between internal chip temperature rising and the heat sink temperature rising is called thermal resistance, and the heat sink compound helps to reduce that.

The 2N3055s and 2N3771s have been found in known-stock Astrons.   The 2N3773s, 2N5301s, 2N5302s and ECG181s have been found in used ones bought at swap meets.   The 2N3773s may have been stock (if they were replacements they were very nicely done, and I couldn't tell), the 2N5301s and 2N5302s were obvious field replacements.   I always check the emitter ballast resistors and I use the 2N5686 exclusively as a replacement as not one supply I have rebuilt with them (over 20) has EVER come back to haunt me (at least for a pass transistor problem).   Yes, they cost more, but the price difference on four new devices for an RS35 is around $25, even if the originals are 2N3055s.   If you decide to save the $25 and use the 2N3771s what is a future failure, the down time, a round trip to the repeater site (remember, gasoline is over $3 a gallon) and ANOTHER power supply rebuild going to cost?   What is your time worth?

Don't mix the transistor types !!!!!!   The emitter ballast resistors do their balancing act only when the transistors are similar. Always replace a dead pass transistor with an identical part number, or if you can't find one, replace all four as a group. And don't go down in ratings - if you find 2N3771s do not replace them with 2N3055s. If you have to replace the group, and you have 2N3055s and don't want to go to 2N5686s at least replace them with 2N3773s. That's 60% more current capacity and twice the power dissipation for an additional 25 cents each (or one dollar for a set of four).

Don't forget to use some good beryllium based thermal compound (the thick white stuff that is the consistency of axle grease), but don't go overboard - you want just enough to put a thin layer between the transistor and the insulator, and again between the insulator and the heat sink. Note that beryllium compounds are known to be human carcinogens when inhaled, fortunately the greasy consistency of the heat sink compound prevents any airborne dust, but you still want to keep it off your skin (i.e. use gloves, and maybe a popsicle stick to spread it thin). See this web page for more details. If you can't get the beryllium based material then use the Arctic Silver™ compound made for computer CPU heat sinks.

As long as I am inside the Astron I also add:

The MOVs and gap-caps can be soldered right across the back of the IEC. Just remember that MOVs do wear out. When the voltage across a MOV reaches the breakover point, the MOV conducts and turns the excess impulse energy into heat. Problem is, the heat affacts the MOVs internal characteristics - its breakover voltage increases. The next impulse comes along and more of it passes through. If a MOV sees enough action, the equipment protection is compromised to zero and you won't know it (this is why the cheap PC "surge protection" power strips are a joke and a delusion - usually all that is inside is one MOV. Tripp-Lite "Isobars" are much, much, much better). MOVs are cheap, however, and better than no protection at all.

If the Astron I'm rebuilding is going to a repeater site I add a voltmeter, an ammeter, and if it's a busy repeater a 24vDC fan (or two 12vDC fans in series) blowing air across the heat sink... a 24v fan run at 12v moves enough air to keep the supply heat sink cool.   Don't forget that the power transformer and rectifier block need additional cooling as well. It's worth punching a 3 inch or 4 inch diameter hole in the top and bottom, put copper screening over the holes and force some air flow over the the internal components with a second 24v fan... if cabinet clearances are tight (i.e. no room above or below), I'll punch both ends of the supply cabinet, add the copper screening and mount the internal fan on the outside over the intake hole (pushing air into the cabinet cools better than sucking air out).

Copper screening is stocked by most model airplane / hobby stores.

All Electronics is a good reputable surplus source for a wide variety of goodies, including voltmeters, ammeters and fans.   If you are going to add a fan (or two) to a repeater site power supply make sure you use a new ball bearing fan - cheap fans use brass or bronze bushings, cheaper fans use plastic bushings, good fans use ball bearings and are worth the extra money - what is a service call to replace a fan going to cost you? Fans with needle bearings are even better (i.e. mil-spec quality) but not too common and when they are found are usually expensive.

