Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Sat 15, 2018 5:28 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 16 posts ] 
Author Message
 Post subject: X-Y IF resonance plotting
PostPosted: Oct Mon 29, 2018 10:08 pm 
New Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 16
Location: San Rafael, CA
I am not understanding my results…
My understanding of seeing a frequency plot on my scope using x-y sweep mode is to input a sweep, hi and low of the expected resonant frequency and there it will be… adjust as needed and away you go.
Extrapolate to reality (all is unplugged of course) and you connect the sweep input to the input of an IF can and attach the output to your scope and you should see a nice display centered (or fairly close to) the IF frequency.
What I am seeing is MILES from the IF. IF should be 460 khz and I’m seeing a 275 khz resonance plot.
What am I doing wrong???
Thanks,
-Marc


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Mon 29, 2018 10:19 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 33570
Location: Maryland 20709, USA
Your IFT might be 275kc. Frequencies in that range were used in early superhets.

Is the transformer connected with input and output tubes installed?
Those will change the resonant frequency, but making it lower, not higher.

- Leigh

_________________
73 de W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Mon 29, 2018 10:21 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 3618
Location: Littleton, MA
Couple the sweep signal output to the IF can input through a high-value resistor right at the IF can, to isolate the input from the cable capacitance (same trick as a VTVM, except in reverse).
Couple the IF can output to the detector or scope input with a high-value resistor right at the IF can, to isolate the output from the cable capacitance.

You'll lose quite a bit of signal using the resistors, so you'll probably have to turn up the scope sensitivity and the generator output.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Mon 29, 2018 10:39 pm 
New Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 16
Location: San Rafael, CA
Leigh;
The schematic has 460khz as the IF. Both the first IF and the second IF are displaying the same.
The first IF has been removed because it was not attached (????) the second is in the set. All tubes are out of the set.

Steve;
I'll give that a try.

Thanks folks,
-Marc


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 3:37 am 
Member
User avatar

Joined: Oct Sun 15, 2006 12:57 pm
Posts: 5941
Location: Liberty, Missouri
What make and model radio are you working on, and what test equipment are you using?

If both cans are showing the same resonant frequency, my guess would be, the schematic must be wrong.

As Steve pointed out, you do want to isolate the signal generator from the I.F. can's tank circuit with a resistor, and depending on the configuration of the output coupling, either a second tank circuit, or a simple link winding, you would want to do the same on the output.

I use a HP3336 generator so I just punch in F-start, F-stop, and the sweep rate, and, I use an Analog Devices AD8307 Log converter IC on the output side.

The resulting displays LOOK PRETTY NICE.

_________________
Pics, Bold & underlined text, are usually links.

https://mikeharrison.smugmug.com/

Mike


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 4:26 am 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3460
Location: Florida
Mikeinkcmo wrote:
..... What make and model radio are you working on, and what test equipment are you using?

If both cans are showing the same resonant frequency, my guess would be, the schematic must be wrong. ..........


A quick test with a second radio will verify the IF frequency.

RRM


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 5:47 am 
New Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 16
Location: San Rafael, CA
The radio is a Gilfillan Bros Inc. 52A Late (AA5)
The scope is a HP2235A and the sweep gen is a 3325A.
The IF has 1.38mh coils (in and out) paralleled with variable caps 30-60pf (or something in that range)
Calculating the resonant freq puts it at 600-800khz!
fo = 1 over 2 pi * sq root LC, if I remember, but I used an online calculator of course…
I am directly connected to the input wires from the IF can with coaxial clip leads (probe & BNC-alligator lead). As I mentioned, the first IF is out of the radio and the second IF is in the radio.
My scope image is the full envelope as I am not using a demod probe. I have tuned both of them at 275khz center freq. I have opened up the scan from 1khz to 1Mhz to check for harmonics. The only response is at 275khz on BOTH IF cans. The schematic has the IF freq listed as 460khz. 200khz off???
So… What is being suggested is to isolate the IF can better?


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 11:00 am 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 10460
Location: Powell River BC Canada
The radio connects the first IFT in series with the oscillator transformer, BEFORE
connecting to plate.

Put the tube and transformer back in, and connect sweep to G1 of first detector.



Attachment:
Gilfilian connects oscillator transformer in series with 1 IFT BEFORE PLATE.JPG
Gilfilian connects oscillator transformer in series with 1 IFT BEFORE PLATE.JPG [ 64.92 KiB | Viewed 424 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 11:09 am 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8068
Location: Long Island
You don't want to "isolate" the IF can. It has to have the same shunt and series impedances (R, L, and C) connected to both windings as it would have in the radio, otherwise all bets are off. Easiest way to do that is in the radio, injecting the sweep into the IF or converter stage, and taking the output from the radio's detector. One of the advantages of a demodulator probe (if you are working with a loose IF on the bench) is that it puts a load on the transformer similar to what it would see in a radio.

