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Author Topic: Blew my 1987 Astron RS-50a  (Read 3227 times)
KD7YZ
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Posts: 8




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« on: March 24, 2013, 11:55:42 AM »

There is/was a SSB-Electronics-gmbh Pre-amp controller 2004b sitting atop my 1988 Astron 50-amp P/S. It's sat up there w/o problems for a good while.
So today I was leaning over the back side of the gear and elbowed the SSB-Sequencer, which has coax-connectors to it, over the back-side of the Astron. I knew immediate to it slipping that I was in trouble, and also dumb. When I heard the slight "Pop" and the Rig(s) went dead, I knew.
So I looked at the casing on the 2n3771, which is on the outside of the right-rear, and I see the tell-tale marks of arc-gouging on the case of the 2n3771.

While nothing looks fried, the main power fuse is still intact and thus something else is fried.

What's the chance that only the 2n3771 is B/O ?

Otherwise, any Elmer's know what else to look for that's kaput? Simply using the multi-meter on the case-mounted transistors didn't tell me much.

thanks, in advance.

KD7YZ
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WA2ONH
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« Reply #1 on: March 24, 2013, 01:04:58 PM »

Check out the WA6ILQ / WA1MIK site at:

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

Schematics and service information on Astron™ power supplies

Good Luck!
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73 de WA2ONH dit dit    ...Charlie
----------------------------------------------------------------------------
"No time is ever wasted that is spent LEARNING something!"
MISTAKES are proof that you are TRYING
AA4PB
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« Reply #2 on: March 24, 2013, 01:08:44 PM »

It could be one or more of the 2N3771 transistors are shorted. That would cause the output to attempt to go over-voltage which would fire the SCR protection circuit and limit the output voltage to a volt or two. You'll have to check each transistor with an ohmmeter to tell which, if any, are shorted.
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AC5UP
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« Reply #3 on: March 24, 2013, 03:35:15 PM »

The outside case of a TO-3 transistor like the 2N3771 is the collector. Which should be wired to the + side of the filter electrolytic. Which in turn comes straight off the bridge rectifier and secondary of the power transformer. Assuming it was a momentary short to ground (as noted by the arc telltale on the transistor case), you should have heard a fairly robust POP... Given the size of the transformer and filter cap. (?)

In any case, when a pass transistor collector shorts to ground the bridge rectifier will be stressed to maximum current from the transformer secondary. That's the first thing I'd check for damage. Believe it or don't, the 2N3771 is probably still good as the short was upstream from the internal junction. Check the rectifier(s) and you should find the fault.

When it's working properly you should see between 18 and 23 volts on the cases of the 2N3771's and 13.8 VDC on the emitters.
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N4CR
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« Reply #4 on: March 24, 2013, 04:17:08 PM »

The 2N3771 on the back IS the crowbar transistor.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
AC5UP
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« Reply #5 on: March 24, 2013, 04:56:26 PM »

http://www.repeater-builder.com/astron/pdf/astron-rs50a-rs50m-rm50a-rm50m-1996-03.pdf

This RS-50A Schematic shows eight 2N3771's as pass transistors with a ninth as the driver. The crowbar device across the output is SCR1, an SO5651 SCR............
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WA1RNE
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« Reply #6 on: March 24, 2013, 04:58:31 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.
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KD7YZ
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« Reply #7 on: March 24, 2013, 05:37:01 PM »

Do I take it then that the 'crowbar' will be the likely blown device?
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N4CR
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« Reply #8 on: March 24, 2013, 06:48:49 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.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KD7YZ
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Posts: 8




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« Reply #9 on: March 24, 2013, 06:51:45 PM »

I am currently reading and digesting the reference link .. at this part "Testing the Series-Pass Circuitry:"

thanks so far for the good ideas guys
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N4NYY
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« Reply #10 on: March 24, 2013, 07:24:27 PM »

Pull the pass transistors and measure them out with a diode test on a DVM. I wonder if the pop is a cap or not. That caps are now 25 years old. Nice thing about these is that even if you shotgun these, your usually fix them.
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K8AXW
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« Reply #11 on: March 24, 2013, 09:51:39 PM »

YZ:  5UP is probably correct.  Suggest you download the schematic for this power supply if you don't already have it and "see" what probably happened when the 2N3771 was shorted to ground.

With this scenario, CR101 and CR102 are probably shot.  These are the large stud rectifiers.

If the 2N3771 that drives the 8 pass transistors was the one shorted then you have a different game.

I would then suspect the smaller diodes, CR1 and CR2 to be shot. 



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KD7YZ
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« Reply #12 on: March 25, 2013, 06:28:12 AM »

OK, this is weird. 13.9vdc on the output.
I'm certain that the gear wasn't, and wouldn't,  powered when I shorted the driving 2N3771 which is located ON the back panel, right-upper-side nd directly above the two fin-mounted 1N1184a's.

Certain! So I unplugged the RS-50a, opened the top lid and peered around for obvious damage. Seeing none and not having the original schematic,  I sought help here.
Downloaded the schematic from the link you posted then started with the repair article on repeater-builder .. So this morning I was ready to take some readings at Points listed on the downloaded schematic from Astron. The first thing I noticed was that the former 'thump' of the large transformer loading up WAS back. The power-switch light was back .. and I tried to remember exactly, but i am fairly certain switching on and off was initially no-light and no thud of the transformer.

The voltage table doesn't list Point#1 yet the schematic shows a Point#1 off the SCR1. #5 and #4 are good. all eight of 2n3771's show circa 22 vdc and the driving 2N3771 has 29 bolts.

I just connected a FT450 to the RS-50a and ran it at 100w for a bit. Key-down, the output of the P/S stays at 13.9vdc ..

I can't explain the fact that nothing powered up immediately after the short happened. Though I didn't press it In all of a minute it seemed obvious to me, then, that nothing worked and to take the P/S off-the-line lest I damage it more.
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K8AXW
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« Reply #13 on: March 26, 2013, 10:20:46 AM »

YZ:  Quite often when something bad happens there is a moment of pseudo-hysteria that leaves us simply wondering what the hell just happened!  Along with this comes confusion and amnesia of certain details.  Sounds as if you might have had one of these.

At any rate, the unit seems to be working fine so give thanks; live long and prosper.
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KD7YZ
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Posts: 8




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« Reply #14 on: March 26, 2013, 03:58:16 PM »

AXW right you may be. If there is a next-time. I think I will video me checking the initial damage.
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