SB220 BIAS/ RELAY PROBLEM

[ghaffly]

Hello all , currently have a problem with an Sb220. Details :

Current setup : Icom 706 MK2 with an Ameritron interface to an SB220.

At cold start linear turn on is Ok and tubes are biased off , no plate
current .Key the linear in Cw mode and interesting things happen.
#1 After releasing the Cw key ,linear's relay should open and plate current
should
be zero. This Sb220 relays stays closed with 100 mils plate current.
#2 The only way to drop the relay is to pull the relay connector from the
rear socket
and then the relay drops out and no plate current. Plug the relay connector
back in
and with zero Rf applied relay closes drawing plate current. There seems to
be a faint
but audible squeal prior to relay closure.
#3 If you turn the linear off for a minute or so after #2 , it is ok until
you apply Rf again.


The voltage on the relay coil looks Ok , about 126v. Plate voltage Ok.
Since the problem is going From Tx to Rx whats keeping the relay closed?
Maybe a bad ZD1( 5.1 v).  Also any ideas on the faint squeal heard before
relay closes?

Any ideas would be appreciated.

Glenn

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