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 Post subject: RF probe for radio work on oscilloscope, best design
PostPosted: Oct Wed 28, 2009 4:12 am 
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What is your favorite RF demodulator circuit for Oscilloscope input (1 Meg, 25 pF)? :?:

Researching this I found many designs for RF probe. I made two with stuff I had on hand and compared. Here are five that look promising (ignore periods). I tested and compared the first two crudely.

Dual Diodes RF Demodulator (tested see pics below)
.....C1.................D2
<----||---------->|-------------------- (+)
............|.......................|
.....D1.____...................___.C2
.........../\.....................___
............|.......................|
<--------------------------------------- (-)
C1 = 20 to 47 pf (I used 38pF & 68pF)
C2 = 50 pf
D1 = 1N60 or 1N34 (used 1N34)
D2 = 1N60 or 1N34 (used 1N34)

Simple RF Demodulator (tested see pics below)
.....C1.....................R1
<----||----------/\/\/\/\/\/\------------ (+)
...............|
......D1...____
............../\
...............|
<-------------------------------------- (-)
C1 = 0.01 uF (recommended, tried other values as well)
R1 = 430K (430K is suppose to be for 1 Meg input)
D1 = 1N34

Recommend by ARF member Chris (from circuits book)
.......C1
<----||-------------------------------- (+)
..................|....................<
.........D1...____..................> R1
................./\....................<
..................|....................>
<-------------------------------------- (-)
C1 = 50pF
R1 = 20 Meg ohm 1/2 watt
D1 = 1N60 (or 1N34)

Internet Demodulator
.....C1...................R1
<----||---------/\/\/\/\/\/\------------ (+)
...............|.........................|
......D1...____.....................___ C2
............../\........................___
...............|.........................|
<------------------------------------- (-)
C1 = 150 pF (seems high)
C2 = 0.001 uF to 0.1 uF (that is quite a range?)
R1 = 1 Meg
D1 = OA79 or 1N34

Like RF Heath Kit probe modified slightly
.....C1...................R1
<---||------------/\/\/\/\/\/\-------------- (+)
............|.....>.........................|
............|.....<........................___
......D1..\/.....>..R2..................___ C2
.........----....<.........................|
............|.....>.........................|
<------------------------------------------ (-)
C1 = 100 pF
C2 = 0.001 uF
R1 = 1 M
R2 = 68K
D1 = 1N60 or 1N34


----------- Here is the "Dual Diode" & "Simple" RF Demod comparison -----------
>Circuits fed into regular X1 scope probe.
>The signals are AM and FM radio stations.
>Test point Grid into 2nd IF tube (Zenith H725).
>For comparison Ch 1 shows a straight X10 probe.
>Changing the input capacitor did not seem too critical.
>No effort was made to shield wires or the proto board.
>The Photography is not high speed so keep that in mind.
>Fed demodulator to scope w/ standard X1 probe (~90 ohms)

------------ AM signal comparing two different RF demodulator's ------------
Image

------------ FM signal comparing two different RF demodulator's ------------
Image

------------ Faster horz time rate 1 uSec (Ch 1 - shows IF freq 455Khz) ------------
Image

------------ Faster horz time rate 0.10 uSec (CH 1 - IF freq of 10.7Mhz) ------------
Image

You can't take pictures of a scope very well, but it gives you an idea. Even looking at it real time, it's hard for me to make a judgment based on what I saw, which RF demodulator is better. Clearly the dual diode has more output at high IF freq (FM 10.7Mhz) than the "simple" RF demod, which also had some spurious spikes at high freq. The lower output of the "simple" RF demodulator at higher freq, is no doubt due to the 0.01 capacitor. For AM (455Khz IF) they the simple RF demod may be better.

I did play with different probe input capacitors. From what I read 100pF is about as big as you want to go. However the 0.01uf on the "Simple" RF demodulator seemed to work fairly well, at least at lower freqs. Trying a 68 or 50pF did not change it much.

