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 Post subject: Sweep Marker Generator.. Question re: use
PostPosted: Feb Sat 19, 2011 4:10 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
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Location: North Dakota
I have never used a Sweep marker generator before, so bear with the noob questions :)

I have recently acquired one and have tested the basic functions and found it to be functional (as far as I can tell). My testing consisted of looking at the output of the marker and sweep generators on an oscilloscope and everything appears to function properly. When looking at the marker output, I see a sine wave which changes in frequency appropriately with the use of the range and frequency controls. The amplitude of the sine wave changes appropriately when the marker-size knob is turned. As far as the sweep generator output, I am not sure what the correct appearance on the oscilloscope should be, but this is what I see: With the oscilloscope horizontal sweep set at slow speed, I see a flat line alternating with a rectangular box something like this ----|||||----|||||----- . The width of the rectangular box changes appropriately when turning the sweep-width knob and the height varies when the step and fine attenuators are turned. When the sweep output is fed back to the input of the sweep marker generator and the horizontal and vertical outputs are connected to the oscilloscope in X-Y mode, I can see the marker pips on a flat line. So far so good.

Now for my question: When I connect the sweep output to a 455 kHz IF transformer and feed the output from the transformer back to the sweep generator, I still see a flat line with the marker pip just as when the sweep output is directly fed back to the generator. I was expecting to see a curve with the peak around 455 kHz. I tried varying the center frequency on the generator but still see only a flat line. What am I doing wrong? If using an IF transformer is not appropriate to test the functioning of the generator, what device can I use to obtain the curve with the marker pips that I have seen in pictures?


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 Post subject:
PostPosted: Feb Sat 19, 2011 4:49 pm 
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Model of sweep Gen? There is a world of sweep Gens.
Do you have a manual for the unit. Functional test are there.
There is a ton of info on these forums. Check archives.

Do you have a RF probe (detector or demodulator probe)?
To properly test you no doubt need one of these.

A few of your comments sound find and a few I don't
understand. Cheers.

Don_S wrote:
Now for my question: When I connect the sweep output to a 455 kHz IF transformer and feed the output from the transformer back to the sweep generator, I still see a flat line with the marker pip just as when the sweep output is directly fed back to the generator. I was expecting to see a curve with the peak around 455 kHz. I tried varying the center frequency on the generator but still see only a flat line. What am I doing wrong? If using an IF transformer is not appropriate to test the functioning of the generator, what device can I use to obtain the curve with the marker pips that I have seen in pictures?


What radio? Schematics? What tet points? Typically you inject at the grid of the tube, before the transformer or ant side. You pick up the signal after the transformer (detector side) with a RF probe, which goes to the scope (assuming you have a post injection marker signal/marker Gen). Pictures, sketch info would help.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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 Post subject:
PostPosted: Feb Sat 19, 2011 6:28 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
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Location: North Dakota
The sweep generator is a Leader LSW-250. I do not have a demodulator probe.

I was not testing the IF transformer in a radio. The test was performed on a spare IF transformer that I had on hand. That shouldn't make a difference, should it? The frequency response of the IF transformer should be the same whether it is in the radio or not, No?


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 Post subject:
PostPosted: Feb Sat 19, 2011 6:34 pm 
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IIRC the Leader LSW-250 is a TV-FM sweeper. Its output would be in the wrong frequency range for a 455 kHz IF transformer.

Bob


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 Post subject:
PostPosted: Feb Sat 19, 2011 7:41 pm 
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Location: North Dakota
Bob, you are absolutely right... I just found some specifications on this thing and the frequency range is 2-250 MHz. The dial is somewhat misleading though because it is labeled 0-250 MHz and therefore I assumed that it would cover the 455 kHz. Subsequently I did try the same experiment with a 10.7 MHz IF transformer but was still unable to obtain the response curve - I am still getting a flat line with the marker pips.


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 Post subject:
PostPosted: Feb Sat 19, 2011 7:47 pm 
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gmcjetpilot wrote:
... Pictures, sketch info would help.


Here is how I have it hooked up..

Image


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 Post subject:
PostPosted: Feb Sun 20, 2011 4:58 am 
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Don_S wrote:
The sweep generator is a Leader LSW-250. I do not have a demodulator probe.

I was not testing the IF transformer in a radio. The test was performed on a spare IF transformer that I had on hand. That shouldn't make a difference, should it? The frequency response of the IF transformer should be the same whether it is in the radio or not, No?

Yes Freq response is the same but the impedance is not. By connecting to the primary with the signal gen you detune it. In the radio you don't connect to the primary of the transformer, you connect to the tube before it. It kind of works but you will not get a pretty classic band pass response. Also as was pointed out not many sweep gens will go down to 455 Khz. This is for FM radio and TV.

