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 Post subject: WR-50-A Signal Generator
PostPosted: May Thu 27, 2021 6:59 pm 
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Joined: May Thu 27, 2021 6:47 pm
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I am restoring a NC-98 Receiver and am ready to re-align it. I do not own a signal generator.

I have been offered the use of a RCA WR-50-A by a friend. He does not have the manual. It will cover the frequency range I require (456kHz to 30 mHz) but I am not sure about one point. In aligning the IF section, a signal of 456 is required then followed by moving up and down 2 kHz.

Is this instrument marked such to make this possible? :?:

An alternative I have (I think) would be to use my Icom IC-7300 transceiver to calibrate the generator. This radio is quite accurate with a digital readout. :?:

Any comments positive or negative are appreciated.

Paul


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 Post subject: Re: WR-50-A Signal Generator
PostPosted: May Fri 28, 2021 12:55 am 
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Location: Long Island NY
For some reason it's not easy to find a downloadable copy of the WR-50 manual (the early ones don't say WR-50A because the 'B' was still in the future), but manuals for the 'B' model are easy to find. The only real difference between the two is the 'A' model does not have limited AM or FM sweep capability while the 'B' does. Other than a few components and a couple more switch positions, they have pretty much the same circuits and chassis layouts. Operating instructions are the same too, except for the features not present on the 'A.' Here's one source for a 'B' manual: http://bama.edebris.com/manuals/rca/wr50b/

Accuracy on the WR-50 series generators is pretty good for analog stuff and you could probably do a fine alignment on your National with one. One tip would be to put a drop of light machine oil in each of the tuning capacitor bearings of the signal generator to make it turn smoother. This makes it easier to reset to the same place on the dial when you have to go back and forth. I am a big believer in verifying the accuracy of test equipment out before using it so a quick check against the receiver in the IC-7300 would not be a bad idea.

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 Post subject: Re: WR-50-A Signal Generator
PostPosted: May Fri 28, 2021 1:09 am 
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Another reason to get a lab quality signal generator. These days they are cheaper than ever. I love my HP 8657B.


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 Post subject: Re: WR-50-A Signal Generator
PostPosted: May Fri 28, 2021 1:24 am 
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Location: Long Island NY
I have a feeling people will still be using and fixing the old WR-50 series generators long after the last HP 8657 blows its microprocessor board and ends up as a doorstop.

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"Hell, there are no rules here--we're trying to accomplish something!"

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 Post subject: Re: WR-50-A Signal Generator
PostPosted: May Fri 28, 2021 1:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
you're going to need a frequency counter to get the frequencies within 2khz. My B model is pretty close, but not that close. cheap chinese frequency counter kit ought to get you close enough to the accuracy you need. or get a nice old counter. I just picked up a leader for $35 delivered on the auction site, and it powered right up and worked like a champ, despite looking a little rough. I checked it against a crystal I had, and it read the first four significant figures the same as my other counter.

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 Post subject: Re: WR-50-A Signal Generator
PostPosted: May Fri 28, 2021 1:36 am 
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Location: Auburn, AL
it looks like the A model can also take the same crystals as the B model. Just switch off the VFO when you want to use a crystal to set oscillator frequency. Use your digital radio to zero beat the signal from the generator to get the generator exactly where you need it to be. this guy does a great job showing how you can do this with your radio if it has an S meter. you can do it by ear, but you'll run into trouble getting the beat perfect within 100hz or so. https://www.youtube.com/watch?v=QCJ4cQGOQLI

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 Post subject: Re: WR-50-A Signal Generator
PostPosted: May Fri 28, 2021 1:37 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
hopefully someone can explain if there's a way to zero beat close to actual frequency successfully without a receiver that has an analog s meter.

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 Post subject: RF Alignment
PostPosted: Jul Tue 06, 2021 7:02 pm 
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I have completed the I.F. alignment . In doing the R.F. alignment procedure all went well until I got to 14 M.C. portion.

:?: I am not able to perform the procedure at 14, 15 AND 30M.C. There is not enough adjustment in the trimmers C41 and C42 to bring the signal generator signal up to the frequency. It is about 5 m.c. off.

:idea: Any thoughts will be sincerely appreciated.

Paul


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 Post subject: Re: RF Alignment
PostPosted: Jul Tue 06, 2021 7:10 pm 
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Probably excessive stray capacitance in the circuit. Or dial misalignment.


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 Post subject: Re: RF Alignment
PostPosted: Jul Tue 06, 2021 8:23 pm 
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I don't believe that there is a dial misalignment. All is okay on the lower bands, but I will look at this.

How can I determine if there is any stray capacitance there and accomodate that?

Thanks


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 Post subject: Re: RF Alignment
PostPosted: Jul Tue 06, 2021 8:50 pm 
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The trimmers are there to make up for stray capacitance. Apparently the trimmers for the problem bands don't have adequate range. If you have to open them to their minimum and it's not enough, that means the inductance of the oscillator coil is too much, or the main tuning capacitor has too high a value at the end of its rotation. If you set the dial to the high frequency end and still can't get the oscillator high enough, the capacitor needs to be checked to see if it's opening completely or perhaps the dial needs a bit of shifting. Otherwise there may be some lead dress in the oscillator section that is adding too much capacitance. Perhaps a part has moved out of place.