A lot of designs are for signal tracers, VTVM's or DMM's which have input impedance of 10 or 11 meg, scope is only 1 Meg. I'll make the other two designs but need some big resistors. Than I'll pick one and stuff it in a shielded housing with the probe right there, as it should be. I'll use 50 ohm coax to a BNC. I would like to know the math behind this. Any EE's out there have the math behind it.

From this you can see that a straight X10 probe will work for troubleshooting, although I'm not sure about the AFFECT on the tested circuit. The main reason I want an RF probe is to do FM alignments with a sweep generator.

Do you have any suggestion of your favorite DIY RF demodulator probe for a scope, good freq range, least affect on tested circuit?


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 Post subject:
PostPosted: Oct Thu 29, 2009 12:05 am 
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Maybe it is me, But I have never had a need for a 'scope demodulator probe. Pete


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 Post subject:
PostPosted: Oct Thu 29, 2009 12:30 am 
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Peter Bertini wrote:
Maybe it is me, But I have never had a need for a 'scope demodulator probe. Pete

Yea that is what I am wondering. However for FM radio Alignments with a sweep generator you (apparently) need a RF probe, other wise you don't get the trace on the scope.


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 Post subject:
PostPosted: Oct Thu 29, 2009 1:06 am 
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There is nothing to demodulate on a swept signal :wink:


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 Post subject:
PostPosted: Oct Thu 29, 2009 2:06 am 
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Peter Bertini wrote:
There is nothing to demodulate on a swept signal :wink:

Not to disagree but the procedures to do a FM radio alignment with a sweep/marker gen, per books, sweep generator manuals (RCA, Eico, Wavetek), specify YOU MUST feed the scope Vert with a demodulated signal (through a detector or demodulator):

The Eico 368 and 369:
"Demodulated output of device is returned via the demodulator cable supplied (incorporating low-pass filter)" - This cable is always missing, but they show you how to make this low pass filter RF demodulator in the manual**.

Wavetek 1801a:
"After the RF signal passes through the RF circuit, of the device under test, it must be demodulated before being connected to the DEMOD IN of the sweep Gen."

RCA WR-50B:
"For any application using sweep signals, use a detector probe for the oscilloscope, since it permits observing the sweep output directly from the generator."

Correct me if I am wrong. Have you done FM alignments with a sweep generator? If you have let me know how you did it and what gear you used.


** Eico 368

..................R1....................R2.....................D1
<---------/\/\/\/\-------/\/\/\/\/\/\----------|<------------------ (+)
.......|..................|.......................|......................>
.C1.___.........C2..____..............C3.___...................<.R3
.....___...............____..................___...................>
......|....................|.....................|......................<
<---------------------------------------------------------------- (-)


R1 = 47K
R2 = 22K
R3 = 47
C1 = 0.001 uF
C2 = 47 pF
C3 = 47 pF
D1 = 1N60 or 1N34

The actual demodulator/filter part is C2, C3, R2 and D1


Last edited by gmcjetpilot on Oct Thu 29, 2009 5:16 am, edited 4 times in total.

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 Post subject:
PostPosted: Oct Thu 29, 2009 2:14 am 
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Not to be picky :wink: But technically, "demodulation" means what it says, to recover modulation from the carrier. The sweep generator output is an unmodulated carrier. You can detect the carrier, and recover a DC level that represents the amplitude of the carrier to drive the scope's vertical channel.

When you mentioned demodualator, I took it as a device that would recover the audio or video waveform from a receiver.

Pete


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 Post subject:
PostPosted: Oct Thu 29, 2009 2:20 am 
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Peter Bertini wrote:
Not to be picky :wink: But technically, "demodulation" means what it says, to recover modulation from the carrier. The sweep generator output is an unmodulated carrier. You can detect the carrier, and recover a DC level that represents the amplitude of the carrier to drive the scope's vertical channel.

When you mentioned demodulator, I took it as a device that would recover the audio or video waveform from a receiver.

