The AM Forum

..hi from tim..if i didnt have enough rigs to werk on...lately, my dx100 is losing grid drive, to the point of complete zero on the meter....it is ok when first keyed-up, but during a tx, it is dropping...ideas?...sk...

first guess is a weak tube. Second guess is bad B+ to the low level stage of the rig where RF is concerned, caused by a leaky cap undergoing a gay sex change and turning into a resistor. Third guess is nasty resistor changing in value either way low ( in which case you will see some smoke at some point, or going high, where you will see no smoke, it just wont work.

Tim, check that multi-section cap in the Low Voltage supply.  If it hasn't been changed it could be leaking like a seive.  If it is, the voltage will go too low to work.

...ok...i'll look at it tonite...funny thing as well, i'm told there is no sig drop, grid current drops, but the plate current, will and does rise, but not right away...audio on my monitor sounds great, rf outpoot is good, but i drop the carrier when the plate current rises too much...next transmission, all is ok, fer abt 4 mins, like nothing happened, then it does it agn...?...sk...

Check the screen circuit which includes the clamp tube.  This includes the 125 uf caps in series along with the 30 k tapped resistor.  Is the clamp tube good?

Hi Tim,

Just working the drive issue,

The multimeter switch has a driver plate current position,  as well as the final grid drive position.  Try to note what happens to the driver plate current as the grid drive is falling ?  Just curious.

There have been problems noted with the drive control pot.  It is wirewound and sometimes  the wire gets bunged up.  This pot is in the driver screen circuit in series with a fixed resistor.  The fixed R could be heating and changing value,  causing the drive to drop.

Do not understand the delay in plate current change with the indicated drive going to zero ...  Try adjusting the clamp tube pot as indicated in the manual. The earlier suggestion to test or change the clamp tube is good,  some thing to think about.

...  the driver tube could be going bad as well.  Have fun   Vic  K6IC

...last night i replaced the  drive pot, 25k, 4 watt...i had an extra from a dx60, and fer now it is werking...i'm looking for a higher wattage pot, one that will handle the heat....when i find one, i will replace this one, and all shud be ok fine...at this point, it is werking...tim...sk..

While you are in there replace the coupling cap that drives the grids of the 6146's. This is a source of drive loss or worse when mine shorted it put several hundred volts positive on the grids of my finals..... Just for a little while.