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
I have had old transistors in TO-3 cases (that is what this sounds like) develop various kinds of chalky oxides on the outside, especially if they are stored in less-than-ideal environments. It probably means nothing. 12-volt supplies are a pretty simple proposition. Any one should do, so long as it is capable of handling the current demands of your transceiver units. Also, because the transceivers are probably designed for automotive use as well, they probably have internal voltage regulation sufficient to handle the swings in voltage that would be experienced in an automobile. It may be that one or the other radio has an internal problem that is causing excessive current drain on the supply. You might see whether they both work when connected to an automotive battery. If one or the other radio also blows the fuse feeding your cigarette lighter outlet, for example, that would indicate that something needs to be looked at in those radios. One other thing to double-check is to be absolutely sure that the polarity of the power cables is correct. A lot of portable devices protect against reverse polarity by placing a shunt diode at the power input which will conduct if the incoming polarity is reversed. This is supposed to blow a fuse in the portable device, but maybe it is blowing the fuse in your power supply first. 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? The power transistor on the back of the power supply is almost certainly being used as a series regulator for the 12-volt output. In most cases, when those transistors fail, the circuit opens. That would not cause your fuse to blow. The outer case of the power transistor is connected to one of the transistor elements, and must be insulated from the chassis of the power supply. Make sure nothing is causing a short circuit to the chassis. Good luck.

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