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Author Topic: Astron VS-35M blowing its fuse  (Read 11943 times)
KD8DEY
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Posts: 352




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« Reply #15 on: March 29, 2012, 07:38:19 PM »

hi John,

Take a look at the area where the power cord enters the case,
there should be a MOV metal oxide varistor there,
I bet it is damaged and or charred.

If it is, cut it out and see if the fuse still fails,
it the supply works fine replace the MOV.

73 james
That is a remote possibility, Especially if he experienced a power surge/lightning strike while the supply was operating.

BUT
he only stated that it had started blowing fuses that morning.

didn't say if he had left the supply on overnight and came back to find the fuse blown or if it blew when he turned it on that morning....Smiley

Also said that he didn't see any signs of physical damage.....

Wouldn't hurt to try it and eliminate the possibility regardless.....
« Last Edit: March 29, 2012, 07:51:53 PM by KD8DEY » Logged
K8AXW
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Posts: 3860




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« Reply #16 on: March 29, 2012, 09:36:23 PM »

Quote
I am having a little trouble following terminology; remember, I'm just an old tube guy. Are the "pass transistors" the four 2N3771's mounted on the rear exterior of the case? And what is the "crowbar circuit"? What is the device that's mounted across the transformer primary, labeled "VR1 V150LA 10A" on the schematic, that looks like a disk ceramic capacitor? I don't see the schematic symbol in the Handbook.


Yes, the "pass transistors" are the four 2N3771 transistors mounted on the heat sink.  The VR1 is a MOV or Metal Oxide Varistor.  It's supposed to handle voltage spikes on the transformer primary (a fault or surge on the power line.....nearby lightning strike, etc.)

The next step is to remove the wire going from the rectifiers to the collectors of the 2N3771 pass transistors.  If the pass transistors are shorted and or there is nothing to CONTROL the pass transistors like the voltage regulator (LM723) which is on the PCB then the voltage to the overvoltage protection circuit will cause the SCR1 to fire and short the output to ground which will blow the fuse. 

However, the MOV in the primary, as DEY suggests it is now a prime suspect as well.

Remember, the game plan is to isolate sections of the power supply.  By removing the wire feeding the pass transistors you have isolated the power supply into two sections.  If the fuse blows then it's the MOV, the transformer, the rectifier(s) or big filter cap.  If the fuse does NOT blow, then it's probably the pass transistor(s).

You eliminate parts in the first part of the supply until you isolate the component that is causing the fuse to blow.  If you remove the wire to the pass transistor collectors and the fuse blows, I'm betting you have a bad MOV or rectifier, in that order.
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K3GHH
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« Reply #17 on: March 30, 2012, 02:20:19 PM »

Now home from work and ready to dive into the power supply again. James, thanks for the varistor suggestion... and DEY and AXW thanks yet again! After my last post I realized that I could have simply looked up what "pass transistors" and "crowbar circuits" were. (I think the QST to which KG6AF refers is in the bunch I just put in the attic, and haven't succeeded in logging into ARRL's new "members only" site yet.) Thanks, especially to K8AXW, for not rapping my knuckles for asking about something I could easily have found (and later did find) in my reference works.

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K8AXW
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« Reply #18 on: March 30, 2012, 02:26:45 PM »

Just be sure to let us know what you found OM.  If you don't, you'll "get your knuckles wrapped" for that!   Grin
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K3GHH
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« Reply #19 on: March 30, 2012, 04:32:19 PM »

Just be sure to let us know what you found OM.  If you don't, you'll "get your knuckles wrapped" for that!   Grin

Ha! And I'd deserve it!

I unsoldered the white full-secondary and the gray center-tap transformer wires from the regulator PC board... fuse still blew. I then unsoldered the yellow wires that connect transformer taps to the rectifiers; at this point there's nothing connected to the transformer secondary. Fuse DID NOT blow.  Smiley

Diagnosis: It's the rectifier(s). Tomorrow is the Timonium MD hamfest, about 3 miles from my house, so I'll see what I can find there!

