[Amps] L-4PS Transformer Buzz

[Michael Tope]

I just installed the Harbach PM-400 in the L-4PS power supply for one of 
my Drake L-7 amps after I popped a rectifier diode in the old bridge:

http://www.harbachelectronics.com/main/page_products_drake_l4l4bl7.html

Electrically, the amp and power supply appear to work perfectly with the 
Harbach board. Plate voltage and RF output are as expected. There is one 
problem, however. Last night about 20 minutes after I fired up the power 
supply I noticed a loud 60 Hz buzz coming from the supply. Pressing on 
the sides of the cabinet or changing the physical orientation of the 
supply didn't seem to help. When I switch to the low-voltage setting on 
the L-7, the buzz stops. Also if I let the supply cool off the buzz 
stops for a while and then returns after the supply warms up. Looking at 
the supply in the dark, I saw no evidence of sparking. Additionally, the 
amplitude of the buzz seems to be totally unaffected when I go from 
standby to full RF output power.

Any ideas on what might be causing this problem?

Thanks,

Mike W4EF......................