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Author Topic: Astron 35M Power Supply  (Read 1567 times)
W4DL
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Posts: 182




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« on: January 26, 2013, 11:28:33 AM »

While experimenting the other day, I shorted the output terminals via some jumpers on the Astron;  now the ammeter pegs and the output voltage is 1 or 2 volts.  I found a shorted 2N3177 pass transistor and changed it and the LM723 regulator chip.  Same results; could I have a shorted SCR?  Any input appreciated, since I'm out of spare parts.

Thanks,  Mike W4DL
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W4VR
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« Reply #1 on: January 26, 2013, 01:22:19 PM »

I'm surprised the fuse did not blow and protect the output devices when you shorted the terminals.  I hope this was done by accident.  I had a problem once with the 35 M; it took a lightning hit.  I replaced all of the output transistors plus that IC and it worked fine afterwords.
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K8AG
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« Reply #2 on: January 26, 2013, 01:26:28 PM »

I believe the 35M has a crowbar circuit.  It is a circuit that shorts the power supply in the event the output wanders too high.  It is to protect your $10,000 radio from a failing $200 power supply.  Changing pass transistors and the 723 may require a crowbar trip level adjustment.  It is also possible that you damaged the crowbar circuit as well.

I am not familiar enough with the schematic to give specific parts to look at.  But Crowbars generally use SCRs to do their work.  My guess is either you damaged the SCR (unlikely), or you damaged the circuitry that controls the SCR to provide the short.

Hope this helps.

73, JP, K8AG
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AA4PB
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« Reply #3 on: January 26, 2013, 01:43:32 PM »

What is supposed to happen if you short the output terminals is that the 723 should go into fold-back and reduce the output voltage to limit the current.
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W6EM
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« Reply #4 on: January 26, 2013, 03:18:55 PM »

Odds are that you did short the SCR.  Since you lost a 2N3771, that brought the full unregulated 25V or so to the crowbar SCR and it fired to ground.  Many times, SCRs cannot take long-time fault energy (I squared-t) and will either fail shorted or burn open.  A burnt-open trace is the worst failure mode since it allows output overvoltage.  This can happen if your 120V side fuse is a tad slow or was upsized and didn't open at all.

Disconnect the SCR and see if the problem goes away.  If it does, you can check to see if it is shorted with an ohmmeter.  Crowbars use a voltage divider on the gate to fire them, so probably not an issue with the voltage divider.  Also, a capacitor from gate to ground to drain off any noise, rf, etc.
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KA4POL
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« Reply #5 on: January 26, 2013, 10:09:04 PM »

This seems to be a bit strange to me. The shortening of the output terminals does just what the internal protection circuit does. So not more than a blown fuse should have occurred. This should be checked after putting the supply back to work.
You say you found the 2N3771 (not 3177) to be shorted. The second one also may be gone.
Other possibilities are the diodes of the bridge and the TIP29 driver Q2.
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W6EM
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« Reply #6 on: January 27, 2013, 05:23:51 AM »

This seems to be a bit strange to me. The shortening of the output terminals does just what the internal protection circuit does. So not more than a blown fuse should have occurred.

Not exactly.  Shorts can produce transients, which can destroy junctions.
There are also some reverse-biased diodes.  One across the 4 2N3771's, and I think one across the output as transient protection.  Either or both of those might also be shorted.  Although, the one across the 2N3771's would have probably been detected when he checked the transistors.

Quote
Other possibilities are the diodes of the bridge and the TIP29 driver Q2.

A bridge diode failure would have produced blown 120V fuses without shorting the output.  If there is still over voltage after disconnecting the SCR crowbar, then possibly the TIP-29 driver.  I think in saying he found a shorted 2N3771, it implied that he checked the other three.  If I find one bad one, I usually replace them all.  Actually, 2N5686's are a more rugged, 50A equivalent and I use them in all of my supplies to replace 2N3771's.  There's a good tutorial on Astrons over at http://www.repeater-builder.com/astron/astron-index.html

The LM723 in Astrons is well protected, unlike some other designs.  
« Last Edit: January 27, 2013, 05:39:03 AM by W6EM » Logged
KA4POL
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« Reply #7 on: January 27, 2013, 05:46:07 AM »

Other possibilities are the diodes of the bridge and the TIP29 driver Q2.

A bridge diode failure would have produced blown 120V fuses without shorting the output.  If there is still over voltage after disconnecting the SCR crowbar, then possibly the TIP-29 driver.
With low voltage at the output there could be one or more open diodes. Where was over voltage mentioned?
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AC5UP
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« Reply #8 on: January 27, 2013, 06:47:53 AM »

Whenever you have a good solid C to E short inside a pass transistor... What I call "welded up"... The new circuit is a direct shot from the + side of the big momma' electrolytic to the + terminal on the output.  There is no regulation, there is no foldback protection, everything the transformer can provide is dumped straight across the crowbar circuit.

