On Nov 14, 2006, at 6:04 PM, jmltinc@... wrote:
OK, I need professional help.
I got a basket-case Henry 3K Ultra some time ago and have been repairing in
my spare time. Amongst the defects it had, was a plate choke flash- over. This
took out the .5 ohm (20 watt?) resistor and diode (on the power supply) that
hold the B- to near ground potential. It also took out a small (. 1?) cap at the
plate current meter (in the desk console). The schematic is not correct for
this unit and Henry was no help. I did what I think is correct.
Now my problem:
The grid meter show excessive current. I am convinced this is not "grid
current", but plate current. See the table below for results:
Grid Current Plate Current Power Out
Amp on, not keyed: 75ma 0ma 0W
Amp keyed, no drive 225ma 150ma 0W
Amp keyed, 50W drive >1,000ma 875ma 1,500W
Notice grid current w/ no excitation and how the grid current follows the
plate current. Hmmm...
If I draw out how the metering is wired it looks right. For the grid meter,
here is a .5 ohm resistor shunt from ground to cathode with a 470 ohm, 3 Watt
series dropping resistor from the cathode side of the shunt to the grid meter.
(The other side of the meter is grounded).
For the plate meter, there are two series .1 ohm shunt resistors from the
glitch R to the cathode. The meter is connected across this with a 150 ohm series
dropping resistor between it and the shunt R / glitch R connection.
Any ideas?
John = The first thing I would do is to connect two back to back 3A diodes across each meter movement to protect them from possible damage. The second thing would be to check the calibration of all metered functions against a calibrated DMM, and do whatever it takes to make them read accurately. Third, I would add a suitable glitch-R in the HV+ lead.
Thanks
-John
R L Measures, AG6K, 805.386.3734
r@..., rlm@..., www.somis.org
