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Author Topic: Astron RS-35A: rebirth? after lightning  (Read 3819 times)
N4EF
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Posts: 31




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« on: December 15, 2012, 01:18:10 PM »


A nearby lightning strike made my Astron RS-35A inoperative.

I've replaced the 4 pass transistors, the SCR 2N681, and the LM723 voltage regulator chip. The power supply now comes to life with 13.8 volts output under no-load testing.

When I connect a Kenwood TS440s (in receive mode), the voltage output drops to about 7 volts, and of course, the transceiver barely energizes.

Similar low-voltage result with a Yaesu 2 m FM rig.

The Yaesu 2 M rig was not involved in the lightning strike.

I tried another LM723, but still get a big drop in output voltage.

What would be the likely culprit for this wimpy output under load and how should I proceed?

Thanks.

Dave
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TANAKASAN
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Posts: 933




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« Reply #1 on: December 15, 2012, 02:12:47 PM »

OK, some ideas for you to try. Firstly though you need to test the power supply on a dummy load and NOT using your rig because if things go bad it could get expensive.

NORMAL MAINS VOLTAGE SAFETY WARNINGS APPLY HERE

1) Check the voltage between pin 12 and pin 7 of the LM723both on and off load. This should drop a bit but certainly not down to the 7 volt level, if you see something silly suspect the rectifier diodes.

2) Move the + lead of your meter to pin 10 and do the same on load / off load test. You should see the voltage on this pin go up under load as the chip drives the transistors harder to cope with the increased load.

3) Next test point is both sides of D5. Again, check that the voltage rises under load. If this is OK then the series pass transistors are being driven correctly by Q2.

4) Check R101 through R104 for correct value, 0.05 ohms. This may appear as a dead short to your multimeter so an alternative approach is to put the power supply under load then measure the voltage at the emitter of each transistor. The voltages should be the same +/- a tenth of a volt or so.

I suspect Q2 or D5

Tanakasan
« Last Edit: December 15, 2012, 02:15:03 PM by TANAKASAN » Logged
K8AXW
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Posts: 3657




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« Reply #2 on: December 15, 2012, 07:37:26 PM »

After a nearby lightning strike and or surge with a subsequent power supply failure, suspect anything solid state.

Quote
Firstly though you need to test the power supply on a dummy load and NOT using your rig because if things go bad it could get expensive

Amen Herr Tanakasan!!  Well said.
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N4CR
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Posts: 1650




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« Reply #3 on: December 15, 2012, 07:48:19 PM »

As said before, you need a dummy load to test against. An automobile headlamp draws a few amps. You can find old sealed beam headlights at the junkyard for free usually.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KA4POL
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Posts: 1910




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« Reply #4 on: December 15, 2012, 10:10:29 PM »

And do a search here for Astron repairs. You'll find quite a few threads with invaluable information.
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KA1MDA
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Posts: 543




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« Reply #5 on: December 16, 2012, 12:03:04 AM »

Since the voltage is dropping to 50% under load, I would suspect an open rectifier diode has essentially converted the full-wave rectifier an half-wave rectifier.
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TANAKASAN
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Posts: 933




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« Reply #6 on: December 16, 2012, 03:48:17 AM »

You may be right about a dead diode but the last linear supply I repaired had a faulty transistor between the regulator chip and the power transistors.

As for the dummy load, I have a collection of old wirewound resistors all fixed to a 12 inch x 14 inch heatsink. This has the benefit of heating the shack on a cold Winter afternoon whilst I work.

Tanakasan
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N4NYY
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Posts: 4742




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« Reply #7 on: December 16, 2012, 05:26:28 AM »

Even shotgunning these is easy. You are half way there. Do what Takanasan says, then see if you got it.
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W6EM
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Posts: 710




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« Reply #8 on: December 16, 2012, 02:47:48 PM »

You may be right about a dead diode but the last linear supply I repaired had a faulty transistor between the regulator chip and the power transistors.

