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Author Topic: Blew my 1987 Astron RS-50a  (Read 2623 times)
AC5UP
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« Reply #15 on: March 26, 2013, 04:12:18 PM »

The LM-723 regulator chip has the ability to self protect by shutting down under certain fault conditions. The reset procedure involves turning the power off, waiting for the filter cap(s) to bleed down to near zero, then powering back up.

You think you were freaked? Sounds like the regulator chip was temporarily traumatized..............    Grin
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W6EM
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Posts: 729




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« Reply #16 on: March 26, 2013, 04:34:50 PM »

Astron designed into its supplies some overvoltage/transient protection that probably saved the day as far as damaging the LM-723 and the pass transistors.  A reversed biased diode across the series-pass transistors and a 40V zener on the input to the LM723, among others.   The repeater-builder article is a good one.  Especially with respect to his suggestion of changing out the series pass 2n3055s or 2n3771s, whatever the case may be, to much heavier devices.

Also, beware of any models that use paralleled "half-bridge" rectifiers.  Current imbalance can cook one of them under heavy loads.

Last, but not least, thank goodness you have an Astron.  I made some major mods to my large 50A Pyramid to include what Astron uses for protection and replaced the paralleled bridge rectifiers with two large stud-mount diodes.  Astronized it.

You also might want to look for a plastic TO-3 case cover.   That way, you'll help keep the collector/case protected from inadvertent contacts.

73,

Lee
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WA1RNE
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« Reply #17 on: March 26, 2013, 04:43:49 PM »

Quote
The 2N3771 on the back IS the crowbar transistor.

The 3771's are pass transistors - the crowbar is an SCR located inside the supply.

There is a 3771 on the back that drives the shunt SCR.

Better check the RS-50 schematic: there is one 2N3771 in the rear that is used to drive the 8 pass transistors - 4 per heat sink mounted on each side of the supply. A 2N3906 small signal transistor on the regulator board sets the crowbar threshold voltage and drives the SCR.


** To KD7YZ:  If in fact you shorted the case of the single 2N3771 mounted at the rear to ground (the power tranistor that is mounted to the chassis and not one of the side mounted heat sinks), then you likely caused CR1 and/or CR2 to fail - not CR101 or CR102.

CR1 and CR2, along with capacitor CR6 provide a dedicated low current DC source for the regulator and drivers, Q2 a TIP29 and the lone 2N3771 on the chassis which I refer to above.

CR101 and CR102 provide a dedicated high current DC source to the pass transistors which should not be involved, unless CR1 and CR2 failed shorted, causing the LM723 to fail, then the drivers and possibly the pass transistors - but I highly doubt this occurred.

This would also explain why the fuse didn't blow which is rated to handle the full output current of the supply plus margin for start up current, etc.

One step at a time - isolate CR1 and CR2 and check them using a DMM with a diode test mode or use a Simpson on the X1 ohms range.

« Last Edit: March 26, 2013, 04:48:20 PM by WA1RNE » Logged
KD7YZ
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Posts: 8




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« Reply #18 on: March 26, 2013, 07:31:51 PM »

Astron designed into its supplies some overvoltage/transient protection that probably saved the day as far as damaging the LM-723 and the pass transistors.
and whoda-thunk? 20 years later and I'm happy they did.
 
Quote
You also might want to look for a plastic TO-3 case cover.   That way, you'll help keep the collector/case protected from inadvertent contacts.

I put some red, good, electrical tape there for the time being. Will search for T0-3 cover(s)

Been using the RS50a all day now. She's just fine after my pre-amp-sequencer startled the poor old gal, hihi

tnx for the support.

now, if anyone just knew anything about POE to my IPCAM. I want to mount it underneath one of the 432 yagi's but power never gets to it over ethernet cable. I will write a new "ticket# in the hopes there is an Elmer for Power-Over-Ethernet. Would be a nice way to know I am pointing at the moon when it's not cloudy anyway.

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W1VT
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« Reply #19 on: March 27, 2013, 06:39:10 AM »

Perhaps you just observed the normal behavior of an SCR crowbar?   Cheesy

An SCR or silicon controlled rectifier is a latch--once you turn it on it won't turn off--until you power the circuit down.  It is specifically chosen for this application to keep the voltage low, possibly preventing damage to expensive gear, until the equipment can be checked out properly. Otherwise, you might see the application of unregulated voltage, which would be very very bad.  

Zack Lau W1VT
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KD7YZ
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« Reply #20 on: March 27, 2013, 08:36:05 AM »

Perhaps you just observed the normal behavior of an SCR crowbar?   Cheesy


You are right. This is the first I've seen of one in action. Thanks for a good 'lesson' on the SCR.

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W6EM
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Posts: 729




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« Reply #21 on: March 27, 2013, 07:49:16 PM »

A crowbar SCR (and the pc board traces) are not in and of themselves capable of a long-time short to ground.  The incoming AC line fuse should never be upsized and should be a fast-acting type, not a slo-blow, to shut down the AC quickly.

In some poorly designed supplies (won't mention any names) the SCRs or pc traces can burn open, allowing the full, unregulated voltage on the output, if any of the regulator transistors develop collector-emitter shorts causing crowbar action.

I've rebuilt quite a few Pyramids that have had some problems.

I found the 2n5686 to be a superior replacement for the 2n3771.  50A and twice the breaksown voltage rating and case power dissipation.  I replaced all 3771's in my main station and shop power supplies.  Worth the cost for the extra margin of safe operation.
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N4CR
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Posts: 1653




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« Reply #22 on: March 27, 2013, 09:41:42 PM »

I found the 2n5686 to be a superior replacement for the 2n3771.

How do you deal with the pin size on those?
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73 de N4CR, Phil

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