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G0BHU-Mike · 24th Oct 2017, 6:19 pm

Hi,
My FT101ZD MKIII is suffering severe “HT Voltage sag” <> 50% on PTT. I have replaced the Rect ‘B’ board and both PA and Driver boards. Problem does not change. Notably, if the 12BY7 is removed the voltages do not Sag. I have replaced both 6146’s and 12BY7 - no significant change.

Observation - I note that there are two differing Grounds (earths)? Chassis earth and transformer ‘0’ (shown on diagram as the same earth). The cct diagram does not differentiate them?? When connected together disaster happens (I destroyed at least one if not both 6146’s) - replaced both with new ones. My problem is I cannot see a dc HT return path to the rectifiers for the Screens and driver HT. Also the transformer HT ‘0’ volt shows it should be at earth. But the above catastrophe happens if you do this.

The voltage sag seems to be related to a no DC return to the rectified HT supply. A scope shows a very large 50Hz rectifier hum with the RF on top of it. Importantly I get power output before the voltage sag.

Has anyone experienced this? Any suggestions would be great fully received.

Thanks
Mike - G0BHU

24th Oct 2017, 6:19 pm	#1
G0BHU-Mike Triode Join Date: Oct 2017 Location: Bourne, Lincolnshire, UK. Posts: 10	FT101ZD Power output issues Hi, My FT101ZD MKIII is suffering severe “HT Voltage sag” <> 50% on PTT. I have replaced the Rect ‘B’ board and both PA and Driver boards. Problem does not change. Notably, if the 12BY7 is removed the voltages do not Sag. I have replaced both 6146’s and 12BY7 - no significant change. Observation - I note that there are two differing Grounds (earths)? Chassis earth and transformer ‘0’ (shown on diagram as the same earth). The cct diagram does not differentiate them?? When connected together disaster happens (I destroyed at least one if not both 6146’s) - replaced both with new ones. My problem is I cannot see a dc HT return path to the rectifiers for the Screens and driver HT. Also the transformer HT ‘0’ volt shows it should be at earth. But the above catastrophe happens if you do this. The voltage sag seems to be related to a no DC return to the rectified HT supply. A scope shows a very large 50Hz rectifier hum with the RF on top of it. Importantly I get power output before the voltage sag. Has anyone experienced this? Any suggestions would be great fully received. Thanks Mike - G0BHU