Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: Linear Power Supply  (Read 6121 times)
KB1GTX
Member

Posts: 459




Ignore
« Reply #15 on: July 03, 2012, 08:16:42 AM »

I have repaired a lot of Astron power supplies and the worst thing I find on them is that the thermal compound is dried out. If you want one to last long, you need to clean off and reapply the thermal compound every few years. If you don't they really aren't anywhere near their stated rating.


Very,true, I've found dry thermal compound on almost every thing.
Logged
K7MH
Member

Posts: 339




Ignore
« Reply #16 on: July 04, 2012, 12:06:24 AM »

Do you really need a 70 amp power supply?
You won't be moving it around a lot!

Yeah, but deadlifting one is a good way to save on a gym membership. Smiley


Or spend more for a physical therapist!! Grin
Logged
WB6DGN
Member

Posts: 619




Ignore
« Reply #17 on: July 04, 2012, 01:31:22 AM »

Quote
That may be true, but you could sure build a much better one than an Astron. Take a good look at the issues with Astron supplies at:

If you want my opinion, for what its worth, I take most of that article with a grain of salt.  Especially the part about the pass transistors.  The 2N3771s are virtually an ideal match for that series of power supplies.  They are rated at 30 amps and 150 watts maximum ratings under "ideal" conditions.  Astron uses 4 or 8 of them in their 35 through 70 amp supplies.  Take the 35 amp supply, for example.  It uses four 2N3771 transistors in parallel for a theoretical maximum current of 140 amps (600 watts for the practical limit).  Take fifty percent of that to compensate for less than ideal heat sinking and heat transfer to the sink, gain imbalances, and therefore current imbalances, in the individual transistors and ripple and other derating factors and you still have the capability to handle 70 amps, more than TWICE the rated continuous current of that supply (25 amps).  How much headroom do you need, for cryin' out loud?  And, besides, they have the additional virtue of being relatively inexpensive to boot.  Compare their price to the 2N5686 (or whatever it is) that's recommended in the article.  NO THANKS, very much!  Same argument for most of the other "recommendations" in the article.  Gross overengineering at best and a large expense and lots of unnecessary work  on average.  Its mighty easy to make all those expensive recommendations when you don't have to pay the bill.  By the way, one point that I DO agree with; replace that old silicone based compound under those pass transistors and if you don't want to do that every few years, use some of the newer NON-silicone based compound when you do.  From memory, I think its Wakefield #136 instead of the old #135.  They still sell the old stuff so grab a catalog and compare.  And, Astron certainly isn't the only one that used the silicone compound, either.
A couple of other recommendations for the 35 supplies that won't cost an arm and a leg to implement;  (1) replace those two half used bridge rectifiers with the 1N1184s (or equiv.) that they use in the bigger supplies (one each leg) and (2)  move that crowbar SCR off of the PWB and onto an insulated pad on the chassis (again, copying the larger supplies).  A quick look at the schematic will make it clear why.  Hint: what's going to happen to those fairly small traces on the regulator board if a pass transistor shorts?  Even if the primary fuse blows, that 64k filter capacitor stores a lot of energy that will discharge through those traces.  Overall, though, I have a great deal of confidence in the Astron linear supplies and they have rewarded me with very reliable service for over twenty years in return; and, NO, I have NO business nor financial connection to Astron or any dealer.
Tom
Logged
W0BTU
Member

Posts: 1685


WWW

Ignore
« Reply #18 on: July 04, 2012, 08:52:24 AM »

Quote
That may be true, but you could sure build a much better one than an Astron. Take a good look at the issues with Astron supplies at:

If you want my opinion, for what its worth, I take most of that article with a grain of salt.  Especially the part about the pass transistors. ... How much headroom do you need, for cryin' out loud? ...

Maybe the transistors are adequate, but what about the other components? I've read so many experiences of Astron 20 amp PS owners that I'm convinced the problems are real, and that the 20 amp rating is for very short peaks, and not a continuous rating. And I have a recent experience of my own to share.

A couple of weeks ago I removed an IC-751A power cable from my RS-35M and noticed another issue: turning the output studs on the back only a small fraction of a turn can short the output! Reason being, there's some crimp-on terminals inside on those studs (bolts, actually, with the heads inside) for the purpose of connecting two bypass capacitors in parallel with the output. And there was no way of bending those terminals so that this wouldn't occur without ripping the short leads out of those capacitors.

When I opened it up, there was this dead short staring me in the face because I rotated the bolts slightly which in turn brought those + and - terminals in contact with each other.

