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 Post subject: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 1:46 pm 
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After getting into radio collecting and restoration this year, a lot of things I might have known when I went to electronics school 50 some years ago, but never used in the computer field, have now come back to haunt me.

One example is just how important is frequency precision when doing AM/SW alignment? I have an old signal generator, as well as a newer one. Both have a frequency tuning knob that is basically a ball park setting. So, I have been setting them to an exact frequency with my digital scope which has a frequency counter. That gets to be a hassle.

I'll separate my above question into RF and IF. I would think the exact frequency is less important when doing RF alignment. What's the point of using a frequency counter to get 600kHz, 1400kHz, 15mHz, etc to exactly that frequency when an old radio's tuning dial isn't that precise anyway? Maybe the tuning dial on my signal generator is close enough for that part of alignment?

But how about IF accuracy? Without setting my signal generator with a frequency counter every time, there is no way to be on exactly 456kHz or whatever the IF happens to be. Is this another case where say 454-457 is good enough?

Lastly, is all of this more important when doing FM alignment?

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 2:16 pm 
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this might stir some discussion, but back in the fifties and prior I doubt the accuracy of typical test equipment used with am radios was more than 10% precise, voltmeters possibly excluded. Hence, 456khz instead of 455 khz was probably never noticed. FM from what I can see can use more precision altho early alignment procedures used similar test equipment as AM. I invested in a cheap frequency counter and incorporated it into my 1960s signal generator.


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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 6:45 pm 
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Well first of all just because your frequency counter has six or eight digits does not mean you have to set the generator that exactly.
The accuracy of the IF mostly effects the tuning range. The IF frequency is the difference between the local oscillator frequency and station RF frequency. The input antenna or RF amplifier tuning has to pass the station RF with minimum loss also. So at the end of the dial if the local oscillator can not be tuned far enough (because of the math with the IF) the radio will not be able to receive a station there.
Frequency has been able to be measured more accurately than voltage even back when wave meters were used. I like to get it within three or four digits because I have a counter. It helps also if your generator has a fine control and does not drift vary much.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 7:36 pm 
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Just to clarify what I meant by precision was something like 598-602 for 600kHz, maybe 1358 to 1402 for 1400kHz, and 454-458 for 456kHz. That's as close as I can get on either of my signal generators with just their dials. To get closer I have to hookup my digital scope and try nudging the sig gen dial. Then by trial and error I can get to the whole number.

That's a good point about lack of precision in field test equipment back in the day. I wonder how close the factory got to their specs? In any case, if I'll going to spend time restoring a radio and then keep it, I'd like to get it as close to specs as possible.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 8:05 pm 
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Don't forget the although the centre frequency of your IF maybe at 455kHz it could be 15 or 20kHz wide in total so a couple of kHz either way ain't gonna make much difference! 99% of the time I tune an analogue radio using the Mk1 ear anyway.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 9:31 pm 
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Radios are aligned with signals they are likely encounter in use.

The second factor is the dummy antenna.

A frequency counter differs from a frequency meter.

Frequency meters measure the effect of an AC signal, by selecting
the strongest component. A frequency counter decides the start and
finish of an individual cycle, and times it. The inverse of the time is
the frequency.

The choice of the means to indicate the frequency of the test source
to align a radio is mainly a factor of cost.

As to what a repair facility uses as an alignment source, the main
thing is repeatability. All the radios passing through the facility will
be adjusted the same way. Some radios will have to be serviced using
instruments with calibration certificates.

If no generator was available, but there were a plethora of stations on the
air, and they all appeared at the correct points on the dial, adjusted
by oscillator tracking , then peaking the IF by rocking the dial between
loud and weak stations would make the radio work well.

An interesting read is use of the BC 221 frequency meter.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 9:48 pm 
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Keep in mind that consumer appliance circuits were designed to tolerate up to a 15% deviation in parts and power values.

Aligning a radio's IF's and tuner gang antenna trimmer with an older signal generator or by ear, is equivalent to, "when in Rome, do as the Romans do". Those methods were good enough "back in the day".

