SB-102 problem

[Kenneth G. Gordon]

On 11 Jul 2009 at 22:01, Brian Wood wrote:

> Hi everyone, 
> 
> I have had several responses, all indicating that it's probably a bad
> solder joint or a corroded tube pin. I poked around some more and
> here's more data: 
> 
> I continued to find it interesting that the voltage on the screen grid
> of the audio power amp was way low. I tried to measure the 22K 1/2W
> resistor that goes from the 312V B+ to that grid and found that I
> couldn't measure it, so I guessed it might be open. The power
> dissipation in that part, if it was working right, would be
> (312-200)*(312-200)/22K, or .6W, and it's a 1/2W resistor! So I pulled
> it and sure enough, it was open (although it looked perfectly good).

Two things: 1) Heathkit often "pushed" the dissipation ratings of resistors; i.e. 
where most of us would have used a 1 watter, they used 1/2 watt, etc. 2) I 
found a resistor that was open, yet LOOKED perfectly good in MY SB-102. 
Mine fed screen voltage to the 2nd receiver mixer. The receiver still worked, 
but gain was very low.

> I
> happened to have a 22K 1W resistor in my junkbox, so I replaced it.
> The voltages are now correct. It's sort of amazing that you can get
> any audio at all when running the product detector and audio amp at
> about 1/3 of their normal voltage, all from "induced" voltage coming
> from who knows where. 

Probably not "induced": tube circuits can appear to operate almost properly at 
extremely low voltages.

> 
> However, the AVC action is still pretty poor. I guess I don't
> understand how, with the RFG pot generating about -1 to -25V for an S1
> - S9+60 signal, you can expect to get much of an AVC voltage from the
> circuit they used

According to the circuit description in the manual, the "S" meter level will not 
be effected by the RF gain pot until you have turned the RF gain down to the 
point that the "S" meter reading due to the pot is ABOVE the reading caused 
by the received signal.

> -- the 2nd IF amp output is fed to a voltage doubler
> made out of tube rectifiers followed by an RC circuit, and diode OR'd
> with the RFG voltage which becomes the AVC. It just can't generate
> that much voltage on a typical signal. I can only see about -1.1V on
> really strong voice peaks. But I guess my meter may not be able to
> respond fast enough to really see anything.

To really see the interaction between signal level input and RFG setting, you 
have to use a steady carrier....like a signal generator. It can't be done 
properly with either an SSB signal or a CW signal.
 
> 
> If I assume that the AVC detector is working, the problem

Please re-explain the problem. I thought you had fixed it previously by pulling, 
then reinstalling one particular tube.

> must be in
> either the metering circuit, which is basically just sampled cathode
> current from the first IF amp, or perhaps the RC network that forms
> the AVC attack time constant (10M ohm, .02uF).

BTW, one can add a FAST AGC setting to the AGC system by replacing the 
audio gain pot with one which includes a push-pull SPST switch on it, and 
connecting that 0.02 uF cap between the end of the cap and ground.

Lifting the grounded end of that 0.02 uF cap from ground gives you FAST 
AGC.

> Or maybe the meter
> resistance. Any further ideas? 

Well, at this point I think you have a bad tube, possibly in the IF 
stages....although that 0.02 uF cap COULD be shorted...

Another possibility could be in the "S" meter zero system, possibly the 200 
ohm meter zero pot. 

Heathkit used several different "S" meter circuits in the HW/SB all banders, 
and at least TWO in the SB-102. The differences were mainly in the "S" 
meter zero-ing circuits. Their FINAL version in the SB-102 was the only one 
that was absolutely stable and didn't drift all over the place.

The "S" meter readings are even effected by how and where you have the 
heterodyne oscillator tuned. If each band is not adjusted to the same, and 
correct, level measureable at the test point, "S" meter zero is different on 
each band.

Ken W7EKB

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