This is a common problem, or at least I have seen it more than once. Usually slipping the dial a bit takes care of at least some of the error.


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 Post subject: Re: RF Alignment
PostPosted: Jul Tue 06, 2021 11:33 pm 
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Paul,

Are you absolutely sure that the band spread capacitor is set properly during alignment? The band spread pointer must be at the set point for RF alignment which is very near the minimum capacitance setting for that capacitor.

Rodger WQ9E


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 1:46 am 
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Joined: May Thu 27, 2021 6:47 pm
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All,

I have set the band spread pointer at "0" as I did for the lower bands. They aligned well.

I observe a relatively small change in frequency as I move adjust the variable cap. I will look at this again tomorrow to see how much I need to move the band spread pointer to get the frequency to where it belongs on the main dial, if this makes any sense. I also want to try and see how I might access the back of the cap. to see if possibly it would be possible to (1) measure its value, (2) see if I might add an external cap to change the readings.

Possible I can access the osc. coil to measure the inductance.

It seems strange that this happens with two different var.caps.

I'll report back tomorrow.

I do appreciate your help!

Paul


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 1:58 am 
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Paul,

That is the problem, the band spread pointer must be at its "set" point for alignment; otherwise you have far too much capacitance in circuit.

Don't worry about measuring inductance or otherwise monkeying with the set. It just needs to be aligned with the controls set properly.

You will need to realign the lower bands also with the band spread properly set. This does NOT impact the IF alignment so you are OK there but the RF alignment must be done again for those lower bands.

All RF alignment MUST be done with the band spread at the "set" point for proper calibration and alignment.

In normal operation, the band spread dial is at the set point and only when you are using the band spread control should it be moved away from the set point.

Rodger WQ9E


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 2:16 am 
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Joined: May Thu 27, 2021 6:47 pm
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All,

Please forgive this but I must ask.

Ok, to be sure that I am doing this correctly, I have set the band spread pointer at "0" for all adjustments. I set the main dial pointer at the setting spec'd in the manual; in these cases in point, 14, 15 and 30 mc.

Possibly I should try moving the band spread until I hear the signal from the sig gen? Maybe leaving the cap midway and then tweaking it?

Ok or not Ok?

Paul


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 2:56 am 
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Paul,

Look at the logging scale which is at the bottom of the band spread set of scales (on the right side of the set). For alignment, the band spread pointer MUST be on the set marking which is in red on the logging scale. This is the proper point.

With the band spread pointer set to 0, you have the maximum instead of near minimum capacitance for the band spread capacitor which is NOT correct. Adjust the band spread so that the pointer is aligned with the set marking on the band spread logging scale and that is the proper point for both alignment and normal tuning of the set.

Once alignment is complete, if you want to use the band spread to tune 20 meters you would set the main tuning knob so that the pointer is in the circle marked 20 meters on band D and then you would move the band spread tuning to tune down through the 20 meter band. When you are through using band spread, then turn it so that the pointer is once again in the set position.

Rodger WQ9E


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 3:05 am 
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Location: Castro Valley CA
Two things:
As already noted, the band spread must be at the set position.
On the “D” band the oscillator operates low side of the signal. Aligning on the high side might be why you are unable to align. Be sure you are on the correct side of the signal. If you have another communications receiver you should be able to confirm the local oscillator frequency. If you do have another receiver you should detect the LO 455 KC (crystal frequency) below the tuned frequency.
Do you have the Sam’s (set 264 folder 14) for the receiver, available at BAMA?

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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 3:14 am 
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Location: Edmonds, WA USA
You are misreading the manual. The point is to set the signal generator to the actual frequency of the crystal filter which will probably over time be a few khz from the nominal 455 IF frequency. The exact calibration of the signal is not important. Just set it for peak output at approximately 455 khz, WITH the Chrystal filter engaged, turn off the crystal filter and don't tough the generator for the rest of the IF alignment. Service grade generators are never precise in their frequency output but the Chrystal filter tells all. Pretty much all old receivers with crystal filters align this way.

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Real Radios Glow in the Dark


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 3:57 pm 
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:( Ok, I totally missed the <set point> on the right hand scale! I need to start over, which I shall do!

Thanks for tolerating my foolish error. I'll go ahead and redo all of the RF alignment and let you all know my results which I expect to be fine!

Paul


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 Post subject: Re: RF Alignment
PostPosted: Jul Wed 07, 2021 4:26 pm 
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Paul,

The alignment instructions with the NC-98 aren't as clear as some about the need to properly set the band spread control for alignment. It is explained in the operating section of the manual and when written, National likely assumed that anyone aligning the set would have been using it for some time before alignment. Most alignment instructions are very explicit in setting for all required control settings.

Hopefully this clears up the issue you are having with your NC-98. And pay attention to Eric's note about the 98 using low side tracking on its highest band (Range D). Listen with an external receiver and you should find the HFO roughly 455 Khz BELOW the marked dial frequency (i.e. with the NC-98 dial set to 28 Mhz, the HFO should be heard at 27.545 Mhz).

Rodger WQ9E


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