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What kind of rectifier diodes? CCA FM

D

DonPerkins

I'm working on a CCA 5000 G Fm transmitter, and I see the diodes on the rectifier boards are marked ASI 10318. There are no MOV's or resistors on these boards. There are 25 diodes in series. I hate to spend $250 on a new board when only one diode appears to be shorted, but I can't find specs for these little guys. Any ideas on what they are or where to get replacements? Also, do they test OK with a Simpson meter only to break down at high voltage? I've got a nasty short in the bridge somewhere.
 
It's unusual for only 1 of the 25 to fail. Here's how to test those diodes..one at a time or the whole string of them. First, be careful as you'll be dealing the 120 volts of raw AC line power--it can kill you if mishandled. I offer this advice based on the assumption that you are a fellow engineer and not an owner trying to avoid calling in your engineer. Take a 120 volt AC power cord and a socket with a 100 watt light bulb in it. Hook one side of the power cord to one side of the light socket. Using clip leads, hook the other side of the light socket to one end of the diode(s) to be tested. Hook the other side of the diode(s) to the unattached wire from the power cord. Step back far enough that you can not possibly come in contact with any exposed electrical connection and briefly plug this test jig into a 120 volt outlet. Look at the light bulb while it's plugged in. If the bulb is at 1/2 brightness, the diode is good. If it's at full brightness, the diode is shorted. If there is no light at all, the diode is open (very rare). This test actually places the diode under power and is therefore a real world test vs using the diode test feature of some ohm meters. I can't tell you what the rating of the diode is, but if you want to try to replace one of a series of 25, a 1000 volt rating would be very safe & a current rating equal to several times your normal plate current will probably work, although the bigger the current rating you can find, the less likely it will short again. It's possible that CCAelectronics.com can tell you what the actual diode rating is. If it were me, I'd replace the whole thing for the $240 and be done with it. Even at a small station, if it puts you off the air for even several hours, you'll blow right through that 'savings'...just my 2 cents worth. Good Luck!
 
Yup. I'd spend the $240 for the new board, and then replace the bad diodes in the current board to put on the shelf as a spare. If it's failed this time, it'll fail again. Having a spare will defiantely be useful to you when you have your next failure. I've used a Huntron Tracker in most cases to look for bad diodes. The light bulb trick does seem like a good way to do it too with less resources.
 
Yup, I'm the engineer, and that's what I thought too, not worth messing with 1 or 2. I still have my 2x4 with the light socket somewhere left over from my days testing SSR's from a Collins 21/E. Thanks for the confirmation.
 
Lots of experience with the diode stacks in an AEL!
After trying to replace only the "bad" ones I eventually ended up replaced all of them in a stack.
The remaining diodes were probably stressed by the initial overload so they seemed more prone to failure. Also went with the highest surge rated diodes that would fit. Had a bunch of those diodes stashed in a bag that didn’t get used. That was nice.
 
I use six stacks of them in a Continental 316F 10 KW AM rig. The ASI 10318 may be replaced with a 6A10. Each diode is 6A @1000 Volts. Pretty much bulletproof, but always a good idea to have MOVs around.

Craig
 
Thanks for the info guys. I replaced all four boards with modules from Charlie Goodrich (very helpful, good guy to do business with!). It turns out I have also found there's only 60 ohms from the secondary to the frame of the transformer! That would 'splain a lot. Didn't blow a breaker when the secondary was disconnected so I assumed all was well there. That'l teach me to assume!
 
Donald L. Perkins said:
there's only 60 ohms from the secondary to the frame of the transformer!
Click to expand...
Put a couple of 2 by 4's under the transformer...I've seen them run like that for a couple of decades...but do put a note on the xfmr indicating that there is high voltage on the frame.
 
DRY wood! Better yet a sheet of plexiglass between the wood and the transformer/choke.
Had an FM20H3 with a leaky power supply choke. After a few days of operation sitting on scrap wood it started smelling like a campfire and throwing overloads.
 
boiseengineer said:
DRY wood! Better yet a sheet of plexiglass between the wood and the transformer/choke.
Had an FM20H3 with a leaky power supply choke. After a few days of operation sitting on scrap wood it started smelling like a campfire and throwing overloads.
Click to expand...
YIKES! I guess I've had dry wood...fortunately haven't had that happen.
 
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