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Astron RS 35M help

74IN

74IN

Well-Known Member
Feb 17, 2003
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It stopped putting out voltage, so I looked inside.

1001745tk2.jpg


What is the broken component near the top of the transformer?

What would cause it to come apart?

I looked at a schematic, it looked like it was a VR of some kind.

To me, the bad part looks like a cap on the primary side.
 

The bad component is fed from the neutral(white) AC wire, and goes to the bottom of the power switch( three terminals).

The switch lights up, but the voltage meter stays nears zero.
 
Can't say 100% for sure,more like 98%, but it looks like a thermistor.They are a form of resistor that changes resistance with changes in heat caused by current flow. When you first turn the power supply on there is a high current surge and the thermistor acts like a high value resistor to help limit current surge. In a split second the inrush current is near zero and so is the resistance of the thermistor. I had to replace a couple on some televisions over the years. You could test it out by jumpering the leads and trying it out by bringing the voltage up slow with a variac if you have one.Even if it works it is still a good idea to replace it as it will help protect the trasformer and especially the rectifiers from a high current surge at turn on.

Excerpt from Wikipedia: "PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device. "
 
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Would its schematic symbol be a dashed box with a smaller rectangle inside, with an arrow through it?
 
OK, I just had a look at the schematic and it looks like I may have been wrong in my post above. that was the 2% part. It looks like a simple MOV or Metal Oxide Varistor, and the PSU should work even without it as it is simply wired across the primary winding and is meant to short out any voltage spikes coming into the power supply. Have you checked the fuse and the switch? (You said the light comes on did'nt you? ) Are you getting any voltage across the filter capacitor? It is possible you may have had a hit on the power line that blew the MOV and wiped something in the supply. I lost a voltage regulator in my Tripp-Lite PR-40a when lightning hit the power lines in front of the house last summer.Sometimes it does not even need to be lightning ,just bad spikes.
 
Are you getting any voltage across the filter capacitor?
Click to expand...

22.6V.

It doesn't seem to discharge the way it did when it worked.

Turn it off, watch the V meter drop as the filter cap was discharged.

Now, no V on the V meter, but the cap retains V.

It's one of three I had when a huge spike occured.

One died right away, another worked for months.
 
Since you have voltage across the filter cap the burned out component on the transformer is not the cause. As I said it is there to shunt spikes to ground. My best guess at this point is the 723 voltage regulator is toast. It is not driving the pass transistors into conduction therefore you will not have any output voltage on the meter and the filter cap would take a very loooong time to self discharge.With any luck the regulator is in a socket and even Radio Shack may still have them in stock. Very common regulator and only cost a couple dollars.I had to replace one in my Tripp Lite PR-40a last year. I had an "oopsie" while poking arouund inside it. :blushing:
 
Thanks so much.:wub:

I've been getting ready to just test every componenet 'till I got to it.:(
 
Discharge that filter cap before you install a new regulator IC. The voltage won't kill you but it might kill a new IC when it is plugged into the socket with voltage present.
 
Anyone know where I could try to get this regulator without a minimum purchase?

I don't care about spending the loot, no idea what I'd buy to make the minimum.
 
Did you try Mouser yet? I don't think they have a minimum. Maybe Digikey too? Try Mouser first.
 

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