This sounds like an accurate diagnosis, from what you describe you have replaced.  It sounds like the collector of the driver transistor is open, so that there isn't sufficient drive to the paralleled pass transistors.  This type of fault tends to fool people because the regulator supplies only enough drive to provide a regulated output for very light loads.

If you think this one's tough to fix, Pyramids are even worse.  They don't have any transient over voltage protection and everything gets fried.  Even with shorted loads.
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WB6DGN
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Posts: 585




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« Reply #9 on: December 18, 2012, 05:47:41 AM »

Well, just about all of the possibilities have been covered but, without the power supply in front of me all I can do is guess.  So, "playing the numbers" based on the repair of a lot of Astron supplies that suffered lightning damage, my guess is a diode or diodes bad in one, or both, of the rectifier bridges.  That seemed to be the most frequent problem that I encountered in repairing 35A Astrons damaged by lightning.  I remove those bridge packages, even if they're not damaged, and replace them with a pair of 1N1184 diodes like the ones they use on the higher current supplies because those bridges have no provision to balance the current in the parallel diodes.  That is a potential source of failure even without the help of lightning.  The chassis bottom pan is an adequate heat sink but I like to keep the diodes fully inside the cabinet and use a couple of "L" brackets fabricated form about 0.100" aluminum mounted in the same area as where the bridges were.  Don't forget to insulate the diodes from those brackets.
One other thing to check is the main filter capacitor (C 5, 64k uF) across the high current supply.  A bad capacitor will cause the supply voltage to the pass transistors to sag quite low as well.  That's real easy to spot with a multimeter, though.
A few more things to look at.
Tom
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K8AXW
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Posts: 3657




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« Reply #10 on: December 18, 2012, 09:15:14 PM »

I would suggest checking the voltage at the + terminal of the main filter cap.  (64K UFD) N/L and the underload.  If this voltage sags more than a few volts, then you have either a bad filter cap or rectifier.  Check the rectifier first. 

I have never had a pass transitor driver go bad. 

In the order of failures, I would list the LM723 #1, pass transistors #2, rectifiers #3, main filter cap #4.
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W6EM
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Posts: 710




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« Reply #11 on: December 19, 2012, 08:03:31 PM »

...... The chassis bottom pan is an adequate heat sink but I like to keep the diodes fully inside the cabinet and use a couple of "L" brackets fabricated form about 0.100" aluminum mounted in the same area as where the bridges were.  Don't forget to insulate the diodes from those brackets.
....

I did a similar thing to a Pyramid supply.  However, I made the sink out of one large aluminum stock angle piece and insulated it from the cabinet on standoffs.  Easier than trying to mess with mica and white "goop,"  since you're tying the diode cathodes together anyway.  And, it provides an easy way to connect a heavy gauge wire terminal.
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W9GB
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Posts: 2598




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« Reply #12 on: December 20, 2012, 04:43:47 PM »

Silicon Rubber insulators have replaced Mica and heat sink compound to insulate power transistors.

Even Sears sells these !
http://www.sears.com/search=nte+electronics+to+3+style+transistor+insulator?catalogId=12605&autoRedirect=false&storeId=10153&yikes_prod=true
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AA4PB
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Posts: 12673




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« Reply #13 on: December 20, 2012, 05:04:54 PM »

Who'd have ever thought it? Can't get parts from Radio Shack any more but Sears has them along side the washers and driers  Grin
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AC5UP
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Posts: 3823




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« Reply #14 on: December 20, 2012, 05:17:22 PM »

Sears isn't exactly Sears any more... It's owned by Sears Holdings which also owns K-Mart, Lands End and a pantload of other retail brand names.

Take a close look at the link for the insulators and you'll find they're sold by CusCus, which could be a Sears holding or a 'web partner'. If you should buy something like that through the Sears web site and later decide to return it, do not take it to your local Sears store... It has to go back through the web site.
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