[CQ-Contest] AL-572B grid current meter dead

[Doug Smith W9WI]

Well, I'm perplexed...

A friend has an Ameritron AL-572B.  While the amp itself works fine, the
grid current meter doesn't...  no reading whatsoever.  Plate current
meter works fine.

I have checked everything that looks like it would have to do with that
circuit -- C140, D117, C128 & 129, C310, R313, even the meter itself. 
Have also checked a few things that I would think would kill the plate
current meter too - R106, R107, Q103, R312, R128, C138, the grid
resistors on the tubes.  

This is a fairly common circuit for a grounded-grid amp: grid current
meter between the negative side of the HV supply and ground, plate meter
in the HV supply negative return lead to the cathode of the tubes.  

Anyone else experienced a similar failure & have any ideas where we
should be looking next?  
-- 
Doug Smith W9WI
Pleasant View (Nashville), TN  EM66
http://www.w9wi.com