If the previous owner had not wired part of the cable internally --so that I had to remove the cover-- I'd have never seen that. Maybe that supply can take a dead short at power-on with no damage, but I wouldn't bet on it.

I suppose it's possible that those capacitors were added after it left the factory, but it sure didn't look like it.
Logged

AC5UP
Member

Posts: 3875




Ignore
« Reply #19 on: July 04, 2012, 10:03:48 AM »

Maybe that supply can take a dead short at power-on with no damage, but I wouldn't bet on it.

A well designed LM-723 based DC regulator circuit is almost bulletproof and will shut down gracefully when the output is shorted. The reset procedure is to turn the supply off long enough for the filter cap(s) to bleed down to a volt or three then clear the fault and turn the supply back on.

Note that the operative concept here involves the words "well designed". Compare the schizmatic of an Astron against the National Semi spec sheet and you will see some variation, possibly not critical, as the chip is tweakable for an application. Personally, if I need to "borrow" an LM-723 design I'd look first for HP, Lambda or PowerOne schizmatics.

Yeah, I'm a bit of a snob when it comes to regulator design...........  Grin

BTW: Ain't a damn thing wrong with a string of 2N3055 or 2N3771 pass transistors on a decent heatsink with good air circulation. And, if you dig a little deeper, you'll find that some of the newer high-perf TO-3 power trannies have a higher Beta value as well... Enough so that self oscillation can become an issue. I think of a pass transistor as a disposable repair part so cheap & proven is usually a good characteristic to look for.
Logged

Never change a password on a Friday                
K8AXW
Member

Posts: 3858




Ignore
« Reply #20 on: July 04, 2012, 10:34:11 AM »

Quote
If you want my opinion, for what its worth, I take most of that article with a grain of salt

DGN:  Agree!  I have a problem with getting into a piece of gear if it isn't broke!  Most will provide many years of average use without a problem.  However, for heavy duty work like a repeater, some things might be worth changing in the Astrons.

I personally feel that several items are borderline adequate.  Specifically he main filter cap, the bridge rectifier and the pass transistors in that order.

Whenever I have to replace any one of these items I go with one that has more headroom in capacitance, voltage and current handling.

Logged
W6EM
Member

Posts: 800




Ignore
« Reply #21 on: July 04, 2012, 06:15:00 PM »

I have repaired a lot of Astron power supplies and the worst thing I find on them is that the thermal compound is dried out. If you want one to last long, you need to clean off and reapply the thermal compound every few years. If you don't they really aren't anywhere near their stated rating.


Very,true, I've found dry thermal compound on almost every thing.

Some of the early white heat sink compounds contained berrylium.  Bad stuff.  Do not get it on you or your hands.  Wipe off and replace with zinc-bearing heatsink compound.
Logged
WB6DGN
Member

Posts: 619




Ignore
« Reply #22 on: July 04, 2012, 08:50:23 PM »

"If the previous owner had not wired part of the cable internally --so that I had to remove the cover-- I'd have never seen that. Maybe that supply can take a dead short at power-on with no damage, but I wouldn't bet on it."

Yes, it can take it with ease.  There's a foldback current limiter that limits short circuit current to 3 amps (determined by R3 and R3x on the control board).  Astron recommends that this value be checked if components that might affect it are replaced (section 5.3.4 of the service booklet).

"If the previous owner had not wired part of the cable internally..."

Uh Oh!  Could this (and who knows what else was done???) have something to do with your dissatisfaction with Astron supplies???

"I suppose it's possible that those capacitors were added after it left the factory, but it sure didn't look like it."

Those capacitors are factory installed and are intended to block RF entry on the DC power leads (hence the short leads, inductance is NOT your friend).  The moral here is to keep those locking nuts (ones nearest the back panel) as tight as reasonable consistent with the shoulder washers under them.  Upon reassembly, I use a bit of blue Loctite on the freshly cleaned studs.  This is a new one on me; never encountered this nor heard anyone comment on it but I have seen the nuts get loose from an "installer" NOT holding the terminals when tightening or loosening the nuts; a rather poor practice regardless the equipment.
Tom
Logged
WB6DGN
Member

Posts: 619




Ignore
« Reply #23 on: July 04, 2012, 09:13:46 PM »

Quote
I personally feel that several items are borderline adequate.  Specifically he main filter cap, the bridge rectifier and the pass transistors in that order.
Whenever I have to replace any one of these items I go with one that has more headroom in capacitance, voltage and current handling."

