On Mon, 12 May 97 00:28:14 -0700 Rich Measures <measures@vc.net> writes:
>I suspect that R1 [0.82 ohm] is open. Reverse grid current is an
>indication that the filament may be out of line and touching the grid
>cage when the tube is warm. [see photo on p. 15 in 9/90 QST]
>Be sure to change the cutoff bias method from voltage to resistive AND
>
>put diodes across the meters or across the meter current shunt
>resistors.
To continue from my prior reply to John:
If R-1 is open, most likely there are other components fried.
- Zener
- A tube
- A grid choke at the offending tube socket
If ur 220 is an early version visually check all of the red 200pf
rectangular mica capacitors at the grid pins....6 total. They have a bad
history of admitting moisture over time and will actually crack the case
from the RF. I have had a few explode with a flash and a bang. Replace
with dipped silver micas.
Also check the parasitic suppressor resistor values. They are originally
47 Ohms 2W; replace if they are over ~ 60 Ohms. If you cant locate 2W
carbons switch to metal oxide from Mouser, DigiKey, etc. Also Rich and
Harbach Electronics sell suppressors.
I recommend ONLY plate suppressors, and no messing around with the grid
or filament circuits, but that is an entirely different thread!
Finally, install a 10-15 Ohm 10W wirewound (not cement) resistor in place
of the VHF choke, RFC-2.
GL....Carl KM1H
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