With a little luck the fuse blows before the SCR smokes.  Because if the SCR opens you'll see ~22 volts DC across the load and that will toast your Yaekencom 756 Pro III Mark V Deluxe Export Edition radio with extreme prejudice.

Note To W4DL:  Yes, you could have a shorted SCR and when the '723 chip goes into protected shutdown a volt or two across the output terminals is not unusual.  You can test this theory very easily by disconnecting the crowbar circuit.  Open the connection between the SCR and the + terminal, then measure the output voltage with no load on the supply.  If it's 13-ish you need to repair the crowbar circuit, if it's 16 to 22-ish volts the crowbar circuit is doing exactly what it's supposed to.

Then you get to figure out what's hosed in the regulator circuit.

BTW:  Do a search here on eHam for " Astron 35 " and see what comes back.  This isn't the first thread about this exact problem and probably won't be the last............  Tongue
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W6EM
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« Reply #9 on: January 27, 2013, 07:06:57 AM »

Other possibilities are the diodes of the bridge and the TIP29 driver Q2.

A bridge diode failure would have produced blown 120V fuses without shorting the output.  If there is still over voltage after disconnecting the SCR crowbar, then possibly the TIP-29 driver.
With low voltage at the output there could be one or more open diodes. Where was over voltage mentioned?
 Nope.  But, if you read about the pegged ammeter before and after attempted repair, that's a VERY strong clue that the crowbar is either 1. doing its job as intended, or 2. providing a serious short across the output to test the high side fuse........  Hard to get observed high voltage across a dead short.
« Last Edit: January 27, 2013, 07:29:12 AM by W6EM » Logged
N4CR
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Posts: 1653




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« Reply #10 on: January 27, 2013, 07:22:02 AM »

There's a lot of Astron documentation and mods on the repeater-builders site.

Good stuff.

http://www.repeater-builder.com/astron/astron-index.html
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W6EM
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Posts: 729




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« Reply #11 on: January 27, 2013, 07:39:51 AM »

While experimenting the other day, I shorted the output terminals via some jumpers on the Astron.....

Always best to "experiment" using a current limiting resistor or a small fuse in line with the supply.  I don't use my modded Pyramid supplies for any experimenting where I don't expect a heavy load.  I have a Lambda bench supply that's current limited to about 2 amps.  Even then, I set the current limiting control very low to make sure I don't cook more than just the widget under test.

Repeater Builder used to have quite a "treatise" on how Astron's ratings are a tad overdone, but I see it's disappeared.  Also, the suggestion to replace all 2N3055s and 2N3771s with 2N5686s.  Especially in almost continuous load situations powering repeater transmitters.  2N5686s are great devices.  50A rating.  Costly, unless you take your chances on eBay.  But, worth the extra couple of bucks for the rating.  Anytime you attempt to really load-down a supply for an extended period, the heat dissipation of the transistors becomes an issue.  Not so with 2N5686s each passing 5-10A in a variety of supplies.  That is, if the unregulated voltage is between 22 and 28VDC or so.  The first time you look at one, you'll notice the pin sizes for the base and emitter are considerably bigger than 2N3771s.  Part of that 50A rating.
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WA2ONH
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Posts: 248




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« Reply #12 on: January 27, 2013, 10:04:13 AM »

On the "Schematics and service information on Astron™ power supplies" page
http://www.repeater-builder.com/astron/astron-index.html

A NEW article was been added January 2013 ...
Repairing Astron 13.8V Linear Power Supplies By Robert W. Meister WA1MIK
"General linear power supply circuit explanation, followed by the most common problems, diagnostic techniques, and repairs for these units."

LINK: http://www.repeater-builder.com/astron/astron-repair/astron-repair.html

Good Luck.
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73 de WA2ONH dit dit    ...Charlie
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"No time is ever wasted that is spent LEARNING something!"
W6EM
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« Reply #13 on: January 27, 2013, 02:06:33 PM »

Here is the link to the article at the top of the Astron page that gives probably the best overall evaluation of the Astron linear supplies. http://www.repeater-builder.com/astron/astron-intro-stuff.html  Mike, WA6ILQ, goes out of his way to talk about the "real world" capabilities of the supplies.  And, where he recommends ditching the 2N3771s for 2N5686s.

Most people assume an ICAS ampere rating is implied by the Astron label number.  Read Mike's article and you'll think twice about how you load them.  That is, with the stock series pass transistors in place and paralleled bridge rectifiers (ugh....).  Not a good practice.  Current division equally?  Nope.  Replace with either one 50A bridge or better yet, two stud-mounted, heat-sinked diodes.

Every supply I repair gets a set of 2n5686s.  Sure, its overkill, but I never again have to worry about them.  And, I mod Pyramids to make them into ruggedized equivalents of Astrons.
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