DX-100B Problems Continued

[Kenneth G. Gordon]

On 6 Feb 2008 at 15:20, Mike Hardie wrote:

> Despite previous help I'm still struggling with a DX-100B.  160 and 80
> meters have about 60 watts out, 40M about 40 watts, nothing on the
> higher bands, no difference on CW or AM.

Hmmm....that's much too low. Although the manual says that the 
OUTPUT should be 100-120 watts AM, and 120-140 watts CW, that 
seems awfully high to me. I would think that 120 watts OUTPUT 
would be the most one could safely get, and 100 watts output is 
more likely.

> 
> Driver current and grid current indicate very low to zero, depending
> on the band, on the front panel meter.

Although the first thing one should do is to suspect the meter 
readings, since the "calibrating" resistors have undoubtedly changed 
value, the low meter readings, coupled with the low output, would 
indicate that there is definitely something wrong in the driver, pre-
driver (12BY7) or VFO.

Have you followed the RF signal through the system with an 
oscilloscope?

> Voltages at the various tube
> pins seem OK, don't have any data on what the signals should be at the
> various stages. Tubes have all been tested and substituted with no
> problems found.

Well, you possibly could ask someone with a properly working DX-
100 to give you some ball-park signal levels with a VTVM or o'scope.

> 
> Where to go from here?

Well, the signal level out of the 6AU6 VFO, according to the VF-1 
manual, should be approximately 10 volts. That will give you some 
place to start.

Output of the 12BY7 should be at least double that, although it 
probably should be even higher, since much depends on the 
impedance the tube is working into. 

If we say 20 volts of RF at the input to the 5763 stage, then ITS 
output should be, again, at least double that, or 40 volts, to the grids 
of the 6146s.

If the outputs of either the 12BY7 or the 5763 are close to the same 
values as their inputs, then the problem is around that stage.

I think you need an oscilloscope.

I am betting that some tuned circuit somewhere in the string is 
WAAAAAAY off the proper frequency, thus preventing proper drive 
levels to one stage or the other.

Ken W7EKB

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