Got a call that the station’s mod monitor was displaying all wrong. Large amounts of carrier shift, excessive negative peaks, none of which were evident with a scope connected to the transmitter sample port.  Bad monitor!  No.  Bad RF amp! 


There isn’t much to these, just a tuned RF amplifer operating in the AM band.  This one was making mincemeat of the AM signal passing through. 

A quick power supply check showed the culprit.  +24v, -38V. 

Very simple power supply. This is how it was done before three-tab regulators became common.  Center tapped transformer feeding a full wave bridge.  Positive and negative voltages applied to a pair of complimentary pass transistors, with a zener diode setting the pass voltage.  This one had a shorted negative pass transistor. 

Those silver Sprague Caps are always trouble. They are guaranteed to have failed at some point over their 20 year service. Probably what took the pass transistor out.