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Old 17th Apr 2019, 1:12 pm   #1
Ian - G4JQT
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Default IF core positions

As a general rule, when there are two positions that peak the IF coil, should the ferrite slug be in the deepest, shallowest, each core different, or does it not matter?

I'm minded to think it doesn't matter as long as the cores are in sufficiently to be stable, but you never know!

(Project paused for definitive answer...)

Thanks.

Ian
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Old 17th Apr 2019, 1:15 pm   #2
Radio Wrangler
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Default Re: IF core positions

Shallowest gives the least coupling between the two resonators and thus the greatest selectivity.

Wideband circuits may need the greater coupling option, but shallowest is most common in broadcast sets.

David
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Old 17th Apr 2019, 1:16 pm   #3
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Default Re: IF core positions

Most commonly the shallowest, the deepest usually results in over coupling between primary and secondary.

Lawrence.
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Old 17th Apr 2019, 1:31 pm   #4
Ian - G4JQT
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Default Re: IF core positions

Thanks guys.

This is for an Ekco A22 and the cores are on separate bobbins in the can, probably about an inch apart. But there must be coupling between those sharing the IF can.

I was trying to be clever and with the use of a wobbulator get a nice (shallow) double hump for greater bandwidth. On strong signals (i.e. my pantry tx) the set was motor-boating with the volume turned up. I fixed it with an extra 47uF across the second PSU electrolytic, but that shouldn't be necessary at all! (The PSU caps are new from the BVWS.)

After days of going round in circles I think the problem was somehow caused by my IF tuning. By following the sheet instructions properly (just peak for max at 465 kHz and don't try to be clever!) I think I've cleared the motor-boating anyway.

Ian
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Old 17th Apr 2019, 2:26 pm   #5
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Default Re: IF core positions

It's generally the rule that when aligning IF's that have cores one above the other, it's the outer peak that is used....in other words you screw the cores outwards so the cores are furthest apart. As stated by Lawrence, if the inner peaks are used (or one outer and one inner) the selectivity will suffer greatly since the coupling between the windings will be greater.
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Old 17th Apr 2019, 2:35 pm   #6
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Default Re: IF core positions

Quote:
Originally Posted by Ian - G4JQT View Post
the set was motor-boating with the volume turned up.
I had a very similar problem with a 1938 Philips 470A. It uses a low IF (120khz I think from memory) and after I'd done the basic restoration I peaked the IF's and it had amazing gain but tended to go very unstable at the HF end of MW. I should have read the alignment instructions properly......

It clearly states that you have to damp the preceding IF secondary with a resistor before trimming the following IF....Everything was fine after doing that. You can also get a similar effect with a radio that's supposed to have staggered IF's when they are all aligned to the same frequency.
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Old 17th Apr 2019, 11:23 pm   #7
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Default Re: IF core positions

I bought some Electroniques I.F to put in an AR77 which had been burnt out. Graphs were supplied showing the response with the cores in different positions.

The amount of coupling will affect the phase of the IF amplifier and if there is some feedback, then oscillation may be promoted. In one set I looked at, the a core had been chewed and the coil had been damaged. I replaced the secondary with a small transistor tuned circuit, top coupled to the primary, this will change the phase. I cut the AGC line to the IF valve and added more decoupling to help the stability. The AGC line was fitted with a ferrite bead, indicating something was not quite right with the design.
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Old 18th Apr 2019, 1:29 am   #8
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Default Re: IF core positions

Another thing worth noting is that single tuned RF coils can also have two peaks that are not the same if there is a coupling coil; one where the slug projects partially from each side of the coil. And in some designs,especially in early transistor circuits (like the Eddystone EC-10) the secondary coupling coil is on one side of the resonant coil on the former (not directly under it) so the result is different depending on which peak (slug position) is selected. In one case the coupling to the next stage is tighter and the gain increases, but the loading on the resonant winding broadens the bandwidth, in the other case the gain is lower due to reduced coupling and the bandwidth a little narrower too.
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Old 19th Apr 2019, 1:18 am   #9
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Default Re: IF core positions

Something else to watch for (that has been discussed quite a bit on the forum) is the metallisation may have failed in the IF valve e.g. EF39 etc, which can cause instability problems. The usual failure mode seems to be either the ground connection to the coating fails, or the coating simply crumbles off. On more than one occasion where I have had IF instability problems, changing the IF valve has cured the fault completely.
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Old 19th Apr 2019, 10:02 am   #10
Ian - G4JQT
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Default Re: IF core positions

Thanks for the extra info.

I did realign the IFs (and LO/Ae ccts) again and regarding the motorboating it is much better than it was . For example I couldn't get it to do this at all with the sig gen on max o/p (about 1V RMS!) and when connected to my active dipole or Wellbrook loop (both expecting 50 ohms loads, but anyway...) there is no motorboating at all on any stations and sounds lovely.

But when putting 3ft of wire on the antenna and sitting it in the house with the pantry tx (about 500mW into a short whip) upstairs at the other side of the house, it does still motorboat slightly at max volume.

Another A22 similarly aligned is fine. Valves all swapped with no effect. All voltages the same, and all paper caps on both sets have been replaced.

I even swapped over the grid connections to the o/p valve so set A was driving the audio of set B and vice versa and the 'fault' remained on the original motorboating set.

So it appears to be a PSU issue. I even swapped over the HT leads (both before and after the choke) but that caused them both to motorboat!

An extra 22uF across the smoothing cap completely stops it, but that shouldn't be necessary because it already has a new (and tested) 16+16uF rather than the original 8+16uF. In addition, extra capacitance here might exceed the switch-on surge of the AZ31 rectifier.

In every other way it seems to be operating perfectly, and in normal use no one would ever notice. It's not really a problem as I don't need to have it at maximum volume, but it is intriguing why one A22 does it and the other doesn't. The 'guilty' one has the 220pF in the 2nd IF rather than 100pF, but apart from that they are identical.

Noise-limited sensitivity of both is about 1 to 2 uV on a 30% mod signal. That seems fine to me.

Ian
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Old 19th Apr 2019, 11:37 am   #11
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Default Re: IF core positions

It may be worth checking the decoupling capacitors (typically 0.1, 0.01 microfarad) across the supply. The low frequency instability can still be caused by RF getting into places it shouldn't. Maybe add a couple more. Has any wiring been re-routed so as not to follow the original design? Just a couple of ideas...from previous experience of chasing elusive faults.
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Old 19th Apr 2019, 10:47 pm   #12
TrevorG3VLF
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Default Re: IF core positions

If the volume control makes a difference, then RF is lokely to be passing through it.
Is the RF bypass capacitor after the detector in good condition?
Is there an RF bypass capacitor in the output stage?
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Old 19th Apr 2019, 11:01 pm   #13
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Default Re: IF core positions

Look at C28 and C29 (manufactures) 100pF
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