Hallicrafters SX-43 2nd I.F. won't peak

[falcon123]

hi ya fella's. i need some help as i'm at a loss to figure this out myself. the primary a.m. transformer will not peak. the upper primary adjuster continues to increase signal amplitude as its screwed in until it runs out of thread NO matter what i do. the secondary on the bottom behaves as normal. my first guess was that it needed a new 220pf cap. i changed it and the two 100pf's on the fm and am secondary winding.ive got two of these 2nd i.f. transformers and both behave the same way both with new 500v 2% cornel dubilier caps.i'd like to point out that both transformers were tried as found with the original caps and worked the same way. resistance values of the windings are nearly identical. sadly hallicrafters didnt list any resistance data. im using a kenwood sg5150 generator. surprising enough.the set performs really well except for about 1mc of drift on fm. its been recapped and all but 3 or 4 of the resistors have been changed.the way i understand things the cap resonates the primary winding when core is in proper adjustment so the fault should be the cap.but not on two separate if cans is suspicious. but 70 year old parts can be at fault.is it possible that something externally is the problem

[Station X]

I've experienced this problem with domestic sets. What happens is that over the years the inductance of the IFT coils changes. This may be due to moisture ingress, deterioration of the wax or other substance used to seal the coils, or possibly a combination of the two.

The answer is to change the value of the parallel capacitor by say 10pF at a time until resonance at the correct frequency is achieved. More capacitance will lower the frequency and less capacitance will raise it.

I suggest swinging the SG's output to check that resonance is achieved at some point before making changes.

[mrrstrat]

What does a replacement transformer look like for this? Is this something that can be replaced with a NOS drop-in?

I found this was the case with my infamous S-20 project. Both IFs were at fault and were both replaced entirely to make the set work on all bands and frequencies.

[falcon123]

hmmm....,would it be practical to bake the transformer in an oven say at 100c and then redip it in beesway as ive two it would seem to be reasonable enough. not sure how much heat the micas can take and i would have to figure a way to prevent the wax from entering the slug adjuster bore. i even have some bees wax.

[mrrstrat]

I think Graham is spot-on: inductance and capacitance also need to take into account the Q factor (quality) as well. High Q factor inductors are needed for proper resonant circuits in radios (as well know). Winding capacitance changes (and permeability) would change where the Q angle is in the inductive/capacitive quadrant. Thats important because its an indication of its inductive reactance with respect to its intended frequency (is it 455Khz?).

Can you measure qualitatively what the coil is doing, such as you can with a LCR meter? I added one of these to my tool arsenal some years ago for measuring the elusive Q easily.

If the issue with the IF is the quality factor, then as Graham suggested increasing the capacitance 10pF at a time might bring it back - unless the inductor has deteriorated unfavorably and cannot be revived.

I do not have much experience in IF surgery - and am somewhat new to radios - but I am starting to think that winding IF inductors is the last frontier for radio restoring guys such as ourselves.

I opted to wholesale replace the IF when I was last down this path, so I would try the manipulation of the capacitance (what you can somewhat control in the impedance formula of an inductor) as Graham suggested as this might be the least amount of effort.

[Station X]

Originally Posted by falcon123

hmmm....,would it be practical to bake the transformer in an oven say at 100c and then redip it in beesway as ive two it would seem to be reasonable enough. not sure how much heat the micas can take and i would have to figure a way to prevent the wax from entering the slug adjuster bore. i even have some bees wax.

I've no experience of doing that and it sounds risky.

Moisture logged inductors can recover if left in a warm dry place, but it can take a long time.

[mrrstrat]

The SX-43 has a lot going on under the hood - the first part of alignment is adjusting 8 different trimmers on coils. I remember I have a couple that really does not seem to do anything and one that did very little if anything at all.

I remember A4 (?) did not have a big effect observable and A7 really did not do anything at all to the observed changes in voltage off the 6AL5 test point. A7 was able to be completely screwed into the IF to have any discernible effect. After a few turns out it did not matter if I had the A7 screw backed way out of the top.

I figured this was because I clipped out C53 (the 22pf lightning crash mica cap). I plan to replace this but it comes down to how (as you know).

This radio was the first one I ever worked on, the first I rebuilt totally, and the first set I ever aligned. The 4 hour alignment did not go completely trouble free but I get great performance out of it. Because now I have aligned and rebuilt several radios (and repaired them) I do want to touch back on the SX-43.