I "somewhat" agree with you on the main filter cap(s); 23 volts is closer to the 25 volt working voltage of the supplied caps. than I'd like to see.  However my only experience in having to replace any of those was from old age (as a precaution), not from failure.

The pair of bridge rectifiers, agree 100%, is a no-brainer.  Not only a poor choice of components but no balancing resistors either; whatever the engineer was drinking that day, I'd like some!

Pass transistors, see my previous comments.  Already gobs of headroom; don't need any more.  Do the math.
Tom
Logged
WB6DGN
Member

Posts: 619




Ignore
« Reply #24 on: July 04, 2012, 09:35:34 PM »

Quote
Some of the early white heat sink compounds contained berrylium.  Bad stuff.  Do not get it on you or your hands.  Wipe off and replace with zinc-bearing heatsink compound.

Agree with you 100%; no sense inviting disaster but, like most things, apparently not all people are affected the same way.  One of my college jobs was at a company making magnetrons for the fledgling microwave ovens to come.  Part of my job was to trim and fit a rather large number of berrylium "donuts" used as insulators into the base casting of the tube.  This was partially done by using a blade or a tiny file to clean up the insulators to fit in recesses in the base.  No breathing protection provided and rarely used the eye protection that was provided (This was before OSHA).  I'm 70 years old and can say with reasonable confidence that, at this point, no noticeable effects.  With what I've been reading recently, I'm more concerned about the effects of the vapors from the solder flux.  Apparently that's some pretty nasty stuff, too.  By the way, the silicone free compound that I mentioned in an earlier post uses zinc oxide as the heat transfer component; their earlier one used aluminum oxide so think berrylium must be a thing of the past.
Tom
Logged
W0BTU
Member

Posts: 1685


WWW

Ignore
« Reply #25 on: July 05, 2012, 07:19:06 AM »

Part of my job was to trim and fit a rather large number of berrylium "donuts" used as insulators into the base casting of the tube.  This was partially done by using a blade or a tiny file to clean up the insulators to fit in recesses in the base.  No breathing protection provided and rarely used the eye protection that was provided (This was before OSHA).  I'm 70 years old and can say with reasonable confidence that, at this point, no noticeable effects.

Were they white? That would have been beryllium oxide. Regardless, I think you're a lucky guy. I don't know about beryllium oxide insulators, but one of my old high school teachers use to tell us stories of people dying from working in a machine shop where beryllium metal was being turned in a lathe.


Quote
  With what I've been reading recently, I'm more concerned about the effects of the vapors from the solder flux.  Apparently that's some pretty nasty stuff, too.  By the way, the silicone free compound that I mentioned in an earlier post uses zinc oxide as the heat transfer component; their earlier one used aluminum oxide so think berrylium must be a thing of the past.
Tom

What did you read about solder flux vapors? I'm not saying that flux vapors are OK to inhale, but I would be more concerned about lead vapors from the solder itself. Here's why I say that.

For many years, I never worried about fume removal when using lead/tin solder. Then I visited the place that wave-soldered PC boards for a company I used to work for, and that all changed. It was pointed out to me that every six months, the 12" to 14" dia. stainless steel fume ducts leading away from the molten solder area had to be taken down to remove the thick, heavy coating of solder that had condensed inside. Wow!

Yes, I'm sure that a lot less solder evaporates from the tip of an iron and the molten solder it's melting. But from that point forward, I made use of fume extractors.
Logged

WB6DGN
Member

Posts: 619




Ignore
« Reply #26 on: July 05, 2012, 08:50:17 PM »

Quote
Were they white? That would have been beryllium oxide

Well, its been over fifty years but, as I recall, they were a light blue color, kinda pretty, actually.  But, then again, hydrogen cyanide gas is a pretty bluish color too!  Its effects are a little more immediate, though!!
Tom
   
Logged
WB6DGN
Member

Posts: 619




Ignore
« Reply #27 on: July 05, 2012, 09:09:54 PM »

Quote
What did you read about solder flux vapors? I'm not saying that flux vapors are OK to inhale, but I would be more concerned about lead vapors from the solder itself.

From the Multicore solder data sheet, " Inhalation of the flux fumes given off during soldering should be avoided.  The fumes are irritating to the throat and respiratory system.  Prolonged or repeated exposure to rosin or modified rosin based flux fumes may lead to the development of respiratory sensitisation and occupational asthma."....."Solder alloys containing lead give off negligible fume at normal soldering temperatures up to 500 (degrees) C."....."Normal handling of lead alloy wires will not cause lead to be absorbed through the skin."
Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!