Astron SS-25

[W6DOM]

Where can I get a schmetic for this supply?

[K0BG]

Apparently, you don't have the manual. I have to assume you can download it from their site. Whatever, but if it isn't working, the very first place to look (or replace) is the 723. I would venture to say, that 99.99% of the time, the chip is kaput!

Alan, KØBG
www.k0bg.com

[AA4PB]

The SS-25 is a switching supply and it doesn't have a 723 regulator IC. Astron is not forthcoming with the schematics to switching supplies nor repair parts for them. You **may** be able to find a schematic from some site with a google search but switching supplies are generally difficult to troubleshoot.

[AA4PB]

Here's a schematic of the Astron SS-30:

http://www.repeater-builder.com/astron/pdfs/astron-ss-30.pdf

[K0BG]

Good eye, Bob!

Alan, KØBG
www.k0bg.com

[KA5N]

Repairing switching power supplies is a formidable task.  Most of the diodes are fast or ultra fast switching types and if one tries to use standard silicon rectifiers, they will soon go up in smoke taking other parts with them.  Even with exact replacements, one must replace ALL bad parts before applying power to the unit or ZAP goes the same or more parts.  With a complete understanding of how the circuit operates and exact type replacements, switching supplies can be repaired.  Otherwise ship it back to the factory.
Allen

[W5DWH]

I have repaired hundreds in consumer electronics and other equipment.

I can offer the following advice.

It is not a problem to use replacement semiconductors from NTE. As was mentioned don't try a NTE 125        ( standard 1N4004 rectifier). You will need a fast switching diode such as a NTE 552, 580 etc.

In most switching power supplies when the main FET blows it takes out the bridge rectifier. Don't be cheap, replace all four diodes even if only two are bad.

After replacing the FET check the source resistor ( it is tied from source to ground). This resistor is typically a .22 Ohm or similiar. This resistor is a precision resistor usually 1 or 2 %. If you don't have the equipment to check it properly, then replace it. If it is out of tolerance the p.s. won't regulate properly.

The main trick in repairing these supplies is to troubleshoot them on a variac. Start out at 0VAC and bring it up slow. Watch your amp draw as well as the DC output voltage.
Most switchers require some kind of load to regulate properly. So I put a load on them as I test them.
For example,the p.s. in a VCR typically needs a 27 Ohm, 10W resistor across the 5V source.
I use a 60W light bulb in a TV. It would be placed across the regulated B+.
What you are trying to do is to put a load on the source that is being "monitored" so that the p.s. knows when to change the freq  or duty cycle of the switcher to maintain regulation.

If your p.s. is designed to maintains its output without a load then just use the variac.

Another common problem is an open startup resistor. Usually around 470k. This resistor supplies startup for the oscillator. You should find it tied to the output of the main bridge.