Astron RM-50M blown capacitor and fuse

[radi0h3ad]

I acquired a RM-50M that blows it's fuse each time as soon as power is applied to it. There is a ceramic disk capacitor that is inline with the power adapter but the top is blown off of it so I can't tell how many farads it should be. I've checked the schematic and it doesn't list the capacitor or it's rating. I have embedded a pic below

Does anyone know how many farads this capacitor is?

Can someone also direct me as to what might be cooking the fuse each time on power up. Replacing the cap most likely wont fix the issue right? Can I just remove it?

[kcbooboo]

The device across the AC input is most likely a transient suppressor or MOV, not a capacitor. This is installed to prevent excessive voltage spikes from damaging the supply. When they do their job, the fuse blows but sometimes the device also shorts out. It can be removed for now, but you probably will want to replace it. Some schematics show it installed directly across the transformer primary while others show it across the back of the AC input connector, which has an internal fuse. So chop this one off and try it again. If the fuse doesn't blow, then just this device is bad and it did its job protecting the rest of the supply.

The schematics show it as a V150LA10A but the current part is V150LA10AP made by Littelfuse. 14mm diameter, straight leads. Replacements can be purchased from Mouser ($0.34) or Digikey ($0.60). Cheap insurance.

If the AC fuse still blows immediately, the main rectifier diodes are shorted. Replace ALL of them. Some supplies use stud-mounted diodes while others use one or two square bridge rectifier packs. Also check the two diodes mounted on the regulator board. Two wires from the transformer go directly to them. They're just 1A diodes such as 1N4000-series. It's just cheap insurance to check them while the supply is apart.

I'm guessing a nearby lightning strike put a surge on the AC line.

Bob M.

[radi0h3ad]

Hey kcbooboo,

Thanks for your guidance on this issue. I thought to remove it but I figured I'd get some help before I started messing with things. I removed the mov and things work fine and dandy and it no longer shoots a huge spark out of the hole that was in the mov. I looked at the schematic tons of times but I keep expecting to so a C followed by farads and I also though that VR was variable resistor, lol. Of course I read the schematic wrong as I'm a dabbler way out of practice when it comes to such things. I'll definitely replace it as it kept the unit from frying due to what ever caused it to pop its cork. I presumed lightening as well...the top of it was rolling around inside of the case like set of dice.

Thanks for your assistance, you gave me the confidence and information that I needed to do what I was going to do blindly...now I'm well informed!

[Will]

Then the next step is to remove one pass transistor at a time and wipe the old heatsink grease off, check the base and emitter pins for signs of poor contact. Put new clear hs grease on Sparingly!. When bolting the transistor back on the heat sink use a external tooth lock washer on the two mounting screws.

Old dried out heatsink grease a known failure point, white grease IS conductive but Astron used it anyway.
Contact failure on the emitter and base leads, another failure point.
The lock-washer helps the contact between the screw and the transistor a known failure point,

[Tom in D.C.]

FWIW, and for possible future reference, I've always found Astron to be an extremely cooperative outfit servicewise when help is needed. I've purchased pass transistors and entire regulator boards from them to fix their power supplies.

[kcbooboo]

You can buy most of the parts used in Astron supplies from regular parts supply houses such as Mouser and Digikey.

I've noticed that Astron is now using snap-in filter capacitors which are soldered to the regulator board, instead of the screw terminal ones that are much bigger, much more expensive, and much harder to get.

Bob M.