B&W 5100 Low Grid Drive

[Chris P.]

I finally got my B&W 5100 together and going, and have run into a low grid drive problem. The best I can get is 4.5 ma on 80 and less on the higher bands. I've checked everything I can think of, swapped tubes, and even swapped in a spare RF/multiplier and buffer module and tried using a crystal with exactly the same results. I've tried aligning the multiplier according to the manual with no luck and hardly any drive on 15m and up. Modulation suffers due to the low drive but otherwise works well on the lower bands. Any suggestions? Does anyone have experience with this multiplier unit and how to align it? Its a slightly strange setup with the tubes in series in order to run off of the high voltage supply.

Thanks

[Detroit47]

I have recapped the whole thing with success. I am referring to the multiplier. You will have to take the module off the chassis and split it in half to do it. While it is open check every resistor or just replace them all. You might want to check the voltage going to the unit also. If the voltage is low so is the grid drive. I use tube extenders to make measurements. Like these  http://k4che.com/tube%20extenders/TUBE%20EXTENDER%20PAGE%201.htm

John N8QPC

[Chris P.]

The resistors were one of the first things I checked. There were a few out of tolerance and the spare multiplier module had good ones so I just swapped in the whole module. It performed exactly the same. The voltages are good. I've split the original module and checked the capacitors around the 80 and 40m meter tube. They check out good for capacitance and have no leakage. Everything checks out so far, but drive is barely adequate on 80, low on 40, barely adequate on 20 and pretty much non existent on 15 and up. Both multiplier modules perform exactly the same. I'm kind of stumped.

[Jim, W5JO]

What is the plate voltage on the first two multiplier tubes?  The plate voltage for the first two multipliers comes from the cathodes of the second two./  The filaments for the second two are supplied from a separate winding at the bottom of the schematic and the rest I will leave to you.  I don't have the diagram and haven't worked on one in years.  I am working off memory.