[CQ-Contest] Need Help Fixin' Collins Amp

[W5ASP at aol.com]

We  are trying to fix an old Collins 30L-1 linear amplifier that had been sitting
on the shelf at a friend's QTH in the Caribbean, having failed several years ago.
We have replaced filter capacitors, diodes, tubes, etc., and finally managed to get
it working... sorta...

It works great for about 20-30 minutes, but then the quiescent plate current
goes away.  This suggests a thermal problem.  When in RCV, the grid voltage
is -170v, and when in XMT the grid voltage is -2.5v.  When the quiescent
plate current disappears in XMT, the grid voltage is still at -2.5v
indicating that the resistive divider circuit and choke are OK.  We suspect
a problem with the center tap of the filament windings of the transformer.
The transformer is a relatively new replacement from Peter Dahl.  

Has anyone ever run across this problem?  What was the cause?  What was the fix?
If ANYONE can offer any suggestions please let us know at w5asp at aol.com.

Thanks...

73, Joe, W5ASP



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