Pete

Right I see what you mean. Thanks for the clarification.


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 Post subject:
PostPosted: Oct Thu 29, 2009 5:37 am 
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The demodulator probe was often used in sweep alignments to remove the carrier. If you do this, then basically the scope only needs to be good enough for audio frequencies. If your scope has enough bandwidth to view the carrier, its not technically necessary to use a demodulator probe.

If you don't use one, the area under the curve will be filled in by the carrier. I like to remove the carrier, so I use one.

Probably any of those you tried will work fine. Set up sweep alignment on something with a response curve (like an IF amplifier) if you really want to compare them.

Demodulator probes for a VTVM or DMM are a different can of worms........

John


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 Post subject:
PostPosted: Oct Thu 29, 2009 5:28 pm 
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AM Demodulator probes were more or less a workaround for the old scopes that couldn't display RF, or sync the trace even if they could display it.

I've had no experience with FM so can't comment there.


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 Post subject:
PostPosted: Oct Thu 29, 2009 9:43 pm 
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blue_lateral wrote:
The demodulator probe was often used in sweep alignments to remove the carrier. If you do this, then basically the scope only needs to be good enough for audio frequencies. If your scope has enough bandwidth to view the carrier, its not technically necessary to use a demodulator probe.

If you don't use one, the area under the curve will be filled in by the carrier. I like to remove the carrier, so I use one.

Probably any of those you tried will work fine. Set up sweep alignment on something with a response curve (like an IF amplifier) if you really want to compare them.

Demodulator probes for a VTVM or DMM are a different can of worms........John

I should make a RF probe for the VTVM as well. Why not. Interesting, info thanks. That puts it in perspective. I am pretty close to doing (attempting) my first sweep alignment, but need a few more cables. I'm not going to be so crude with my next iteration of RF probes. I have to stuff the circuit into a shield container with the probe sticking out of it and connect it with prober cable and BNC connector, verses unshielded wires. I can make a prototype and swap out the circuits and compare. When I find the best one, I'll make a permanent dedicated probe.

Alan Douglas wrote:
AM Demodulator probes were more or less a workaround for the old scopes that couldn't display RF, or sync the trace even if they could display it.

I've had no experience with FM so can't comment there.

Interesting! :D Thanks I have a "modern" scope, and the plan X10 is a pretty good tool. The demodulator assists in seeing the response curve and markers as I read, I'm sure.

I wounder if a properly designed RF probe could be another TOOL to use in trouble shooting if used properly for the right application. For typical AM radio Freqs a RF probe is less useful (I think?). With FM radio (VHF), the oscillator is in the 98-118Mhz range (not .955-2.1 Khz), plus or Min's. From my "tests", a RF probe seems to show more response in VHF?

A good goal is to NOT affect the circuit being measured. I'm not sure but I think an RF probe can do that better? A X10 into a 1 Meg scope is may be not ideal?

Besides FM alignments with a sweep gen, making a probe that has less affect on the circuit being tested would be a bonus. From my above experiment, a X10 probe was about the same, may be not quite as good, as the RF demodulator. There was some affect with both at the test point I used (grid input to 2nd IF). However with a properly made probe (no long unshielded wires to the bread-board) it might load the circuit even less. I think there is a good chance of that.

They also have ACTIVE probes, with Op-Amps or FET transistors right at the probe. Their ability to not load the circuit is even better I hear. It has infinite impedance. They are better for ultra high freq and digital signals, but I'd think it would benefit work at lower freq as well, giving similar benefit? There are plans for those floating around for active probes. Seems simple to make one. They just have a few active components. It needs a small DC power supply, battery or wall wart.

There is nothing like experimentation. I'll make one or two RF probe designs, can't hurt. I'll try them. See what works and doesn't work. There is some benefit to an RF probe, for the right application (I think?), just not sure it is worth it or not. From what you all say, you can live with out it. :wink:

Since I'm more focused on FM radios late 40's and 50's a RF probe will help I think. I should make one for the VTVM as well.