Does the radio work at all? If so, it might be worth starting from a simpler place; i.e. if you know the radio lines up at 460 kHz and works normally, then the IFs cannot be bad or misaligned.

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

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 12:53 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 24069
Location: Annapolis, MD
If you want to test tuned circuits on the bench, you have to have interfaces that match the circuits in the set. That said, it's still maybe surprising it would be off that much. Just using "manual sweep", I've been able to verify if a transformer was in the ballpark before putting it in the set. I simply connected the generator to the input through a resistor, and monitored the output with a 10X probe. If motivated, one could compare the probe capacitance to what the transformer sees in-circuit, and make a correction.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 1:18 pm 
Member
User avatar

Joined: Oct Sun 15, 2006 12:57 pm
Posts: 5941
Location: Liberty, Missouri
Sweeping should be done on working assemblies. In the case of receivers, the front end can be swept, or the I.F. strip(s), and should be done as a block.

This, in general, is the way I set up the equipment to sweep I.F. strips. You have to keep track of the input levels, to keep the entire assembly operating within it's linear range. Don't know about the 3325, but the 3336 sweeper only goes down to -70dBm and sometimes an additional attenuator is also needed.

The local oscillator is disabled, as is RF input from the preceding stages.

Image

And I use scope probes to make the connections in the radio.

Image

_________________
Pics, Bold & underlined text, are usually links.

https://mikeharrison.smugmug.com/

Mike


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 2:12 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4729
Location: Montvale NJ, 07645
mbruvry wrote:
I am directly connected to the input wires from the IF can with coaxial clip leads (probe & BNC-alligator lead).

Your test equipment is becoming part of the tuned circuit and throwing you off.
You need to follow what Mike outlined above. Connecting to the mixer tube is standard procedure.

Often you can test tuned circuits by loosely coupling, like just clipping leads OVER the wire insulation or laying the lead parallel to a wire in the circuit. The idea is to couple as loosely as possible so as to not throw off the resonant frequency. I can use my HP 3585 spectrum analyzer to peak an IF by just clipping the tracking generator to the mixer grid input lead insulation. If the output of your sweep generator doesn't go low enough, try clipping to the insulation.


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 3:09 pm 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 10460
Location: Powell River BC Canada
Also, watch out when using equipment that never was intended for use
with vacuum tube devices. A slip of a probe could damage an input in
a flash. Old tube test gear is far more robust.

You could get away with blowing a chunk of metal off a test lead.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 3:24 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 3618
Location: Littleton, MA
mbruvry wrote:
I am directly connected to the input wires from the IF can with coaxial clip leads (probe & BNC-alligator lead). As I mentioned, the first IF is out of the radio and the second IF is in the radio.
My scope image is the full envelope as I am not using a demod probe.

To be very clear:
The 50 ohm coax cables, since they are unterminated, present a considerable capacitance at their clip leads. You are connecting this capacitance in parallel with the IF transformer windings, so of course it lowers the resonant frequency.

If you desire to simply see the response of the IF transformer alone, you need to isolate the cable capacitance from the IF transformer. On the scope side, using a demodulator probe will do this, or use a 10X scope probe.. On the signal generator side, use a high series resistance or small capacitor to couple the end of the cable to the IF transformer.

If you want to measure the actual response of the IF transformer in the circuit, then you'll have to do as the others have explained, and measure it in-circuit in the receiver, with the tubes installed. You'll still need the demodulator probe or the 10X scope probe, but others have said, you can connect to the grid of the mixer to isolate the capacitance of the cable on the signal generator side.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Oct Tue 30, 2018 4:49 pm 
New Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 16
Location: San Rafael, CA
Such knowledge and experience!
This, as has many times before been exclaimed, is why many come with our questions!

I will install the loose IF transformer that I discovered 'floating' in this radio, re-cap, replace out of tolerance resistors. Take a good look around at everything and fire it back up (well, bring it up slowly on a variac) and see what else I've missed.
I'll also get out my demod probe and follow Mike's advice.

So, I've got some 'homework' to do...

Thanks again all.

-Marc


Top
 Profile  
 
 Post subject: Re: X-Y IF resonance plotting
PostPosted: Nov Fri 02, 2018 3:25 am 
Member

Joined: Jul Thu 29, 2010 5:35 am
Posts: 1005
Location: Simonton, Texas
This is a very informative thread and I have learned a lot from it. Thanks for the info guys.

_________________
Knowledge is useless unless you can share it. KA5OBI


Top
 Profile  
 
Post New Topic Post Reply  [ 16 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB