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Author Topic: Astron VS-35M blowing its fuse  (Read 13692 times)
K3GHH
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Posts: 42




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« on: March 28, 2012, 03:49:25 PM »

This behavior just started this morning. I bought a package of 8a slow-blow fuses, disconnected everything, put in a new one, threw the power switch...and it still blows. I've removed the top cover and don't see any obvious signs of burning or physical leaking. This supply has been in use for many years. I have a schematic, but no other literature about it. Any suggestions as to where to start searching for the problem?

--John K3GHH
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KD8DEY
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Posts: 352




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« Reply #1 on: March 28, 2012, 04:01:55 PM »

have you checked output voltage?
it keeps blowing fuses covers a lot of territory.

most common problem is those darn 723's blowing

tried removing the regulator board to see if the transistor mounted on the case is cracked?.......
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W8JX
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Posts: 6666




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« Reply #2 on: March 28, 2012, 04:19:15 PM »

I tend to think that there is a failure of regulator and crowbar is blowing fuse.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
K3GHH
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Posts: 42




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« Reply #3 on: March 28, 2012, 04:27:50 PM »

Lots of help from 8-land tonight!

Is "regulator" the UA723 integrated circuit to which DEY referred? The transistor looks fine, and if by "output voltage" you mean the supply's normal setting, I use 13.8v. 
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KD8DEY
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Posts: 352




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« Reply #4 on: March 28, 2012, 04:48:33 PM »

the 723 is the regulator ic mounted to the board.
it cannot handle the current to regulate the 4 output transistors alone so the output from the IC goes to
what is usually a TIP-29 type transistor mounted on the bottom of supply cabinet.

if you remove the 2 screws on top of the capacitor, you should be able to lift up the
board enough to see the TIP-29 (TO-220) type transistor.

If you see any type of damage to the transistor case (crack however how small) it's pretty much a sure bet that the transistor (and likely the 723) is toast. some versions may or may not have a 2.4 ohm resistor in one of the 3 wires going to it. one end attached to the board the other end directly connected
to one of the 3 wires going to the tip-29.

I was asking if you see any voltage at all at the output terminals or how high it was


Forgive me i'm bored
« Last Edit: March 28, 2012, 05:10:57 PM by KD8DEY » Logged
NA4IT
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Posts: 893


WWW

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« Reply #5 on: March 28, 2012, 06:15:09 PM »

You might look at the Astron portion of http://www.repeater-builder.com/rbtip/. They have a lot of info (and some downloadable schematics, etc) on those power supplies.
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K8AXW
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Posts: 4001




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« Reply #6 on: March 28, 2012, 09:19:42 PM »

John:  When faced with a problem such as you have the answer is simple.  Divide and conquer! 

You can break down the Astron power supply into groups.  There is the first group which consists of the transformer primary wiring....the transformer itself.....the rectifiers.... the large filter capacitor.....the circuit board....the pass transistors and then the output area consisting of the crowbar circuit and the 2200uFD cap across the output terminals.

Start in the middle by removing the wire from the 70K uFD capacitor feeding the pass transistors.  This will leave the transformer, bridge rectifier (The black block with 4 terminals on it) the 75K uFD capacitor, the PCB rectifiers and the PCB in the circuit

Power it up and see if it blows the fuse.  If it doesn't, then you know the problem is the pass transistors (very likely suspect) the regulator (LM723 which is usually the main suspect) or a defective crowbar circuit, which is unlikely.

None of these devices will show any defect.  They must either be replaced or tested.

Assuming now that the fuse doesn't blow, then the first thing I'd do is replace the LM723 which can be obtained at Radio Shack.  If after connecting the pass transistor line and the fuse blows again with the new chip in place, then the main suspect, the pass transistors should be either checked or replaced.  I think you will have found your problem in this example.

Now, if the the pass transistor line is removed and you power it up and the fuse blows, then I would suspect the big black block rectifier with the 4 terminals on it first and the 75K uFD cap second.  It's possible the transformer could be shorted but highly unlikely.

This can be checked by removing the output of the block rectifier from the + terminal of the 75K uFD cap.

Try this and then come back with the results.  Good luck!
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KD8DEY
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Posts: 352




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« Reply #7 on: March 28, 2012, 09:38:39 PM »

None of these devices will show any defect.  They must either be replaced or tested.

I beg to differ Smiley

http://i228.photobucket.com/albums/ee58/bubbathegimp/Radio/Crowbar.jpg

http://i228.photobucket.com/albums/ee58/bubbathegimp/Radio/Resistor.jpg

http://i228.photobucket.com/albums/ee58/bubbathegimp/Radio/fixed.jpg

Parts may not always show damage and need to be tested, BUT sometimes its just so obvious Smiley

PS
Not all Astrons use bridge rectifiers. There are variations depending on year of manufacture and parts availability. (The one from the photo (older RS-35A) used a pair of stud mounted rectifiers) newer ones may have a bridge Smiley
« Last Edit: March 29, 2012, 12:14:06 AM by KD8DEY » Logged
K3GHH
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Posts: 42




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« Reply #8 on: March 29, 2012, 01:48:30 AM »

John:  When faced with a problem such as you have the answer is simple.  Divide and conquer! 

You can break down the Astron power supply into groups.  There is the first group which consists of the transformer primary wiring....the transformer itself.....the rectifiers.... the large filter capacitor.....the circuit board....the pass transistors and then the output area consisting of the crowbar circuit and the 2200uFD cap across the output terminals.

...

Try this and then come back with the results.  Good luck!