If you are a stickler for precision however, then modern day digital equipment would be the route to take, although as posted previously, it most likely won't make much difference in performance.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Tue 19, 2019 10:10 pm 
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fifties wrote:
If you are a stickler for precision however, then modern day digital equipment would be the route to take, although as posted previously, it most likely won't make much difference in performance.


Agreed, but I have to make a comment about trying to use a digital scope on antique analog radios. I have a Siglent SDS 1102X scope. It works great on computer logic, which is digital. But in my opinion it is worthless on old radios. I can hook up my 60 year old Tektronix analog scope and have a display in a couple of seconds. It can take me several minutes to get the same display on my digital scope. Then it disappears if I touch anything. The best way to put it is the digital scope has to figure out what it's looking at before it can display it. That's not easy with an AM radio signal with all kinds of noise and harmonics. My old analog scope doesn't care what it's looking at - it doesn't know, it doesn't care, it just displays it.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 12:21 am 
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Attachment:
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IMG_20170301_211608_resize_54.jpg [ 521.06 KiB | Viewed 279 times ]


The problem I often have with these dials is they'll be accurate on the lower or upper parts of the band,but not around 800-900 KHZ. But not always. Each radio is different.


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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 3:45 am 
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Radio repair men would frequently align the IF to a different frequency to eliminate image interference in their particular locality. As long as it isn't too much different than the specified number, it worked fine.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 4:09 am 
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In most (all/a lot?) of cases, really accurate alignment is difficult (impossible) to achieve. Not all inductors and capacitors are created equal, they all have tolerances and so do all the other components around them so where a particular set of components may have really good alignment, in the next chassis down the line that good alignment may not be achievable.

I had a gummint surplus Marconi R1475 from when I was about 11 years old. It had 4 wavebands to cover 2 to 20MHz, the lower 2 bands were about 4MHz wide and the top two about 6MHz wide, the dial drum rotated under a line on the transparent cover. A pointer followed a spiral cut into the drum which was marked with the frequency in kHz with 600kHz crystal calibration points along each scale. You would have thought that it would be a piece of cake to get this very closely aligned but even so, each radio was hand calibrated and then a wiggly line was painted along the inside of the transparency. Notice the difference in the two wiggly lines on either radio - it's interesting to see the similarity between the bands on each radio, but totally different from radio to radio. I wonder what they used as a standard? I should have asked my Dad or Fred Turner - they both worked at Marconi Chelmsford from 1943 to 1964 although during the war they were quite busy in designing and prototyping the Type A Mk3 spy radio.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 4:53 am 
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majoco wrote:
In most (all/a lot?) of cases, really accurate alignment is difficult (impossible) to achieve. Not all inductors and capacitors are created equal, they all have tolerances and so do all the other components around them so where a particular set of components may have really good alignment, in the next chassis down the line that good alignment may not be achievable.


That also brings up the fact that component values can drift with age. One can almost always increase the gain of either a tube or Transistor set, by tweaking their 50+ year old IF cores.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 5:14 am 
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And... if you can't get it close enough with hi-tech, you might consider these fine instruments.


Attachments:
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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 6:13 am 
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Location: Powell River BC Canada
A two channel digital oscilloscope can be put to good use in
alignment of a radio.

First, read up on the use of an output meter, in the old literature.

Next, connect one channel of the oscilloscope to the loud speaker
or any other audio output point. Set the channel to AC coupling.

Run the signal generator, and see the audio output wave. You will
be looking at the peak to peak deflection as the target.

While doing this the free channel can be set to DC. This probe can
now be connected to the AVC system, any point you choose.

This trace, of course will be a flat line which will indicate by position
what the AVC voltage is.

With a study of the circuit using the DC channel along with AC, and
trying various RF output levels of the generator, delayed action can be
seen.

There is a note in the operating manual for my GR 1001 signal generator.

It mentions that the same voltmeter used to measure output voltage
of a receiver to obtain sensitivity data, should be used to measure the
voltage used as the modulation source voltage for the signal generator.