Last edited by gmcjetpilot on Oct Fri 30, 2009 1:10 am, edited 1 time in total.

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 Post subject:
PostPosted: Oct Fri 30, 2009 1:07 am 
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When doing a sweep alignment of the Fm IFyou are using the detector in the radio to send to the scope and the scope is set a the 60 hz setting. On the Sencore SG-165 when doing the 10.7 sweep you use the direct isolated probe connected to the detector load resistors in the radio( the electrolytic has to be temporarily disconnected) to get the signal for the scope. The Sencore has built in markers sent into the mixer making it easy to see where the peaks should be.


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 Post subject:
PostPosted: Oct Fri 30, 2009 7:55 am 
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Lou deGonzague wrote:
When doing a sweep alignment of the Fm IFyou are using the detector in the radio to send to the scope and the scope is set a the 60 hz setting. On the Sencore SG-165 when doing the 10.7 sweep you use the direct isolated probe connected to the detector load resistors in the radio( the electrolytic has to be temporarily disconnected) to get the signal for the scope. The Sencore has built in markers sent into the mixer making it easy to see where the peaks should be.

The Sencore SG-165. There was one for sale on ARF classified. I could have bought it, but decided to pass. I should have bought it. I have markers on my other ones, but the RCA is only a single marker with crystal (which I have). The Eico and Wavetek I can add a marker with another signal gen, which I have. The EICO also has internal marker gen, so I hope to get at least one or two markers. Thanks for the tip, this is good to know. Disconnecting the electrolytic is mentioned for factory alignment on my RCA 8R71. The Zenith Radios do not get into alignment with a scope, but allude to it as preferred, while giving the single Freq/VTVM method.

I just did this. I had to rebuild an I.F. (455Khz) due to intermittent problem (note cracked capacitor film)
Image

Replacing dead caps and a cracked slug (so it was way off & need to be set to 455Khz)
Image

RCA wr-50b (Sweeping 455Khz) with scope & straight X1 probe - SWEEP not single Freq
Image

With my DIY RF demod circuit (out of frame) - SWEEP not single Freq
Image

It was easier to set the IF with an "RF probe" (my crude setup). This is what it looked like when diddling the IF and I was too high or too low.
Image
Image

I did not see this with the straight probe, which gave that cool ribbon. With the RF probe I had to set the scope to DC. With a straight 455Khz signal (not a sweep), the probe made little or not difference.[/img]


Last edited by gmcjetpilot on Oct Fri 30, 2009 10:01 pm, edited 1 time in total.

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PostPosted: Oct Fri 30, 2009 3:16 pm 
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If you sync your scope to the sweeper, rather than to the input waveform, you should see the same envelope for the direct probe and for the detector.

I never use an RF detector. Whenever I need to probe a sensitive RF circuit, I use an active probe. It's the probe capacitance that affects a circuit, not its resistance. Probe capacitance is very low for active probes, normally < 2 pF and often < 1 pF. An ordinary x10 probe usually exhibits at least 10-15 pF, sometimes more. You can find active probes on eBay at reasonable prices. The older HP probes are particularly cheap. I got one for < $20 delivered. HP active probes require two supply voltages, Tek active probes three. You can find dedicated active probe power supplies on eBay or you can build your own. My spectrum analyzer has a front-panel socket for powering an HP active probe. I believe some Tek scopes have a socket for powering Tek probes.

Active probes have a maximum linear signal level (often around 1V, but sometimes lower) and a maximum nondestructive level (60V for my Tek probes, as I recall). Because of the latter limitation, I never use an active probe inside tube equipment, even when what I want to probe is at a low voltage (one slip and I've blown the probe). You also have to worry about saturating the probe with DC if it doesn't have an AC coupling switch. So you can't use an active probe in many places where a x10 probe works fine. But I find them invaluable, as you might conclude from the fact that I have five active probes but just two passive probes.