Looks like progress to me.
« Last Edit: March 30, 2012, 04:33:51 PM by K3GHH » Logged
K8AXW
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« Reply #20 on: March 30, 2012, 08:54:18 PM »

GHH:  Very good!  You have not only found your problem but you have also found your way around in an Astron power supply.  Good feeling, eh?   Wink

While at Timonium you might want to find another set of 2N3771 transistors and perhaps even an LM723 to hang on your spare parts wall.  Something took out the rectifier..... or it could have just died.  At this point you don't really know!  I'm still suspicious of the pass transistors.
« Last Edit: March 30, 2012, 08:58:12 PM by K8AXW » Logged
KD8DEY
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Posts: 352




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« Reply #21 on: March 31, 2012, 01:09:33 PM »

I was told long ago, when replacing semiconductors, Don't replace just 1, replace in "sets" if applicable since the other semiconductors paralleled in the circuit may have been compromised.

better safe than sorry......
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K3GHH
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« Reply #22 on: March 31, 2012, 03:40:17 PM »

Got back at 11AM after 4 hours at the Timonium hamfest, a shadow of its former self, unfortunately. I was looking for KBPC2502's, and found only two vendors offering bridge rectifiers. One had 25-amp units, but their number started with a G and I didn't know if the G and K were significantly different. The other vendor, Tom K8CLA (operating Debco Electronics), said his KBPC5010's would work in place of my 25-amp lower-voltage originals, so I bought a couple and two 2N3771 transistors (the "pass transistors") from him; he was very helpful. It took about an hour this afternoon to install the rectifiers and reconnect the PC board, and the VS-35M seems to be working fine!  Smiley

Thanks for your help and encouragement, especially AXW and DEY. It's a nice supply, and maybe now it's good to go for another 20 years.
« Last Edit: March 31, 2012, 03:59:50 PM by K3GHH » Logged
K8AXW
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« Reply #23 on: March 31, 2012, 05:24:23 PM »

GHH:  Rgr on Timonium.... never been to that one but the hamfest thing seems to be dropping off everywhere.

Glad you got your rectifiers.  On the smaller units it have been replacing the rectifiers with the 50A bridge rectifiers available on eBay.  Of course with yours that use individual rectifiers you pretty well need to stay with the program.

Glad you got it going.  The 2N3771 "wallhangers" shouldn't be considered a waste of money.  Just look at them as an insurance policy.

Did you have to replace the LM723?

Was both rectifiers bad?

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K3GHH
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« Reply #24 on: March 31, 2012, 05:57:47 PM »

AXW: I forgot to look for a spare 723 (so no, I didn't replace it), and simply replaced both rectifiers without learning how to test which of the 2502's is bad. They're both sitting here on the desk --- guess I'll look into testing them at my leisure!

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K8AXW
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« Reply #25 on: April 01, 2012, 11:09:33 AM »

GHH:  Simple test.... just put the ohmmeter across each diode and see what the meter reads.  Then reverse the leads and take another reading.  One should be very nigh and the other very low.

If you show a very low resistance or zero resistance in both directions the diode is shorted.

If you show a infinite resistance in both directions, the diode is open.

If you draw a schematic of a bridge rectifier (one of those square blocks with 4 terminals) you can test the bridge rectifier by simply testing each diode the same as above, one diode at a time.

Al
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WX7G
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« Reply #26 on: April 03, 2012, 05:34:59 AM »

The problem is a shorted rectifier diode.
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K8AXW
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« Reply #27 on: April 03, 2012, 09:28:22 AM »

Quote
The problem is a shorted rectifier diode.

I understand this.  I was simply asking if both went or just the one. 
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WX7G
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« Reply #28 on: April 03, 2012, 11:44:08 AM »

Just one.
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KD8DEY
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« Reply #29 on: April 03, 2012, 12:25:58 PM »

Just one.
Glad you got it up and running. Did you replace just the one or the pair?
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