There was a recent post on RF Probes. Cheers.

As to your picture I got it! This is a solid state version of the Eico 369. It is post injection marker which is good. Should work well. If you don't have the manual for it, down load the manuals for both the Eico 368 and 369. The 369 adds post injection markers to the 368. These manuals (BAMA) will help you greatly, but try and get the original manual. It is a great help when learning. Also you should get a FM tube radio to practice one WITH VISUAL ALIGNMENT INSTRUCTIONS. I find that Sam Photo Facts to be invaluable for this. Some of the factory instructions give you the "Visual Method" but most do not, especially Zenith.

To TEST there should be a TEST SET UP in the manual. Just looking at your picture, you would replace the TEST POINT OUT cable going from the DUT to the LSW-250 with a demodulator RF probe. The DUT is not there. Then connect the RF probe from the SWEEP CABLE OUT to the RF probe. You should get a rectangular box on the scope with a marker. This should give you some info as far as the test set up. (This is for the Eico 368 and does not have post injection markers, but it shows you were the "demodulator" or "detector" or RF probe goes.)

antiqueradios.com/forums/viewtopic.php?t=129357

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


Last edited by gmcjetpilot on Feb Sun 20, 2011 6:52 pm, edited 1 time in total.

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 Post subject:
PostPosted: Feb Sun 20, 2011 6:35 pm 
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Location: Long Island
The issue here appears to be that you are feeding the swept RF output of the signal generator (with or without the IF coil in line) into the oscilloscope. If the scope is capable of responding to the RF frequency directly, you'll only see the RF, not the characteristic curve of a tuned circuit.

The input on the generator is marked "Rec TP" (receiver test point), which is usually after the demodulator or detector stage in the radio or TV. At that point, the RF is gone, and there should be a DC voltage that varies in amplitude at the sweep rate. The amplitude at any particular instant corresponds to the frequency response of the circuit you are sweeping, so the beam of the scope traces out the frequency response of the circuit over the range of the sweep.

If you want to use this unit to test coils, you need to provide an external detector or demodulator. The subject has been covered in great detail recently on ARF, so if you do a search on "demodulator probe" or "detector probe," you should get plenty of good background material. These devices can be nothing more than a germanium diode and a resistor, or they can get quite elaborate. They're available commercially as well.

To verify if this is the problem or not, try injecting a small amount of DC into the "Rec TP" jack with the scope running. 1.5-volts DC from a penlight battery should be sufficient. What should happen, when you connect the battery, is the trace on the scope will jump up or down vertically. It's OK if the trace doesn't stay up or down, because the circuits may well be AC coupled, and once you've charged the capacitors with the battery, nothing more is going to happen.

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

Thomas A. Edison


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 Post subject:
PostPosted: Feb Sun 20, 2011 7:32 pm 
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Joined: Jul Wed 22, 2009 8:32 am
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Go to the BAMA web page and down load the Eico 369 manual

On Pdf page 8 (page 7) you will see the test set up.
On Pdf page 9 (page 9) you will see the classic test pattern.

The test pattern is a flat line. Unlike you previous test where you see the Sine wave getting closer, low Freq changing to high, the RF probe converts it all to DC. Thus you get a flat line. This level amplitude, level across the Freq sweep the key or characteristic of a well working seep Gen. Any slope will cause distortion in your band pass. The wider sweep width and flatter the better. This is as perfect as it gets (forgot the sweep freq and width but it's pretty):

Image

This is an older tube sweep, RCA WR-50B (it is as good as manual)
Image

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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 Post subject:
PostPosted: Feb Sun 20, 2011 10:04 pm 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1524
Location: North Dakota
Thanks gmcjetpilot and Chris108 for very informative posts. I went ahead and built myself a simple demodulator and was able to get a test pattern that looked somewhat, but not quite like the one shown in gmcjetpilot's photos. The rectangle in mine is not quite so well formed. Both the top and bottom lines are somewhat irregular and wavy but overall has the rectangular pattern and I can see the marker which moves to the right or left when the marker frequency is changed. So I have verified that the sweep generator is indeed working, albeit not perfectly. The next step is to see if I can get a bell-shaped response curve using an FM IF can (outside a radio)


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 Post subject:
PostPosted: Feb Mon 21, 2011 1:47 am 
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Joined: Jul Wed 22, 2009 8:32 am
Posts: 3805
Don_S wrote:
The next step is to see if I can get a bell-shaped response curve using an FM IF can (outside a radio)

Read what I wrote above and suggested. You are wasting your
time with sweeping the IF transformer outside the radio. I have
heard people suggest it, but never seen it done. I suspect people
who suggest it never did it. I tried it and it did not work. I would
love to be proven wrong. You will see some kind of response, but
nothing like a band-pass, due to the impedance mismatching.

