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 Post subject: Hallicrafters SX101A Alignment- "Electric Radio"Instructions
PostPosted: Jan Tue 23, 2018 12:43 pm 
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Instructions appeared in DEC 2005 edition of Electric Radio- pg. 42.

The author suggests alignment for the 50.75kc and 1650kc IF amplifiers using only a VTVM and frequency counter. This was based on a service bulletin from Hallicrafters in the 1970's.

On the step- "connect the frequency counter across the speaker output"- not sure if that means the 3.2 ohm output or the 500 ohm output?? And is this done with the speaker connected?


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Jan Tue 23, 2018 1:52 pm 
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First, be careful with that article. Hallicrafters didn't apply AGC to the highly selective final IF because of the "Miller effect" of capacitance changes in vacuum tubes when the operating conditions are changed. It is a fairly minor effect but it will cause the tuning of the transformer to change slightly as the AGC system controls gain. The original engineers of these vintage radios were pretty darned competent.

I have never found the original Hallicrafters alignment for these radios to be confusing and there are more sources of error in that revised procedure. The only change I suggest to the original is to first carefully measure the frequency of the 1600 Khz. and 1700 Khz. conversion oscillator crystals and if one or both are slightly off then you can revise the alignment procedure to account for this discrepancy. The final IF must be aligned at exactly 1/2 of the absolute difference between these two oscillator frequencies + .75 Khz. (this provides the desired passband removed from carrier and alternate sideband rejection). The BFO is aligned at exactly 1/2 of the absolute difference (don't add the .75 Khz.). For example assume the 1700 Khz. oscillator has drifted to 1702 while the 1600 is on frequency. One half of the absolute difference is 51 Khz. so the new BFO zero pitch frequency is 51 Khz. and the target alignment frequency is 51.75 Khz. This preserves the proper sideband switching with the radio.

The 1650 Khz. IF alignment isn't terribly critical, the ~50.75 Khz. alignment is but just do it carefully and it will come out fine. There isn't anything complex about this alignment unless someone tries to make it more difficult than it really is.

When adjusting the 1600 and 1700 Khz. oscillator coils make sure that you set the coils so that they are just past the peak on the slowly decreasing side of peak. Once the alignment is otherwise finished then choose whichever sideband has the highest gain and further reduce its output (again staying on the slowly decreasing side of peak) so that its output is reduced so that both sidebands provide the same conversion gain in the final mixer.

The SX-101A is a very competent receiver and does not need modification unless one is interested in making it perform more like a lesser receiver. If you haven't used a Hallicrafters selectable sideband on AM receiver before practice tuning by turning the BFO on with pitch set to zero and zero beat the desired station then switch to AM mode and choose the desired sideband. You will quickly get the hang of proper tuning on AM without this step but it can be useful if you aren't used to highly selective sideband receivers for AM. 5 Khz. provides the best response (equivalent to a traditional AM receiver with 10 Khz. IF bandpass) but 3 Khz. is also very useful. Choose the sideband with the least interference.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Jan Tue 23, 2018 2:24 pm 
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Location: Buffalo, NY
Thanks Rodger!

One of the reasons, I was hoping the Electric Radio advice was OK is that I don't have an audio or signal generator that outputs 50.75 KHZ. I have a small audio generator that goes as far as 50 KHZ and it's in steps that are not adjustable.

I guess I'll have to hold off until I can get a proper generator.


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Jan Tue 23, 2018 2:48 pm 
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If you have two signal generators (one could be a simple crystal oscillator) you can "beat" them together in a mixer to create the 50.75 Khz. output frequency. For example use a 5 Mhz. crystal controlled oscillator and mix that with your signal generator set to 5.05075 Mhz. using a simple diode mixer. Or if one of your audio oscillator's selectable sub-steps is an even sub-harmonic of 50.75 Khz. clip its output with a diode to create strong harmonic generation and use that output.

The method in ER will work and is similar to what Drake suggests for the Drake R-4 series through using its crystal calibrator and BFO. Note that some counters have greatly degraded accuracy at the audio range so proceed with caution. To use the method outlined in ER it is absolutely critical to start at perfect zero beat and close isn't good enough. With your counter connected use enough audio drive to get a good stable signal.

I suspect Hallicrafters published the procedure because most people didn't own a generator that goes that low in frequency. I used an old HP audio oscillator for several years before picking up a HP-606 generator later replaced by an 8640B with external converter for LF but the old HP audio oscillator worked as well as anything once I had it set up properly. I had to hand select a "night light" bulb for least distortion in its Wein bridge circuit and then undo my own sabotage. I cleaned up and lubricated the variable capacitor like I always do with vintage gear but this caused the capacitor in the HP-200A to rotate so easily that it would move by itself. Adding a felt washer between the knob and panel provided sufficient friction damping to undo my cleaning and lubrication induced issue :)

That is one piece of test gear I will never dispose of since it was inherited from my father.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Jan Tue 23, 2018 9:29 pm 
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How would I construct a "simple diode mixer"?


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Jan Tue 23, 2018 11:52 pm 
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If you google simple mixers several circuits will come up; you are basically coupling your two oscillator inputs to a non-linear element (diode) which acts as a mixer. Typically you would use tuned transformers for the coupling but with what you need I suspect capacitor coupling will suffice since you aren't shooting for high performance. Or a very easy solution is to pick up one of Mini-Circuits' pre-built balanced mixers which can be acquired cheaply from many sources including ebay. With these all you need to do is connect the inputs to the correct pins.

What audio generator do you have? It is likely the output can be skewed enough to hit 50.75 Khz. by slight adjustment of the master clock if it is a synthesized unit and if it is using switched passive components then you can easily change one value slightly to hit the needed frequency.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Feb Sun 18, 2018 9:24 pm 
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Location: Buffalo, NY
Rodger,

Finally acquired a UMR-25F- a beautiful piece of military equipment. Now I can do a proper alignment using the Hallicrafters manual.

Any advice on freeing the various inductors? Some have wax seals, other have their screw shafts that are soldered.

Steve


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Feb Mon 19, 2018 8:41 pm 
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Steve,

The wax is soft and your screwdriver should easily push it aside and the slugs will turn. I would test the screws for the oscillator slugs to see it they turn as well. The solder is probably not meant to permanently prevent them turning, but only to hold them from moving with little pressure.

Norm

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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Apr Wed 18, 2018 11:36 pm 
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Rodger: will these 50.75KHz alignment instructions also work for an SX-100?


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 Post subject: Re: Hallicrafters SX101A Alignment- "Electric Radio"Instruct
PostPosted: Apr Thu 19, 2018 1:38 am 
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stahlhart wrote:
Rodger: will these 50.75KHz alignment instructions also work for an SX-100?


Yes they will. And the SX-100 is a great general coverage receiver.

Rodger WQ9E


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