From another email to repeater-builder:

The stock Astron power transformer has the center-tap grounded, a high current secondary, and a outer AC winding just for the voltage regulator board.   The secondary is one winding, with the center section being the heavy (high current) wire and the two outer sections much thinner wire. See the diagram below:
-----------------    -----------------   thin wire
                 )  (
                 )  (
                 )  (  thin wire section of the secondary winding
                 )  (
Primary winding  )  (
                 )   -----------------   thick wire
                 )  (
                 )  (
                 )  (  thick wire section of the secondary winding
                 )  (
                 )  (
On the 120/240   )   -----------------   center tap (thick wire)
models the       )  (
primary is in    )  (
two sections     )  (  thick wire section of the secondary winding
that are in      )  (
parallel for     )  (
120vAC and in    )   -----------------   thick wire
series for       )  (
240v.            )  (
                 )  (  thin wire section of the secondary winding
                 )  (
                 )  (
-----------------    -----------------   thin wire
I've seen two with one of the outer sections opened up.   There is no way to fix it short of a new transformer.   The inexpensive fix is to abandon the thin-wire section and move the two thin wires to the secondary of an added small separate 24vAC transformer. There is plenty of room inside the cabinet for it.

Linear Power Supply Design and Theory

      These next sheets are oriented towards the 3-terminal regulators:
(these are not used in Astrons, but until we have a generic power supply page here at repeater-builder this page is a handy place to stash them)

Reset circuits for Astron Linear Series Power Supplies

Float-Charging Modification for Astron Linear Series Power Supplies

Battery Back-up Modification for Astron Linear Series Power Supplies

Schematic Diagrams of some popular Astron Linear Power Supplies

Please realize that you will find multiple different schematics listed below for the same supply as the designs changed over the years - due to parts availability, circuit improvements, etc.   You may have to download more than one schematic to get the one that matches your supply, and you may not find your schematic at all (as we only have the ones that were donated to us).

When (or if) you find the right one then I suggest you print it and stuff a copy inside a plastic page protector, and taped to the underside of the lid of the power supply cabinet!   Several folks have mentioned in emails and on mailing lists that you can call Astron on the phone and you will hear them tell you that they don't have electronic copies of their drawings and they don't know how to email them.   Trust me, the person that answers the phone will be amazed when you tell them that the drawings from different years for the same model power supply show some different component ID's and values.   Unfortunately this is important because if one chooses to buy replacement parts (from Astron) they (according to Astron) need only to supply the model and component ID's....  Fortunately everything but the filter caps, transformer, and sheet metal are common Mouser or DigiKey parts.   And I bet you could find the caps if you tried hard enough.


So leave off the prefixes and suffixes and first look for the basic model as opposed to the specific model.

Don't even bother asking if the metering option can be added later on...   At one point I was visiting a client in in Irvine and used the opportunity to stop in at Astron to pick up a RS-20A and RS35M schematic, and casually asked if I could buy the lower half sheet metal of an RS-20M plus the meters to upgrade my RS-20A to an M series.   Yes, but the price was over 2/3 of the cost of a new supply - plus shipping! ("Sorry, you'll have to pay in advance, and we'll have to ship them, they aren't in stock")   For that price I can buy a matching pair of surplus meters and cut the holes myself. By the way, adding the metering to most of the Astron designs is not hard - acquiring matching voltmeters and ammeters, and cutting the meter holes is the hardest part. For the wiring just refer to the -M version of your supply or a similar model.

Several manufacturers have had Astron build supplies for them - for example a Kenwood KPS-12 is based on the Astron RS-12, the Motorola HPN1007A is an RS-10 derivative, and the Motorola HPN9041 is an RS20 variant.

Donations of additional schematics are always welcome !!
Send them to Mike WA6ILQ at: (callsign) -at- repeater-builder -dot- com - and thanks in advance!
(you will be credited unless you tell us to assign it to A. Nony Mous)

See the "Notes" section above for an explanation of the suffix letters

Schematic Diagrams of some popular Astron Switching Power Supplies

More from Skipp May WV6F:

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 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 & 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.

If you are interested contact Skipp at Skipp025 -at- yahoo -dot- com

And an email from someone who bought Astrons' 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 a 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 / 8:00PM EST.   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

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This page originally created August 2000 by Kevin Custer W3KKC
Totally rewritten 10-14-2004 by Mike Morris WA6ILQ
Contributions from Skipp May WV6F, Ron Rogers WW8RR, Mike Perryman K5JMP, Bob Meister WA1MIK, Jeff Kincaid W6JK, Doug Marston WB6JCD, Don Best N6ALD, Will Martin KA6LSD, A. Nony Mous and her cohorts.
Copyright © 2000 and and date of last update by

The Astron logo/image is a registered trademark and is used within this page with permission from the Astron Corporation.

The schematic images are copyright © Astron Corp.   Each one is dated in the title block on the bottom right of the individual drawing.

This web page, 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.