[Boatanchors] DX-100 80/160M Grid Drive Problem

[Ron Thomas]

Before I go tearing into this old DX-100 again, I thought I would describe a problem I'm having recently in hopes that some of you have already experienced it and can tell me the fix.

I recently had one of the 5R4 high voltage rectifiers short out while I was tuning up the transmitter on 80m with PA plate voltage applied.  Of course, this resulted in a blown fuse immediately.  Once I determined the cause of the high voltage problem and replaced the bad 5R4 and blown fuse, the transmitter operates normally on all bands EXCEPT 80 and 160 meters!  With the meter switch in the DRIVER position, I get good upscale indication on all bands, including 80 and 160 meters, and it increases and decreases as it should when the drive pot is rotated.  However, when I switch the meter to the GRID position, I get no grid drive on 80 and 160 meters (and no peaking using the grid variable cap).  Drive is normal on all other bands (and peaks as it should using the grid variable cap).  I can load the transmitter up on 40 meters, for example, to full power... so power supply components and RF stages seem to operate OK.  I just can't get grid drive to the 6146s on 80 and 160 meters!

My first thought was that there is an issue with the driver stage, but again operation is normal on all other bands.  If the driver bandswitch was open on 80 and 160, I don't believe I'd see any current indication with the meter switch in the DRIVER position.  I don't see any other components between the driver and the 6146 grids that could fail and still allow normal operation on all other bands. 

Any ideas before I pull this beast out of the cabinet again?

Thanks & 73,

Ron  N4RT