Adjusting IF's with seized slugs.

[dr peppers]

I know this has probably been covered I hope no one minds me mentioning this again.
I seem to be having trouble with IF's.
I have a HMV which is out on the second IF adjustment, but the ferrite slug is cracked and if I poke it I'm going to ruin it, I read somewhere that you can insert another material in the core as well as the slug to trim the tuning, but I cant remember what, I think it was brass to increase the tuning and ally to decrease it, can someone remind me please.
I have some lengths of brass brazing rod, and ally tig welding rod, I was thinking to make an adjuster I'd drill a hole down a length of nylon bolt a little smaller than the rod and shove a piece in, the section can then be screwed in and out of the can to tune it, the brass could be used to tune one way and if that didnt work swap it for the ally which would tune the other, whichever tunes correctly.

[MichaelR]

Personaly, I would try and remove the broken slug. Not easy I know but from my own experience with care it can be done.

You are right about using brass with the coil that you have got I am not sure about the implications of the Q of the coil being affected such that it would end up as a not very good solution.

If you had come across such a repair in one of the IFT , I bet you would be thinking "somebody has bodged this".

Mike

[Billy T]

Brass will raise the resonant frequency, and to lower it, either pad the winding with capacitance or screw another ferrite in on top.

I presume you know whether it is currently tuned high or low, but I used to use a plastic wand with a small brass insert at one end and small piece of ferrite at the other to check whether it needed to be high or lower, then if adding a slug of the correct material was too coarse, I used padding caps or a lower value cap to get it in tune. If it is only a little off, provided sensitivity is adequate it can often just be left as is. If the core is cracked and loose, causing intermittent alignment shift, a drop or two of melted wax will settle it down.

This approach is not a bodge, it is sensible pragmatic servicing because you run the risk of damaging a possibly irreplaceable IF transformer by attempting to remove a cracked ferrite core. Iron-dust cores sometimes yield to very patient drilling, but ferrite is much harder and is more likely to fracture more and split the former. Of course if it is a vintage am radio, the cores should hopefully be iron dust but they can split too.

I worked with a very good technician once whose mantra was 'beware of creeping elegance' and many radios passed on to him for 'expert attention' yielded to the simple approach, which always started by checking all the 'repair work' done so far and very often putting back parts that had been incorrectly or unnecessarily replaced. His motto was 'if it worked like that for years, we don't need to redesign it, let's just find the fault'.

A good example was an auto-tuning car radio that wouldn't! The auto-tune detector circuit was unstable and would stop everywhere but on stations. He disabled it, checked that the radio was otherwise ok and correctly aligned, which it wasn't, then put the original transistor back in and away it went.

That was nearly 40 years ago and I've never forgotten that lesson.

Other may know it as the KISS principle: Keep It Simple, Stupid!

Cheers

Billy

[JimMac53]

On a Murphy that I repaired, I was able to remove the cracked slug (Yes, I cracked it.) and since it had cracked into two pieces, I was able to epoxy them together and rebuild the slug. I then carefully cleaned the IF former threads, realigned the IF, and tweaked the VHF oscillator so it gets Classic FM. The radio works fine (it's on as I write this).

[Station X]

I have managed to remove broken slugs using a pair of tweezers with the tips filed down to a fine point. If the slug has split longways you need to twist first one half then the other. You'll soon know whether it is going to move or not. With care it's also possible to crush the slug and pick it out piece by piece.

[Sideband]

The biggest problem is when the former shrinks around the slug....it's doesn't have to shrink by very much but it makes the slug impossible to move by normal methods. Sometimes the only answer to this is to leave well alone and add or reduce inductance by the methods previously mentioned.


Rich.

[Leon Crampin]

If you have a standard superhet with just one seized slug, tune the IF to the resonant frequency of that coil, then align the others to it.

If the resonant frequency of the "problem" coil is too far out (unlikely) then tweak the parallel capacitor to suit. This will affect the bandwidth, but unless there are other problems, the effect will be very small.

The actual IF frequency is not absolutely critical although a wide divergence from the design value will result in poor tracking and band coverage. I aligned a Murphy A170 with a seized slug in this way (IF I think came to about 468kHz) with excellent results and good tracking.

Just swing your signal generator to find the resonance of the stuck coil - but remember the process is iterative and you will have to do this a few times.

Leon.

