Urei LA4 dead output at Clare MRB2A12

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JimboJohnson

Member
Joined
Apr 15, 2010
Messages
17
Hi all. I'm repairing a pair of Urei LA4's at the moment - havn't worked on a pair before - one needed a fuse holder replaced, and sounds fantastic. The other had all the diodes of the bridge rectifier blown up - replaced, and have a nice +15 and -15, and everything works great until right at the output where there is a  a Clare MRB2A12 assembly; which doesn't appear on any schematics I've come across. According to the label, it's an inductor, and two capacitors, all independant; and should merely be a cap in series to the output where the problem occurs.

  The signal never makes it through to the output terminal; but when the component is swapped from the the working unit, the same result occurs, and both measure the same off board; but significantly different in circuit. (0.6nf in not working unit, .13nf in working.) I figure that I can simply bypass the offending area; as schematic has ouput directly connected to the transformer,  but would be nice to work out exactly what the issue is, and keep things stock.
Any ideas?
Cheers! jim
 
The Claire box is a relay. Sounds like you are working on an old black face. Its basically a timing circuit that mutes the output on power up. Replace the 555 timer chip and all should go back to normal
 
Well, I certainly feel like a ********.  Those darn tricksters drawing pictures of capacitors on their relays....
But thanks!  Will try that tomorrow.  It is actually a Silverface, but i guess probably an early one then?
 

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