I may very well find I have the same issue with mine though the set is highly performant in its current state.

[trh01uk]

Well, here's a few ideas that I ran through, which I will list for completeness:

1. You are testing at the wrong IF frequency. (I couldn't find the actual AM IF frequency in the information on the net).

2. The decoupling capacitors on the IF primaries are faulty in some way. I refer to components nos 32 and 35 for instance (as numbered in the file at https://bama.edebris.com/download/ha...PF_Sep1948.pdf). You say you recapped the whole set. Did you replace these capacitors with something very close to the originals? Do they have the same lead lengths as the originals? These components are part of the primary IF tuned circuit. (I note the secondaries also have decouplers which are apparently working OK - are they different in some way?)

3. As others have suggested the primary inductor has developed a fault which has changed its inductance. I've seen coils go "green" due to conversion of the copper wire to copper dioxide (I think), and that's likely to change the coil characteristic. The trouble with this theory is that is very hard to think of a mechanism that only affects the primary, and not the secondary!

4. There is something wrong with your test set up. Unlikely, but you might want to describe to us exactly what gear you are using and how you have it connected. Where, for instance, are injecting an IF signal and how?


Richard

[ms660]

AGC chasing can also produce a similar effect.

Lawrence.

[trh01uk]

Originally Posted by ms660

AGC chasing can also produce a similar effect.

Lawrence.

How does AGC chasing work only for the primary side of an IF transformer - but not the secondary?

Richard

[ms660]

Originally Posted by trh01uk

How does AGC chasing work only for the primary side of an IF transformer - but not the secondary?

Richard

The secondary is peaked and at the peak the input to the AGC detector might be below the AGC threshold so no AGC action, then tuning the secondary towards its peak the input to the detector increases as does it's output but at some point the AGC threshold might be exceed, then further tuning of the coil towards its peak makes little difference to the output because the increasing AGC action tends to mask the effect of the tuning adjustment both towards the peak and after the peak.....Not saying that this is the problem with the OP's receiver but I thought I would mention the effect anyway, a lot of alignment instructions say to keep the signal input to a minimum to avoid that effect.

Lawrence.

[trh01uk]

Yes, interesting Lawrence. If I understand your point correctly, the order of tuning the two halves of an IF transformer would effect the result. If we started with the primary and peaked that, then the secondary would be the coil apparently failing to tune?

Presumably the effect can be avoided by switching off the AGC?


Richard

[falcon123]

you have asked a lot of questions. the generator is set (kenwood sg5150) to 454.5kc which is the crystal frequency. i have also tried it at 455kc. the mvc is turned on so the avc is off.the hallicrafters manual stipulates that the set while designed at 455kc is aligned to exact crystal frequency which can be slightly more or less. the generator is connected to the low capacity gang directly and i set it for a minimum tone and back it it off as it increases during alignment. the procedure follows the hallicrafters service data NOT sam;s but in this area its the same. the caps you refer to 32-35 (sam's) c70.c55 (hallicrafters) are cornell dubilier poly axials are identical, installed the same as original, and are .01 mfd 630v. perhaps the item on the 2nd i.f. is defective? i did bake the extra transformer and re dip in bees wax after i finish my coffee this am i will have another go.i'm not sure how to check the 2nd i.f. for a different resonate frequency. if i merely tune the generator up and down to see if 2nd i.f. output increases i decrease the output from the 1st and 3rd. how would i isolate it?? anyway lets hope that the baked transformer fares better and failing that ,i will try another .01 cap, i noted NO corrosion in the 2nd i.f. by the way.

[mrrstrat]

Baked IFs and Coffee - sounds like a nice breakfast!

[falcon123]

sadly i installed the hopefully desicated and rewaxed i.f,and the song remains the same. i do take comfort in the knowledge that baking this i.f. at 100c did no damage but didnt fix anything either.the only thing i didnt do was change the decoupling cap c70 as my fluke said it was ok,but thats not the same in circuit either. i guess its the only thing left to do.

