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Posted: 10/21/2014 6:25:45 PM EDT
Hello, I work at a school, and am trying to help the principal diagnose our school's radio system.
The Astron RS-20A power supply is blowing the 5 AMP fuse as soon as the power switch is flipped, and for the last few weeks the base made a horrible buzzing sound anytime it was keyed up. It received fine and the handhelds work fine, but the base buzzed like it was shorting out. Anyone have any suggestions? I found a thread in the archives with schematics, but that is all voodoo to me. I got the cover off, and can see no obvious signs of a short or a blown transistor etc. I am a gun guy not a radio guy, so can any answers be a little less technical than the schematics I found online? 
Thank You, BigHunt |
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Posted: 10/21/2014 6:40:49 PM EDT
[#1]
There is an M.O.V. (spike protection device) on the transformer. It is hidden under a few layers of the wrapping on the transformer winding. It may be shorted. Mine shorted out a few years ago and I replaced it with one from radio Shack.
Easy 10 minute job if you have a soldering iron. ETA - pic. Unit looks like this.. 
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Posted: 10/21/2014 6:46:33 PM EDT
[#2]
Will it hurt to remove the black tar paper wrapping to get to it?
ETA: Oh, I can see that from the outside. Just replace it? Is there a way to test it to see if its bad? We have a cheap multi-meter. |
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Posted: 10/21/2014 7:20:03 PM EDT
[#3]
You can check it for continuity. It should have very high resistance. The "quick and dirty" way is to remove it, replace the fuse (with one of original value) and power it up. If it blows the fuse, then further trouble shooting is required.
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Posted: 10/21/2014 8:02:31 PM EDT
[#4]
Open it up and remove the Homotrons
Tighten down the screws to the big cap under the board, they can work loose and you loose the filtering from the cap. |
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Posted: 10/21/2014 9:05:08 PM EDT
[#5]
Snip one side of the M.O.V. lead, in such a way that it could be soldered together again if not bad.
If it stays up, MOV is bad. ETA, usually a storm or spike is what kills them, never head a 'slow death' on one. |
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Posted: 10/21/2014 9:25:07 PM EDT
[#6]
Quote HistoryQuoted:
You can check it for continuity. It should have very high resistance. The "quick and dirty" way is to remove it, replace the fuse (with one of original value) and power it up. If it blows the fuse, then further trouble shooting is required. But first disconnect the load... that is, the base rig should be disconnected from the power supply. If it blows the fuse with no load, then the problem is in the power supply. I would replace the MOV anyway. Just because. Now turn it on with a fresh fuse and no load. All OK? Then the problem is in the base rig. |
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Posted: 10/21/2014 9:33:16 PM EDT
[#7]
Check the bridge rectifier. Mine went out a few weeks ago.
I think the MOV is to protect the rectifier from transients when the power supply is turned on or off. Also, if you hear a "bump" when you turn the PS on, check the tightness of the bolts holding the transformer together, and to the chassis. They tend to work loose. |
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Posted: 10/21/2014 9:43:50 PM EDT
[#8]
Quote HistoryQuoted:
Open it up and remove the Homotrons
Tighten down the screws to the big cap under the board, they can work loose and you loose the filtering from the cap. I assume homotrons are some kind of joke? 
The screws on the big blue capacitor were super tight no that's not the problem. I'm gonna try clipping the mov in the morning. I hope that fixes it! Thanks for the suggestions guys. Arfcom knoweth of all things! |
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Posted: 10/21/2014 11:41:27 PM EDT
[#9]
Yes,the MOV (metal oxide varistor) is a sacrificial component. A triggering event (voltage spike) causes it to go short, blowing the fuse and protecting the equipment. A very cheap and effective surge protection device but they are one shot and check out sort of like that old boy that hooked up with Anna Nicole Smith.. It is going to be your prime suspect, but beyond that just about any shorted component on the secondary side can blow the fuse too.
(I hear,if you want to avoid a repeat occurrence of something like a lightning strike taking it out, you can hook it up to a battery...) 
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Posted: 10/22/2014 9:21:34 AM EDT
[#10]
I snipped the M.O.V. it still blew the fuse.
I guess I will just get us a new one on the way. |
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Posted: 10/22/2014 10:17:33 AM EDT
[#11]
Samlex SEC-1223
A no-hernia solution to your problem. |
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Posted: 10/22/2014 12:19:02 PM EDT
[#12]
Quote HistoryQuoted:
I snipped the M.O.V. it still blew the fuse.
I guess I will just get us a new one on the way. Bummer. My RS 20A got spiked a year or two ago and replacing the MOV fixed it. The Astron linear supplies are normally bullet proof and I have been using them for well over 20 years. |
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Posted: 10/22/2014 11:04:20 PM EDT
[#13]
Quote HistoryQuoted:
[The Astron linear supplies are normally bullet proof and I have been using them for well over 20 years. I will vouch for that. I have a RS-50M that was powered on for decades. The meters are actually STUCK at the position they were in while operating, but the supply works great. |
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Posted: 10/23/2014 8:57:18 AM EDT
[#14]
If you do decide to replace the power supply, you should keep the old one to fix in your spare time. Those linear power supplies are really not complicated. Astron provided service help in the past and still might so you might contact them. There is an RS-20A schematic at http://www.repeater-builder.com/astron/pix/astron-rs20-1988-09-24.jpg. Should be relatively simple to diagnose from there.
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Posted: 10/23/2014 11:24:32 AM EDT
[#15]
Quote HistoryQuoted:
I snipped the M.O.V. it still blew the fuse.
I guess I will just get us a new one on the way. Check the bridge rectifier. It's used in a conventional full wave circuit, so lifting one of the transformer leads will do it. |
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Posted: 10/24/2014 12:04:32 AM EDT
[#16]
The Astron RS35 I was given was blowing the AC fuse. Turned out to be one of the full wave rectifiers was shorted. That power supply used two diodes with a center tap secondary. You would need to lift both "hot" leads to see if the diodes are bad. Even if you buy two diodes. the LM723 voltage regulator and the op amp output chip (I can't recall the chip number). You shouldn't have that much in the repair.
RS |
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Posted: 10/28/2014 11:12:35 AM EDT
[#17]
Got a new power supply. Hooked it up and everything works fine and the buzz is gone.
One of the custodians snagged the old one. I didn't need it anyway. I have too much junk the way it is. |
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