Brian


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PostPosted: Oct Fri 30, 2009 9:57 pm 
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k6sti wrote:
If you sync your scope to the sweeper, rather than to the input waveform, you should see the same envelope for the direct probe and for the detector. Brian


Thanks Brian. The RCA wr-50b is 60hz rate (AC power freq). The scope has a external line sync/trigger. I'll try that this week end.

Total newbie with scopes and probes, so this is all new to me. I'm just exploring the potential uses and limitations. I definitely will look at making or buying an active probe just for fun and learning. For now I am going to try some passive stuff.

Too bad about the low voltage limitation on active probes. Is there a way around that? I guess you could use a mini tube?


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PostPosted: Oct Fri 30, 2009 10:26 pm 
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gmcjetpilot wrote:
Too bad about the low voltage limitation on active probes. Is there a way around that?


Not that I know of.

I forgot to mention that some vacuum tube FM tuners have a built-in detector for use when sweeping the IF. Look for a diode, capacitor, and load resistor somewhere near the end of the IF strip with no other obvious purpose.

The other trick is just to hang your x1 or x10 probe near the circuit you want to monitor. I do this all the time for tube equipment. If you crank up the scope sensitivity, you should see plenty of signal. Even an active probe can't beat the circuit loading. Often this is all you'll need for RF and IF alignment.

Brian


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PostPosted: Oct Sat 31, 2009 3:51 am 
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k6sti wrote:
I forgot to mention that some vacuum tube FM tuners have a built-in detector for use when sweeping the IF. Look for a diode, capacitor, and load resistor somewhere near the end of the IF strip with no other obvious purpose.

I am not sure any of my radios have that built in detector, but I WILL LOOK! Thanks!

Yea for the Zenith AM/FM radios I have (H723, H724, H725, Y832, 7H820, C730, G730) no sweep/scope method of FM alignment is given in the alignment notes. It does refers to it as preferred and to some long lost Zenith Signal Gen Model 800 manual (form Z8001). I think I can figure it out however with a little trial and error (I hope). :roll: The manuals for the Eico and RCA sweep/signal generator give generic info on how to do an FM alignment. I'll drop you a PM when I get closer to doing it, if I have questions. My RCA 8R71 AM/FM does give more info specifically for using a scope for alignment.

Quote:
The other trick is just to hang your x1 or x10 probe near the circuit you want to monitor. I do this all the time for tube equipment. If you crank up the scope sensitivity, you should see plenty of signal. Even an active probe can't beat the circuit loading. Often this is all you'll need for RF and IF alignment. Brian

Thanks Brian. That is a great trick. Yea on high sensitivity the probe is like an antenna, picks up lots of signal, even static electricity on my body.


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PostPosted: Oct Sat 31, 2009 3:21 pm 
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I am not sure any of my radios have that built in detector, but I WILL LOOK! Thanks!

Don't forget than you can use the AM detector stage for the DC level to drive the scope; it is negative going (for AGC use) so you will have to invert the DC channel on the scope to use it.


Thanks Brian. That is a great trick. Yea on high sensitivity the probe is like an antenna, picks up lots of signal, even static electricity on my body

If you do that technique inside the set, you'll have to be careful of signal "blowby" from earlier stages. Sometimes the incidental radiation from earlier stages will swamp the peaks on downstream stages. A lot of this is experience, you'll get the feel for it the more you do it. Also, when coupling into an IF transformer as you showed earlier, you have to be careful not to detune or load down the device with the measuring instruments. That's why its usually better to tune them in the circuit they are used in. Pete [/b]


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PostPosted: Nov Sun 01, 2009 10:21 am 
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Peter Bertini wrote:
Don't forget than you can use the AM detector stage for the DC level to drive the scope; it is negative going (for AGC use) so you will have to invert the DC channel on the scope to use it.

OK professor you will have to dummy that down. :wink: What? Ha ha. Not sure I understand, I mean I'm sure I don't understand. :) My scope can invert. Frankly I am not 100% sure when to use DC/AC or what trigger to use. I kind of experiment and see what works.

