The AM Forum

Had to replace the screen resistor on the osc/buffer tube in my Ranger. After replacing the resistor, the drive on 80 meters will go up to about 2 mills with the drive control but as I increase the drive with the drive control, the drive (grid current on the final)  will start to go down. The control works well on all other bands. Before replacing the resistor, the radio worked well on all bands. The radio has the Timtron mods. Any ideas?
Bill

Bill, look for the reason the screen resistor burned out.  Unless it drifted out of tolerance due to age, you're just treating the symptom by replacing the resistor - not the cause.

73 John

Hi John,
Yup...I think your correct. The funny thing is that when the toob was drawing all that plate current....actually glowing red......the radio still worked and I did not have the drive problem. The other stages seem to be ok using the front panel meter to check the cathode current of them. But the osc/buffer is still drawing more current than it should but I attribute that to the fact that I solid stated the power supplies and the low B+ is rather high. Im wondering if something in the plate ckt of that toob is bad. I have to recheck the ckt but I believe there is some sort of resistor/coil combination. Whats wierd is that the drive starts to go down after the drive control reaches a certian point....but only on 80. The drive control varies the screen voltage on a stage AFTER the osc/buffer I worked on....hmmmmm.
Ill keep ya posted.
Bill

Check the grid voltage, if it's positive, there's either leakage or the toob is gassy.  It almost sounds like something in the plate circuit has gone south, though, for the screen to start drawing all that current.  Could be that the plate circuit is way out of resonance so it's acting as a multiplier.  I don't have a skizmatic handy so I can't be of too much more help here at work.

73 John