[Amps] L4 power supply problem?

[Jason Buchanan]

hi,

I have an L4 with its original power supply.  Been working fine for a 
long time.


Today, I turn it on and after a few minutes there is a large pop and 
blue spark from near the HV interlock, or located behind the rear tube.

The .82 ohm resistor fried - I replaced it with a new one and carefully 
inspected here and there in the amplifier cabinet.  I couldn't find 
anything that burned near the interlock.

I powered it back on in CW low voltage position and the plate voltage 
reads around 1300V.  On SSB it reads around 2000V, maybe a little less. 

That was all the experimentation I wished to try.  I opened the L4-PS 
and looked around but can't find anything on the top side that looks 
burned or stressed. 


The thing I noticed that was peculiar was that when I powered the L4 
off, the plate voltage would begin to decrease as expected, but then 
ROSE about 200 volts and then slowly decreased again as expected.  When 
I took the amplifier cover off there was still a bit of voltage in the 
HV supply, around 150V on the meter.  I used a pair of alligator clips 
to ground the cabinet to the top of the plate choke and then removed the 
clips - what was odd was that the voltage rose up to about 100V - when I 
grounded it the meter read 0 again.  When I removed the clip it rose to 
about 50V.  Finally I put the clip on and left it on and it finally 
stayed at 0 with the clips off.

Has anyone seen this before?  Any idea what is wrong...  and what I can 
do to repair it?


73 Jason N1SU