In the radio you input at your converter tube grid or the grid of the
first IF amplification tube. The primary is seeing super high
impedance. Your signal gen is 50 ohms and becomes part of the
transformer if you connect directly. When you slap that 50 ohm
signal right on the coil you de-tube it. When you want to test a
sweep and see a bandpass with-out a "live radio", you will have a
band pass FILTER with a 50 ohm impedance.
I am sure you can
make a LC circuit to do this (from the HP 8601A Signal/Sweep Gen
manual).

Image

Commercially made filters:
http://www.allenavionics.com/LC_F/BP400-5M.htm

The oscilloscope is also not high impedance, 1M ohms. If you pick
up right on the coil it affects it, but to a much less extent. Normally
you pick-up, with your RF probe, right after the limiter tube,
depending on the FM radio, before the discriminator transformer. If
the FM radio has a ratio detector, it's done slightly different. This
is where my #1 advice comes in. Don't wing it!


If you want to accelerate your learning trust me, get yourself a
AM/FM radio and the alignment instructions for that radio, with
visual alignment procedures, aka sweep alignment. If you get a
AM/FM Zenith, get the Sam Photo Fact for it.

You can not align a radio IF transformer outside of the circuit it
was designed to work in, at least meaningfully. It is useless to
align an RF transformer in your hand. An IF transformer in your
hand can do nothing useful. When you ALIGN that means all the IF
transformers and parts must work together. That means you have
to TUNE or ALIGN them in the circuit with voltage on it. They are
dependant on each other. Again I could be wrong, but I tried it. It
was not meaningful. I would test a transformer outside the radio
with my better equip I have now, but I don't a IF transformer to
test. You can use a STANDARD single Freq signal Gen and just peak
the response of a transformer for ball park adjustment.

Image

Forget sweeping AM it can be done, but not with your rig. Also
sweep alignment does not result in dramatically better alignment,
but it does have some advantages, you can play with gain and
bandwidth. Regardless it is moot you can't do it with that sweep Gen.


Image

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


Last edited by gmcjetpilot on Feb Mon 21, 2011 3:28 am, edited 2 times in total.

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 Post subject:
PostPosted: Feb Mon 21, 2011 2:12 am 
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Joined: Dec Sun 02, 2007 3:20 pm
Posts: 1524
Location: North Dakota
Your point is well taken. I was not going to sweep the IF coil outside the radio to peak it, but just to look at a response curve. The only tube FM radio that I have, I cannot find the model number (only got the chassis) and therefore cannot locate any documentation on it, let alone visual alignment instructions.

Are you using a Balun to connect the sweep generator to the radio? ( I am referring to the black thingie with white lettering and the red, blue and black alligator clips)


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 Post subject:
PostPosted: Feb Mon 21, 2011 2:49 am 
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Another issue is the phase adjustment between the marker generator and the scope, which gmcjetpilot went over at length in a series of posts last year. Older scopes had an adjustment for this, but newer ones generally don't. Without it the marker won't be accurately positioned in the sweep output indicated on the scope.

Bob


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 Post subject:
PostPosted: Feb Mon 21, 2011 3:30 am 
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Joined: Jul Wed 22, 2009 8:32 am
Posts: 3805
Don_S wrote:
Are you using a Balun to connect the sweep generator to the radio? ( I am referring to the black thingie with white lettering and the red, blue and black alligator clips)

No that is the RF detector or demodulator Probe. The signal is the alligator with the blocking capacitor.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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 Post subject:
PostPosted: Feb Mon 21, 2011 3:46 am 
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Joined: Jul Wed 22, 2009 8:32 am
Posts: 3805
sofaslug wrote:
Another issue is the phase adjustment between the marker generator and the scope, which gmcjetpilot went over at length in a series of posts last year. Older scopes had an adjustment for this, but newer ones generally don't. Without it the marker won't be accurately positioned in the sweep output indicated on the scope. Bob

He is set there, the Leader LSW-250 is 1970's vintage solid state
and has the sweep sync output, made for syncing the scope in the
X-Y mode. The one you are thinking of is the RCA WR-50B, which
has no sync so to speak, it is "line sync". I was able to work that
out with a tap on 1.5 volt AC sync signal. I used that for the X part
of the X-Y mode. It worked pretty well. There is a sweep sync pot I
became aware of to "calibrate it" and work as designed. However
it was a bit non-linear and needed to be calibrated to be in phase
as you say. I never had a proper 10.7 Mhz crystal to get a marker.
I never had a 455 Khz. Trying to mix another signal Gen in the mix
for a marker was a pain. It worked, but too much fiddling for me. I
never had confidence with it, but it's a nice little unit for what it is.

Image
Image

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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