Of course, a modern two channel digital scope would meet this need.
Attachment:
General Radio GR 1001.JPG
General Radio GR 1001.JPG [ 18.19 KiB | Viewed 222 times ]

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 5:50 pm 
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Lots of deeply technical responses given above. For 99.9% of the old radios that we work on the answer is simple. One question, can you tell the difference on the old radio dial between 1405 and 1410 or 1395? ON 99.9% of the radios we work on the answer to that question is NO !! The same is true if you tune in WWV at 10 mhz or 15 mhz can you tell if that dial is dead on 10 or 15 mhz? At those frequencies on most of our old radio dials you couldn't tell if you were 25 to 30 khz off in either direction. As to the IF frequency if your off 4 or 5 khz again in 99% of our old radios it won't make any difference at all. As to Mid Dial for the most part the engineers designed and built the radio first and then the graphic's guys created the dial to match the frequencies.

On FM yes some parts of FM alignment need to be reasonably close.

Yes I use HP-8640B Signal Generators. When they were new about 40 years ago they were 20K each. Today you can get a good one that works in the $300 price range. Expect to also replace power supply Electrolytic capacitors. Also the nylon gears need to be replaced. Accurate yes but also for the most part accurate way beyond what is usually needed to work on these old radios.
John k9uwa

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 7:07 pm 
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Location: Panama City, FL 32401
fifties wrote:
Keep in mind that consumer appliance circuits were designed to tolerate up to a 15% deviation in parts and power values.

Aligning a radio's IF's and tuner gang antenna trimmer with an older signal generator or by ear, is equivalent to, "when in Rome, do as the Romans do". Those methods were good enough "back in the day".

If you are a stickler for precision however, then modern day digital equipment would be the route to take, although as posted previously, it most likely won't make much difference in performance.


In some Zenith's it may not be possible to tune up to 1600 KHZ. Possibly because of drifting values. It makes me curious how accurate the dial was when it left the factory. We have a station here on 590 KHZ. That helps when doing an alignment. Often to include 1600, 590 has to be much below the 600 mark. You can't always win,but you can get close as possible. Recently I had a Royal 50 I brought out of storage. Its reception was good but could have been better. I did an alignment just by ear,and what a difference. More signals even during the day. The best part was hearing KCWJ 1030,From Kansas City, MO. Never knew they existed and they only run 500 watts overnight!


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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 8:55 pm 
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jburnsctnwd wrote:
And... if you can't get it close enough with hi-tech, you might consider these fine instruments.


No! The chisel and hammer belong to another trade:

Electric motor repair.


My old instructor in motor winding once said, if there is any doubt
about the insulation in the middle of a pole coil, drive a chisel through it,
and look.

All doubt about whether it needs rewinding will be removed.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 9:27 pm 
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Well you know that crystal radios are about as precise on tuning as it gets. :mrgreen:

I use the frequency dial as an approximation.
Then there's the radio stations below the border in the 1960s like XERB (Wolf man Jack) Whose frequency was nothing but wide band splatter.
The little imported table tube radios had no problem with all the stations in southern California (L.A. Basin).
I had aTRF Philco Console that as wide banded as it was could pick up San Diego and San Francisco stations.
And then the dials were still spot on. In my experience anyway.
I never had any test equipment other than a VOM back then and just twiddled with the I.F.s and oscillator with my calibration ears.

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 Post subject: Re: Question about frequency precision during alignment
PostPosted: Nov Wed 20, 2019 9:45 pm 
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Zenith Royal wrote:
fifties wrote:
Keep in mind that consumer appliance circuits were designed to tolerate up to a 15% deviation in parts and power values.

Aligning a radio's IF's and tuner gang antenna trimmer with an older signal generator or by ear, is equivalent to, "when in Rome, do as the Romans do". Those methods were good enough "back in the day".

If you are a stickler for precision however, then modern day digital equipment would be the route to take, although as posted previously, it most likely won't make much difference in performance.


In some Zenith's it may not be possible to tune up to 1600 KHZ. Possibly because of drifting values. It makes me curious how accurate the dial was when it left the factory. We have a station here on 590 KHZ. That helps when doing an alignment. Often to include 1600, 590 has to be much below the 600 mark. You can't always win,but you can get close as possible.

Exactly. Sometimes it's simply about a balance between each end of the dial in order to get a happy medium, where neither is dead on, but close.

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