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eHam Forums => Misc => Topic started by: K3GHH on March 28, 2012, 03:49:25 PM



Title: Astron VS-35M blowing its fuse
Post by: K3GHH 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


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY 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?.......


Title: RE: Astron VS-35M blowing its fuse
Post by: W8JX on March 28, 2012, 04:19:15 PM
I tend to think that there is a failure of regulator and crowbar is blowing fuse.


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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. 


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY 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


Title: RE: Astron VS-35M blowing its fuse
Post by: NA4IT on March 28, 2012, 06:15:09 PM
You might look at the Astron portion of http://www.repeater-builder.com/rbtip/ (http://www.repeater-builder.com/rbtip/). They have a lot of info (and some downloadable schematics, etc) on those power supplies.


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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!


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY 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 :)

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 :)

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 :)


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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.


Title: RE: Astron VS-35M blowing its fuse
Post by: KA1MDA 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


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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.




Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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


Title: RE: Astron VS-35M blowing its fuse
Post by: KG6AF 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.


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY on March 29, 2012, 05:36:23 PM
sounds like everything is pretty much disconnected except the transformer, rectifiers and the output transistors........


Title: RE: Astron VS-35M blowing its fuse
Post by: KE4DRN 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


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY 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....:)

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

Wouldn't hurt to try it and eliminate the possibility regardless.....


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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.


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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.



Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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!   ;D


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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!   ;D

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.  :)

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.


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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?   ;)

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.


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY 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......


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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!  :)

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.


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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?



Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH 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!



Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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


Title: RE: Astron VS-35M blowing its fuse
Post by: KH6AQ on April 03, 2012, 05:34:59 AM
The problem is a shorted rectifier diode.


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW 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. 


Title: RE: Astron VS-35M blowing its fuse
Post by: KH6AQ on April 03, 2012, 11:44:08 AM
Just one.


Title: RE: Astron VS-35M blowing its fuse
Post by: KD8DEY 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?


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH on April 03, 2012, 02:40:11 PM
Since I didn't know how to test the 2502's, I replaced the pair. As I think about it, though, my replacements are 5010s, and the reason Astron used two 25-amp rectifiers was to wire them in parallel for the extra current capacity... so I bet I could have replaced the two 2502's with ONE of the 5010's.

But I'm obviously a novice at working with solid-state components, and just wanted to get my PS working quickly, so I wasn't inclined to experiment.


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW on April 03, 2012, 08:21:09 PM
GHH:  You made the correct move.  Whenever you replace one of two identical components with a different model number you run the risk of failure.  In other words, by replacing the parallel diodes with the one high value diode, it's a good move to do the same with the other pair.

I feel as you do.... let's just get the darn thing fixed!  Good job!


Title: RE: Astron VS-35M blowing its fuse
Post by: KB5ZSM on April 03, 2012, 10:36:59 PM
If I understand you right, you still have the rectifier in circuit. Do you have an Ohm Meter? Why hasn't anyone suggested using an Ohm meter? Disconnect one of the A.C. leads from the transformer to the rectifier and then check the rectifier for shorts (I assume the red and black lead on the rectifier are still disconnected). The bridge rectifier is just four diodes in one package with four leads (+,-,AC,AC). No pair of leads may show a short circuit. Every pair of leads you test should show a diode action where the leads in one direction will show infinity and in the other direction a much lower Resistance (at least a ten-to-one ratio if not a lot more). Any shorts and you found your culprit.
I also like the idea of checking the MOV that was mentioned earlier. That was the disk looking thing on the primary side of the transformer that you mentioned also.
If the pass transistors were shorted, they would cause your output voltage to go way up causing the crowbar circuit to engage but since you disconnected that part of the circuit, you already eliminated that as a probable cause.
Good luck!

Win (KB5ZSM)
Tip Top TV


Title: RE: Astron VS-35M blowing its fuse
Post by: K3GHH on April 04, 2012, 01:59:26 AM
If I understand you right, you still have the rectifier in circuit. Do you have an Ohm Meter? Why hasn't anyone suggested using an Ohm meter?
.
.
.
Good luck!

Win (KB5ZSM)
Tip Top TV

Win, the power supply has been repaired and is working fine. The original KBPC2502 bridge rectifiers are sitting on my desk; I replaced them with two KBPC5010's from a recent hamfest. K8AXW (and probably KD8DEY) explained how to test the originals with an ohmmeter, and I'll do that just for curiosity. But this little adventure is OVER!

--John K3GHH

Edit: Win, I should add that I very much appreciate your willingness to share your expertise (expertise I obviously do not have!) and the time and thought you devoted to your advice. The fact that the PS is now working doesn't change that in the slightest!


Title: RE: Astron VS-35M blowing its fuse
Post by: N4EF on September 22, 2012, 05:42:18 AM
I had the same trouble with my RS-35A blowing fuses after a lightning strike.  I found the technical hand-holding very helpful and the comments from experienced electronic techs were awesome. Thank you.

My Astron RS-35A is now working again: one of the 2N3055 pass transistors was bad. For the sake of clarity for those (like me) who haven't dissected their Astrons before, that the 4 pass transistors (on the back of the power supply, within the heat sink) can be replaced in 4 minutes without soldering as they unscrew and can be pried out. The new transistors plug in only one way. Easy.

Dave
n 4 e f


Title: RE: Astron VS-35M blowing its fuse
Post by: K8AXW on September 22, 2012, 09:07:36 AM
ZSM:  Win, you're obviously correct.  It would have been a simple matter of checking the diodes with an ohmmeter..... for you, me and many others faced with the same problem.  However, with GHHs lack of expertise in solid state electronics, I felt it was easier to suggest what components could be bad and just replace them.  Assuming that he did properly test the rectifiers and found one or all bad, he was going to have to find replacements anyhow.

As it was, it worked out.  No doubt, after experimenting with the defunct diode(s) he'll learn and know the next time. 

Your good advice will no doubt be noted and remembered by other readers of this forum.