Henry 2K-4

3-500Z tubes are drawing ~300 ma of plate current

when HV is applied, not keyed, no drive

By: K2MF

06/03/2014

 

 

ISSUE:

 

3-500Z tubes are drawing ~300 ma of plate current when HV is applied with no drive

 

FIX:

 

While troubleshooting the problem it was discovered there was .2 ohms to ground on the center tap of the filament transformer, which definitely was not correct. More investigation led to suspect C40 and/or C41, which are .1 mfd 50V RF bypass capacitors on each leg of the filament transformer line in the RF deck.  It turned out that C40 was the culprit, when removed from the circuit, had a resistance of exactly .2 ohms.  It was replaced it with a good .1 mfd 100V bypass capacitor I had laying around in a parts drawer. C41 was ok.

 

 

 

 

Pictures provided by: Joe, IT9BLB/KF6FBC

CONCLUSION:

If one of these RF bypass capacitors short, a symptom CAN be that when plate voltage is applied, the amp will draw significant plate current without key and without RF drive.  We surmise this is what happens when the filaments in the 3-500Zs are somehow brought to ground potential and that anything that causes that has the ability of creating the same symptom.  This is likely why the troubleshooting procedure for this behavior in the manual says that we should look for a grid-filament short in the tube(s) because that would be the likely reason for this to happen.

​73 de ​

Barry​ K2MF >>​

k2mf.bgs@gmail.com