[MichaelR]

Being very careful I managed to remove a slug that would not move using a dremel. Starting with a small drill size I built up a central holewhich in time allowed me to breakaway the core wall that had established, using one of those soldering tools that are common which have a pick.
The real care is need in the final stages in getting the remnants of core out of the former thread.

Time and care but doable

Mike

[Boom]

The last seized slug I had was on my Ekco A44. The Phantom Twiddler had been at it with a screwdriver. I got the slug out by careful use of my modelling drill then Aralditing an old drill in the hole and leaving it to set before twisting the bit with a pair of pliers. It came out cleanly.

[MichaelR]

Dave,

You must have been a dentist, that is nearly a root extraction !

Mike

[Boom]

Originally Posted by MichaelR

Dave,

You must have been a dentist, that is nearly a root extraction !

Mike

Ah right. Maybe we need Nick to sort it out?

[dr peppers]

Why thankyou for your replies.
I dont know if the core is ferrite or iron, the sets 1947 so maybe its ferrite, theres a definate possibility that I broke the core trying to adjust it, I dont know, shrinkage of the former isnt something that I'd considered, thats going to make it hard to adjust even if it wasnt broke.
I've tried to dremel ferrite before, its as hard as anything, if I have to remove it then I'll try the sharpened tweezer method.
Leon your idea is good, if I can get the set to work reasonably well I'd be pleased, there isnt much work involved with your idea, the tuning isnt far out, its about 490knz, I'll try aligning the other IF to it, at least I'll be able to get the thing working and appraise whether I want to spend any more time on it or not, I spose theres a trade off here, realign to the out of tune 2nd IF and worsen the tracking, or put brass in the core and affect selectability, the re-tune is easy so I'll start with that, and I listen to a station on 1.2mhz which is close to a french station so selctability is importantish, getting it to work is the first thing then see if it gets barred from the living room.

[Leon Crampin]

If you shunt your "offending" coil with about 10pF, you can then apply my method and the IF will be nearly correct. It's harder to put right when the frequency is too low - unless you open the IF can and change the capacitor.

Leon.

[dr peppers]

Yep ok then going to try that.
On the same score doing the same thing with another set, that is removing the caps in the cans and fitting external ones, might sort out my other set thats playing up with suspected silver mica disease.

[Colin]

Originally Posted by Leon Crampin

If you have a standard superhet with just one seized slug, tune the IF to the resonant frequency of that coil, then align the others to it. Leon.

I like this idea too. A nice bit of lateral thinking and I guess it's probably what a busy radio repair engineer would have done back in the day anyway? Thanks for the tip Leon

[dr peppers]

The set works.
Its nery near spot on, a 10 puff and a 2puff2 accross the primary of the xformer makes its resonate within a few hundred hz of 470kcs without touching the broken slug, the rest of the set is re-aligned and works really well.
I also followed the advice about sealing the slug, I chucked a load of scrap wax paper caps in a home bashed melting pot made from an electrolytic can, melted them with a little micro torch and poured the wax into the primary side, all ok now.
Thanks for your advice people.

[DAVEHALL]

Dodge we used on Clansman IFT ,where the slug had been sealed ,was to apply heat ( sonething like hair dryer) .

[Billy T]

Originally Posted by dr peppers

I don't know if the core is ferrite or iron, the sets 1947 so maybe its ferrite, there's a definite possibility that I broke the core trying to adjust it,

As far as I know, ferrite cores were not commercially available for domestic products until after WWII (about 1947) because TDK Japan was the only manufacturer, so the age of your set may indicate the core type. Generally speaking ferrite cores have a hole right through from top to bottom, presumably to spread the strain of the trimming tool, while iron dust cores simply have a slot in the top and bottom.

Iron-dust is much easier to drill and even if it cracks it does not cause secondary damage quite as often as ferrite. When a ferrite cracks, the drill tends to grab and break off a larger piece. The drill then spreads the two halves, often cracking the former or doing even more damage.

If you can avoid removing the slug by retuning the coil itself, or the entire IF chain in the manner described by various posters, you may save yourself a lot of unnecessary work, not to mention grief. As I said in my previous post 'beware of creeping elegance'.

Cheers

Billy

[dr peppers]

Creeping elegance, I've sorted it now.
Bodged.
It might be a bodge but it looks ok and it'll work for another 50 years.