I have two Astron SS-25M and two SS-30 switch mode power supplies (SMPS). A couple of years ago one of my SS-30 supplies quit with no voltage out during Field Day. Later one of my SS-25M supplies had the output voltage drop to 6 - 8 volts. About a year ago I took a look at the SS-25M to see if I could figure out what was wrong. After a little while with no success I gave up and put it away.
Last fall I decided to take another look. I researched SMPS's on the internet and got a bit better understanding of how they work. I read and reread many articles. I started with the SS-25M. I picked up an isolation transformer off of eBay so I could work on the high voltage side safely and use my oscilloscope.
I took voltage readings of the TL494CN controller IC and found I had Vcc supply voltage. I checked the output of the IC with my oscilloscope and saw I had a nice square wave. There was very low feedback voltage on the IC. I took voltage readings at various points in the supply but couldn't figure out how the feedback circuit worked. I wound up replacing a number of components that tested good thinking maybe they weren't working at operating frequency/voltage but that didn't solve the problem. I started checking each diode, transistor, resistor, and capacitor looking for a short or out of spec component. I did find D5 had a high resistance reading rather than infinity in the reverse direction. I replaced it but that didn't solve the problem. I couldn't find whatever was causing the problem.
Finally it occurred to me take my good SS-25M and select a number of points to take voltages. Those points are shown by the circled numbers on the attached schematic. I took voltages with it operating, with 13.8 volts applied to the input with another supply, and operating but with the TL494CN IC removed. Those voltages are shown on page 2 of the attached schematic. In my research I read applying voltage to the output of the defective supply would help in determining if the feedback circuit was operating properly. Of course, the defective supply must not be powered.
I then took the same readings on my defective SS-25. The voltages with it operating weren't too far out of line with the good supply, considering the output was about half what it should be. With 13.8 volts applied to the output the voltage at test points 6 & 7 stood out with both points reading about 1.5 volts vs the good supply having a voltage of 4.75 volts. Those points are at the cathode of D17 and D18. So I pulled out D17, D18, Q3, and Q4. Testing those components I found one of the diodes had low resistance both directions, basically a short. I didn't know if it was D17 or D18 since I took them out at the same time and didn't keep up with which was which.
I replaced the defective diode and reinstalled the other components and powered up the supply. Success! The output voltage was now 13.8 volts! One down one to go!
On the SS-30 I had previously connected 13.8 volts to the output and found the 13.8 volts dropped to zero. Checking the output terminals with my DVM I found the resistance was zero. (Luckily the supply I was using to provide the 13.8 volts had short circuit protection.) Testing the output rectifiers showed D7 was shorted. When I replaced D7 I still had no output voltage. Checking Vcc on the TL494CN showed I had no startup voltage. I had just purchased on old EICO 1020 variable power supply at an estate sale so I set it to 15 volts and touched pin 12 (Vcc) on the TL494CN. That kicked off the controller IC but I only had 8 - 9 volts output. Hmmm, similar to the SS-25 problem.
With my success repairing the SS-25 I went straight to connecting 13.8 volts to the SS-30 output and checking the voltages. Like the SS-25 I had low voltage at test points 6 and 7 though not as low as on the SS-25. Rather than 1.5 volts I had close to 2 volts vs the 4.75 volts on the working supply.
I first removed D17. It had low resistance in both directions. I replaced D17 and fired up the supply. Success! Again! The output voltage was 13.8 volts!
It's been educational trying to repair these supplies. Plus it gave me an excuse to obtain some test equipment I didn't have. It's very important to use an isolation transformer when working on these supplies since the high voltage side is not isolated from the 120v mains. Also when taking voltage measurements be sure to use the correct ground connection with your meter. When measuring the high voltage side use the ground side of C7 shown as RTN1 on the schematic. For the low voltage side you can use output ground connection. IF YOU'RE UNSURE DON'T DO IT!
In addition to replacing the defective components I also replaced the output capacitors in both supplies. I discovered the only difference between the SS-25 and SS-30 is the SS-30 has one additional output capacitor and inductor. The SS-30 also has a larger fan. All other components are identical.
I started out using the schematic found at http://www.repeater-builder.com/astron/astron-index.html. As I worked on the supplies I made note of the component values and added them to the schematic. I also noticed there are several discrepancies between the schematic and the as-built supplies. For example, the schematic shows a D23 and D24 that aren't actually present. There are also a number of jumpers shown on the schematic that aren't present. I have modified the schematic to account for these discrepancies. I'm sure there are others I didn't discover.
My SS-25 and SS-30 are older supplies. I think Astron came out with updated versions a few years ago. These have a hole on the side of the case to adjust the output voltage. I think these may have surface mount components.
Below are PDF files of my modified schematic and scans of my test voltage worksheet with each supply. If you're trying to repair your Astron SMPS best of luck! I hope this info helps!
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