SB400 - Low Grid Drive on 40 Meters

[Mark Johnson]

Heath Team;

I've got an interesting problem on an SB-400. The transmitter is paired up with an SB 300 receiver.

 I had the exact problem on an SB300/400 pair a few years ago and never resolved it to my satisfaction. I happened to find this pair a few months ago and thought I'd bring them back to life as I always enjoyed using the Heath twins. Upon nearing completion and doing the final tuning tweaks, I ran across this problem again. 

The net of the issue is there is no,or very little, grid drive showing on the meter when tuning up, or when the rig is in operation. I get full output into a dummy load and a nice clean CW/SSB signal on the SB-610 scope, but there is no grid drive showing on the meter. The rig has great output on all bands and on air signal reports are good. 

I know the 8.65 mhz trap setting can be tricky and cause 40 meter output problems so I adjusted mine my feeding the transmitter, through attenuators, into my spectrum analyzer and I adjusted the trap variable cap for minimum. It adjusted fine. 

The LMO, Het Osc and BFO cables from the SB300 to the SB400 are proper RG-62 and cut to exactly 24 inches. 

The SB300 Het Osc coils are peaked OK. 

I changed out the meter dropping  resistor, 4700 ohms, for the grid circuit as it was high. All other voltages are OK.
,
As mentioned, I had this exact same problem years ago on a previous pair. Output on 40 meters is a nice 95 watts into my dummy load, just not showing any grid current on 40. All other bands, there is grid current showing. 

Any ideas?

Many thanks

Mark V Johnson VE3DJU/VE3DDI

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