Batboard

I have a RS-35A power supply that is driving me insane!!! I bought it off a [clears throat] member[/clears throat] here and it hasn't woked since I got it. I've replaced the 2n3771's, the TIP29, and the 723IC, and now I get 14.9vdc @ the terminals with a meter, but under even the slightest load (backlight for a switch panel that is known good), it drops to 5-7vdc. WTF???????

Rick,

Common on the ASSTRON PS. There is some information and a road map at http://www.repeater-builders.com look under Astron.

Skip, (repeater builders Yahoo group) has a replacement circuit board that eliminates the problem.

Did you check the emitter resistors in for the 2N3771's?

Nand.

Will wrote:Rick,

Common on the ASTRON PS. There is some information and a road map at http://www.repeater-builders.com look under Astron.

Skip, (repeater builders Yahoo group) has a replacement circuit board that eliminates the problem.

Well, Will, apparently Skip doesn't want to do business with me- I went to the site the day you posted it, found his email after a bit of reading, emailed him about the board, and have yet to hear a peep from him.

Nand, I have no idea what you're talking about, so if you can help me figure out what exactly you're speaking of, it would help alot.

The Astron power supplies that I known of have a ceramic power resistor (10 or 15 Watt) directly wired to the emitter of each pass transistors (2N3771). They are about an inch long and ¼ inch square. The resistance is about 0.05 Ohms or some other very low value. You can’t miss them; they are directly connected to the final pass transistors on the back of the supply. These resistors will burn open if the pass transistors shorted out for some reason and cause the problem that you seem to have.

In most cases when the pass transistors short out because of high current these resistors will burn open. There purpose is to allow better current sharing between the pass transistors.

Nand.

OK, yes I have 4 of them, now if I can figure out how to test them...

There is no need to test these. When the 2N3771 transistors failed, these resistors failed as well. It is the most common failure with these types of power supplies.
But if you like, use the Ohm meter section of the tester you used for measuring voltage. These resistors should measure like a short if they are ok and open if not.
If you don’t have an Ohm meter, remove the resistors and place them across a battery in series with a light bulb. If the bulb lights, the resistors are ok.

Nand.

HI!
I don't know this PS but know other Astron PS...
Check the driver voltage over the TIP29 colector (this transistor have a separate voltage supply). If the voltage drops, u have a bad eletrolic cap. The over current circuit protection also.
Goof luck!
Antonio - PY5FT