Quote:
If you do that technique inside the set, you'll have to be careful of signal "blowby" from earlier stages. Sometimes the incidental radiation from earlier stages will swamp the peaks on downstream stages. A lot of this is experience, you'll get the feel for it the more you do it. Also, when coupling into an IF transformer as you showed earlier, you have to be careful not to detune or load down the device with the measuring instruments. That's why its usually better to tune them in the circuit they are used in. Pete

Lots of leakage as you say. I took my digital LF/MW/SW portable radio tuned to the IF freq and sniffed for signals. The radio was like a radio station on the IF freq. May be some shield (aluminum covered in insulation) to block stray signals. The AM/FM table top Zeniths I'm working on are very tight, CANS and tubes are side by side, all together with point to point wiring (often un-insulated).


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PostPosted: Nov Sun 01, 2009 3:45 pm 
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If you want to pre-align an IF transformer out of circuit using an RF generator, you must use a series resistance between the generator and the primary of the transformer. The value of the resistor should be equal to the plate resistance of the IF tube used with the transformer. For most pentodes probably around 100k ohms would be a good guess.

You will then be able to get the alignment "in the ballpark" with your detector probe (with very small loading) connected across the secondary.

If you don't use the series resistance in the primary circuit, the bandpass characteristics of the transformer will be completely different than when it's in the radio. Even when it's back in the radio you'll have to align it again anyway due to the capacities of the tubes and wiring in the set.

Long story short; bench testing of IF transformers is tricky and only gives you an approximate idea of the bandpass.

_________________
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PostPosted: Nov Mon 02, 2009 6:54 am 
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Cdoose wrote:
If you want to pre-align an IF transformer out of circuit using an RF generator, you must use a series resistance between the generator and the primary of the transformer. <snip>

You will then be able to get the alignment "in the ballpark" with your detector probe (with very small loading) connected across the secondary. <snip>

Long story short; bench testing of IF transformers is tricky and only gives you an approximate idea of the bandpass.


Thanks, Cdoose, great detailed info. I never thought I could "tune" it perfect out of the radio, but of course I had to do something. The only original part in the IF were the two Pri/Sec coils and one of two slugs. The top slug and three capacitors were replaced. The one original bottom slug was stuck. So it need a drop of LPS and some exercise to free it up, so it was not even close to it's original position. I'm sure I could have used the radio to adjust it, provided the rest of the radio was close. This was the AM section, not as critical as the FM (as I understand). I can see if you had ALL newly rebuilt IF's, you would have no clue where they resonated or their bandwidth (or pass), it could be ugly? :roll:

Appreciate you detailed answer. I see exact alignment of an IF is not possible out of the radio. Reading the word alignment means the IF's are all aligned together, in the radio with the other IF's and discriminator, in ALIGN, not individually.

The radio played right out the gate with my newly rebuilt IF. So I did something right. The new top slug (primary) needed almost not adjustment. The secondary bottom slug was off, but it needed nominal tweaking. I don't remember, half or 3/4th a turn. May in the ballparks parking lot? :wink: So I did get close. Here I thought the results were because I was good? :shock: Now I see I was just lucky! Ha ha! :lol: Cheers.

PS: Reading the theory of slug tuned IF transformers ......is there some secret in the order to tweak the slugs :?: My Zenith IF's are consider "permanent" tuned type I.F.'s. I assume that's because the capacitors are fixed and the distance between the coils are fixed as well. The slugs change the "permeability". Is there only one setting that's ideal. Does each side of the transformer (a tank circuit in it's own right) have one ideal setting, or is there a range based on the other side of the transformers setting :?: I assume this might change bandwidth, gain?

I ask because aligning the FM section (not AM) with basic methods, the Pri slug on the DISCRIMINATOR I.F. is right near the top. It needed lots of adjustment, seems like I might have missed it. I realize band-width is an issue with FM and a limitation of the single Freq method, thus the sweep method. Sorry for the rambling qustion.


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