Question Details	Asked By	Asked On
Power supply	jocase	03/18/04
I have two radios connected to a ASTRO power supply. The power supply no longer works.(Fuse blows whenever I turn the power supply on). There is a white substance around the large transistor on the back panel. The radios haven't been operated in about 2 or 3 years. Is it ok to run this power supply without a load to test it with a VOM? I would like to get back on the air as my General Class License is still good. Thanks!
Clarification/Follow-up by jocase on 03/18/04 8:11 am: Yes, it is a 12 volt supply. The white substance is chalky, similar to what you sometimes see as a battery is bad and left in its case for a long time. I will disconnect the power supply and see if it is putting out. Thanks! John Clarification/Follow-up by jocase on 03/18/04 1:30 pm: Yes, it is the TO-3 case. (Some of this stuff is actually coming back to me!) "Also, is the fuse that is blowing one that protects the incoming AC side of the power supply, or one on the 12-volt output?" It is a 5 amp fuse labeled as such on the back and positioned directly above the AC cable. It is the only fuse on this Astron RS 20A supply. Polarity is right as radios worked for years and then quit. The low band transceiver is a Kenwood RS 440 S. It can be used in a car, but I have never done that. The other radio is an Alinco DR 570 2m and 70 centimeters rcv. I will check the TO-3 case with an ohmmeter first as that is the easiest to do. Let's see now, where are some batteries for my ohmmeter? John Clarification/Follow-up by voiceguy2000 on 03/18/04 2:09 pm: I looked at the Astron web site to see whether they published tech manuals for their units, but I did not see any. They are located here in Southern California, and you might want to give them a buzz tomorrow to get some troubleshooting ideas before you get too deep into the project. I had been assuming that the TO-3 device was a series-regulating transistor, but it occurred to me that it might actually be a three-terminal regulator packaged in the TO-3 form. You might look and see whether you see a familiar 2NXXX designation on the transistor. If I was troubleshooting the device, I would be starting at the transformer, disconnecting things, and trying to isolate the location of the problem. I am assuming that this is a relatively simple device.
Answered By		Answered On
voiceguy2000		03/18/04
The white substance is almost certainly silicone grease, designed to improve the conduction of heat from th transistor case to the chassis. I am assuming that this is a relatively low-voltage supply (i.e., 12 volts). Chances are you can operate it with no load for troubleshooting purposes. As to why the fuse is blowing, external wiring is the first thing to suspect. Other than that, the next most likely candidate with devices that have not been used for a while is large electrolytic capacitors used to filter the DC. You may have to replace one or more of those if they have deteriorated from non-use (they lose their polarization, which leads to breakdown of the electrolytic effect and causes them to conduct DC when they are not supposed to).

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