SB-100: S-meter Sensitivity

[Kenneth G. Gordon]

On 30 Apr 2008 at 19:40, Skipp Tullen wrote:

 
> Despite my having replaced the 6AU6 RF amplifier in the receiver 
> section of the SB-100 with a 6HS6, the rig requires 80 microvolts of 
> signal on the 40 meter band to produce an S-9 signal strength 
> reading.

That is not right. And changing the 6AU6 to the 6HS6 would have no 
effect on the signal strength. What that change does is lower the receiver 
generated noise. It does NOT enhance the signal strength.

> In addition, the AGC threshold seems rather high, as 
> evidenced by little or no signal strength reading on ordinary 40 M 
> background noise that reads S-4 to S-6 on, for example, a TS-450S-A/T.

Again, that is not correct. It appears that you have some other problems 
in the IF or 1st or 2nd receiver mixer, or elsewhere.

It has been my experience that bad or weak tubes in many different 
places, not necessarily directly connected with the IF or RF amps and 1st 
receiver mixer, can cause big problems with sensitivity and AGC action.

> Are these low S-meter readings characteristic of the SB-100 design, 

Absolutely not.

> or do I need to look further into the circuitry for some subtle 
> problem?

I think that the answer here is "yes".

One thing that immediately comes to my mind: alignment. Proper 
alignment in these rigs is very important, and not really intuitive.

For instance, problems of the kind you are experiencing can be (and 
have been) traced to improper levels, or mis-peaking, of the heterodyne 
oscillator coils, and this is definitely not intuitive. First of all, the coils have 
to be peaked on the correct "side" of the peak, and secondly, the levels 
should all be set to the identical value. I prefer -2 VDC.

I think if I were in your position, I would first do a COMPLETE and 
ACCURATE alignment. Then repeat it. After that, I would carefully 
examine the operation of the rig, and start doing some systematic 
"looking".

I prefer to do my alignment in the transmit mode, at low power, and with 
the VFO set for the exact center of each band. Set the driver/preselector 
control to the settings in the manual.

Good luck, and vy 73,

Ken Gordon W7EKB

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