[BC] Interesting phenomenon

[donroden at hiwaay.net]

Harris FM-7.5  following large lightning storm.

Transmitter screen over-current circuit protects transmitter.

Turn off screen breaker and transmitter makes 10% power

Screen blocker shorted on the 4CX7500.

Order parts and leave on 10% power with screen supply turned off.

A week later, power is down to 5% and getting noisy in areas.

Replace screen blocker and slowly bring up screen voltage.

At apx 20% fwd pwr,  screen over-currents and shuts down transmitter.

Backed power down 10 % and then brought screen up slooooowly
( keeping current under 100ma )

Screen current bouncing around between 50 and 100.
Power output bouncing around between 10% and 20%.

Tuning control dips screen current and peaks output power meter at same point.

After 10 minutes, screen drops below 50 ma.

Increase screen voltage until screen current starts bouncing again
Now power output is at 50% and bounces around in sync with screen current.

Still keeping screen current below trip-off point.

After 10 minutes, screen current stabilizes below 50ma.

Increase screen voltage upwards again and screen current rises
( but no bouncing )

Take tx to 100% power , re-dip screen current to under 100ma.

Turn off screen breaker and power out goes to 10% like before the week of
operating with no screen voltage.

Turn breaker back on and transmitter makes  100% and screen current
stabilizes below 100ma with no bouncing.

Did an element in the tube get "poisoned" by running the transmitter
with no screen voltage ?

I've never seen a warning about this type of emergency operation.

Don R