Thanks! This looks very helpful. I did a lot of building as a kid, but that was tube stuff and I've never even worked with transistors, much less ICs. Compared to circuits and components of the 1950s this is all quite mysterious.

I downloaded and printed a schematic from the repeater site (my original is rather blurry) and will slowly follow K8AXW's approach, keeping in mind DEY's caveats, etc.

In the meantime, I've fired up my old Century 21 (built-in power supply), found an antenna tuner that doesn't need 12v, and an RCA cord for my bug (no 12v needed for that), so I'm still on the air. For several years in the late 1980s that C21 was my main rig.
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KA1MDA
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Posts: 543




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« Reply #9 on: March 29, 2012, 04:01:35 AM »

Most of the linear power supplies I have repaired which blew fuses on power up were all caused by the same failure-  a shorted rectifier diode. Every time the supply is turned on, the rectifier diodes take the biggest stress as the discharged filter caps draw a huge inrush current as they charge up.

The best way to diagnose is the divide and conquer method. Disconnect the leads from the DC terminals of the rectifier, and try switching the supply on. If the fuse still blows, disconnect the transformer leads from the rectifier and try again. If the fuse does not blow, it's the rectifier that's bad. If the fuse still blows, the problem is in the transformer.

If the fuse didn't blow with the DC leads disconnected from the rectifier, reconnect them, then disconnect the leads going from the filter cap to the pass transistors, leaving just the rectifier and filter cap in circuit.  If the fuse blows, you have a shorted filter cap. If the fuse doesn't blow, check the pass transistors.

As I mentioned in the first paragraph, I've never had to troubleshoot beyond the rectifier- it was a shorted diode every time. I usually replace them with a bridge rectifier rated for 40A @ 800V. Sure the ratings are overkill, but it only costs a few bucks more than a much lower rated unit, and guarantees it will never fail again!

Tom, KA1MDA
www.ka1mda.org
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K8AXW
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Posts: 4001




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« Reply #10 on: March 29, 2012, 08:58:36 AM »

DEY:

I stand corrected!  I made a blanket statement when I should have been more specific.  If you have a bad component on the PCB, it MIGHT be visibly bad.... like a burned resistor.

What I was referring to was the bridge rectifier, input filter cap, voltage regulator IC and pass transistors. 

As for
Quote
PS
Not all Astrons use bridge rectifiers. There are variations depending on year of manufacture and parts availability. (The one from the photo (older RS-35A) used a pair of stud mounted rectifiers) newer ones may have a bridge
I stand corrected again.  Although I'm familiar with many of the Astrons, I will admit that I haven't seen them all or all of their variations.  Actually, on the schematic I have of the RS-35M it indeed does use stud rectifiers.  Although I looked at this last night I really didn't see my mistake.

It's been my experience with Astrons that the order of failure is: 1 - The LM723 regulator chip  2 - The pass transistor(s)  3 - The rectifiers  4 - The input filter capacitor.

I've never experienced a failure of the PCB as shown in the photos provided by DEY nor have I experienced a failure of the crowbar circuit. 

One more thing GHH:  Some of the Astrons had the LM723 chip soldered onto the PCB.  Some use sockets for the chip.  If yours has the soldered in chip and you find you need to remove it, I suggest you install a socket. 

As for troubleshooting these power supplies, I like to start in the middle, not the beginning.  I find this save a great deal of time.


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K3GHH
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Posts: 42




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« Reply #11 on: March 29, 2012, 05:01:29 PM »

Progress report: I have not yet followed ALL of the steps suggested by AXW and MDA... and the fuses are still blowing. (1) I disconnected the red wire from the rectifiers to the positive terminal of the large blue 64KuF capacitor (one schematic shows 70KuF and another 64; mine's the 64); this wire appears to take the rectifiers' DC output to the regulator board and the rest of the power supply. [Fuse still blows.] (2) I also disconnected the PC board from the 64KuF's negative terminal, and disconnected the black wire that runs from that terminal to the supply's negative output lug. [Fuse still blows.]

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.

Next step looks as if it will involve the soldering gun. I don't know if this photo will come through, but I'll give it a try. The PC board is obviously unscrewed from the top of the 64KuF capacitor. The red lead from the rectifiers comes down to the bottom of the photo; the grey wire soldered to the negative part of the PC board (which mounts on the 64K cap's negative terminal) is from the transformer's center tap.

PS: I found a little old hamfest-special 6A regulated power supply that puts out 13.2v. It's enough to run the Orion, etc. though obviously not with much power on transmit. Better than that Century 21.

http://i44.tinypic.com/vfhyjb.jpg
« Last Edit: March 29, 2012, 05:25:34 PM by K3GHH » Logged
KG6AF
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Posts: 373




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« Reply #12 on: March 29, 2012, 05:30:12 PM »

If you want to get a better idea of what's going on in the circuit you're debugging, there's a good article in the December, 2005 QST's Hands-On Radio column, which takes you through the Astron RS-35 power supply block by block.  It's not exactly the same as your supply (there are no voltage/current adjustment knobs on the front panel, for one thing), but I bet it's substantially the same.  The article explains crowbar circuits, pass transistors, and other things of interest to anyone trying to get a linear, series-regulated power supply working.

The article is free to any ARRL member, and can be downloaded from their web site.
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KD8DEY
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Posts: 352




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« Reply #13 on: March 29, 2012, 05:36:23 PM »

sounds like everything is pretty much disconnected except the transformer, rectifiers and the output transistors........
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KE4DRN
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Posts: 3734




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« Reply #14 on: March 29, 2012, 07:02:24 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
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