[Radio Wrangler]

One other thing to add in is the possibility that the IF design may be such that one coil in an IFT is fairly heavily damped/loaded and that as a consequence the tuning is very broad, so broad that it doesn't matter where you screw the slug. In this case the IFT is working as a single pole resonator and the un-peaky coil is just acting as a lightly coupled driver (if it was other than lightly coupled it would kill the Q of the peaky resonator)

This could explain the improving IF gain as the slug moves to one end (usually inwards) as this increases the coupling to the true resonator and decreases the insertion loss of the IFT.... it also widens the true resonator and you may not want that.

Can you find alignment instructions? Or someone with another set.

This sort of thing is why sweeping/wobbulating IFs is so good.

David

[mrrstrat]

My SX-43 does about the same as his: drifting FM. The FM seems to wander after a few minutes and I chase it with the dial - just a slight adjustment.

[trh01uk]

Originally Posted by Radio Wrangler

One other thing to add in is the possibility that the IF design may be such that one coil in an IFT is fairly heavily damped/loaded and that as a consequence the tuning is very broad.........

That doesn't quite fit with what the OP originally observed:

"
......the primary a.m. transformer will not peak. the upper primary adjuster continues to increase signal amplitude as its screwed in until it runs out of thread ......."

So what we have is a change of response from the coil but apparently the peak (if there is one) is outside the available travel of the core.

Having said that, what may be helpful is to turn this around and ask "What frequency does this primary coil actually resonate at?" And find that by taking the sig gen over a range of frequencies until a peak is found. Of course the secondary will contribute its own peak, but I am expecting to find two peaks, based on what was said above.


Richard

[trh01uk]

Originally Posted by falcon123

you have asked a lot of questions. the generator is set (kenwood sg5150) to 454.5kc which is the crystal frequency. i have also tried it at 455kc. the mvc is turned on so the avc is off.the hallicrafters manual stipulates that the set while designed at 455kc is aligned to exact crystal frequency which can be slightly more or less. the generator is connected to the low capacity gang directly and i set it for a minimum tone and back it it off as it increases during alignment. the procedure follows the hallicrafters service data NOT sam;s but in this area its the same. the caps you refer to 32-35 (sam's) c70.c55 (hallicrafters) are cornell dubilier poly axials are identical, installed the same as original, and are .01 mfd 630v. perhaps the item on the 2nd i.f. is defective? i did bake the extra transformer and re dip in bees wax after i finish my coffee this am i will have another go.i'm not sure how to check the 2nd i.f. for a different resonate frequency. if i merely tune the generator up and down to see if 2nd i.f. output increases i decrease the output from the 1st and 3rd. how would i isolate it?? anyway lets hope that the baked transformer fares better and failing that ,i will try another .01 cap, i noted NO corrosion in the 2nd i.f. by the way.

Yes, sorry for bombarding with questions! Just trying to cover as many possibilities as I can, because I think you have us all scratching our heads here......

It all sounds as though you have perfectly restored the parts, and yet we are left with the puzzle of one half of the transformers resonating somewhere else than the IF frequency.

Can you find out a) if there is any resonance in the primary tuned circuit, and b) where that resonance is? I suspect you can do that by tuning the sig gen far enough to find any peak from that primary. When you do find a peak, you should be able to confirm its due to the primary coil by adjusting the slug and observing output falling either side of resonance.

I am wondering if this is a tolerance problem in the fixed capacitor tuning the primary coil? Are these original - or replaced with new?

We may not get an answer as to why the primary tuned circuit is off frequency - but the solution is fairly obvious: adjust that parallel fixed capacitor up or down in value until you have resonance in the middle of the adjustment range of the slug. Sometimes the quick and dirty solution is the best you can do.....


Richard

[Station X]

Yesterday I was aligning the IF stages of a Hallicrafters SX-24. Hallicrafter's alignment procedure calls for this to be done at the "XTAL CW" bandwidth setting using an unmodulated signal with the BFO set for a 1kHz audio note. The generator is "rocked" to determine the Xtal frequency and one IF secondary is tuned for a notch rather than a peak. This isn't easy at all and I'll probably use a sweep generator instead.

The point is though that if the Xtal is switched out, all the IFT's show very distinct peaks on both primary and secondary windings. There is no running out of adjustment before a peak is reached. The difference between the SX-24 and SX-43 is that the former uses trimmer capacitors to tune the IFT's whereas the latter uses tuning slugs.

I agree with previous posts suggesting that the OP checks to determine what frequency the IFT's are actually resonating at.