Why does alignment drift?

[GMB]

Given no loose parts, i.e. all trimmers and slugs sealed in position - why do tuned circuits sometimes go out of alignment after a few decades?

I have usually found they stay in tune, but apparently not this time.

How common is it to find realignment required and which components do you think are mainly responsible (and which way do they usually drift)?

[paulsherwin]

I usually find inductors stay in tune too, if they haven't been disturbed.

I guess sometimes the inductance of coils changes with moisture ingress or chemical changes in the insulation or former. There's also a chance that associated capacitors will have degraded.

Paul

[Station X]

Capacitors and inductors change their value due to ingress of moisture. Wire on inductors expands and contracts due to temperature cycles. Vibration due to sound from the speaker or mains transformer hum shifts the wire in the inductors.

[Station X]

Of course most misalignment is due to the phantom twiddler tightening loose screws, but that was excluded in the first post.

[Steve_P]

Well, apart from the Phantom Twiddler, the other cause is moisture - it depends where the set has been stored. Put it somewhere warm for a couple of weeks.

Apart from the paper caps, nothing else really affects alignment.

Cheers,

Steve P.

[murphyv310]

Hi.
I can't really say Iv'e had a great deal of problems with alignment, my collection is mostly tellies and 78's (the records never have alignment problems ). The phantom twiddler is the biggest problem certainly with sets I do for other collectors and funnily enough with cap replacement in IF stages and tuners. One thing many collectors do when replacing decoulplers in IF's the replacement cap is usually smaller than the original and often they are tacked onto a different earth point, this changes the earth current at RF frequencies and can alter the alignment and can also make the stage unstable or even affect the gain of that stage.
I rigidly stick to the layout the manfacturers used, sometimes a pain in the ae but necessary. The results though are worth it all my sets give excellent results.
Trevor

[Skywave]

Plus the valves - ageing effects & replacement of.

Al / Skywave.

[PJL]

I have had quite a few with IF capacitor drift/failure. There is also the documented silver migration problem which USA sets are prone to. Pre-war radios with capacitor trimmers usually benefit from realignment.

[GMB]

Has anyone noticed which why drift tends to be?

I guessed that moisure might eventually slightly affect even mica and ceramic capacitors - my thought is that would increase the capacitance so the tuned circuit would have dropped in frequency - and that is what I'm finding.

I wasn't sure about the stability of coils and their tuning slugs.

I always worry when confronted by highly sealed tuned circuits that seem to be out if alignment that maybe the real fault is something else. In this case I'm faced with trimmers that were soldered by the manufacturer after alignment - the obvious concern is that attempting to obtain perfection might be more damaging that living with the error.

[XTC]

Apart from damp, silver migration, shock and twiddling, plastics oxidise a bit over the years and waxed paper formers are a natural product. Just few thou of movement would be enough to take a tuned circuit off-peak. I think there was a thread a time back about the inductance of cores changing over time. You also see cracks in the wax of some wax coated IF coils which lets in damp of course, but also suggests some change in the wax, and maybe stresses and shifting.

Anyway, unless you have performance figures at hand (unlikely with a domestic set), or a similar set to make a comparison with, how do you know whether IFs are properly peaked unless the performance is dreadful? I must admit, I always tweak them as per the service sheet, unless it's an IF stage which needs a specific response curve.

Pete.

[XTC]

Originally Posted by GMB

Has anyone noticed which why drift tends to be?

I guessed that moisure might eventually slightly affect even mica and ceramic capacitors - my thought is that would increase the capacitance so the tuned circuit would have dropped in frequency - and that is what I'm finding.

With mica trimmers, I've noticed that some have to be slackened off to a point which seems wrong, but rarely if ever, one which has to be tightened hard. Where there are fixed 1 and 2 pF caps, I've had to take them out rather than add more capacitance.

As for tinkering with things that are best left alone, my worst experience was adjusting a MW RF coil in a tatty Philips domestic set. It seemed to adjust a bit then MW went dead as I was adjusting it. Opening it up showed that the core went through a former like a plastic drinking straw, and had obvious got damp at some stage and stuck the core to the former. Adjusting the core turned the former inside the coil, unwinding it from the inside and eventually breaking a wire.


Pete.

[GMB]

how do you know whether IFs are properly peaked unless the performance is dreadful?

Well in my case one coil was a trap for a crystal harmonic and that had to be backed off 1/4 turn to get it back on the peak.

Another is a band pass filter used by the 1st mixers of both rx and tx which is the worrying one. It's a 5.2-5.7Mhz band pass and it seems to be shifted down by at least 0.2MHz. The inductors are mainly little toroids I think so not adjustable. It seems a big error and it's close to "no user adjustable parts inside" the way the trimmers are sealed up. I can't see what else could cause this shift of a complicated little batch of Ls and Cs. The crystals and VFO seem to be spot on so it isn't that the filter is right with the oscillators wrong.

[kalee20]

Originally Posted by XTC

As for tinkering with things that are best left alone, my worst experience was adjusting a MW RF coil in a tatty Philips domestic set. It seemed to adjust a bit then MW went dead as I was adjusting it. Opening it up showed that the core went through a former like a plastic drinking straw, and had obvious got damp at some stage and stuck the core to the former. Adjusting the core turned the former inside the coil, unwinding it from the inside and eventually breaking a wire.

Yes that happened to me too, it was a little Philips Philetta radio. I had to strip the IFT apart and I made a replacement former with cigarette paper soaked with Araldite, wrapped around a waxed drill bit as a former which I slid out after it had semi-hardened. Then re-winding with multi-strand 48SWG wire. It did work!

I've never noted exactly which way alignment is out, I will do so in future. Certainly circuits trimmed with mica compression trimmers do seem to benefit from a tweak more often than slug-tuned inductors.

[dominicbeesley]

Fag paper soaked in araldite - wish I'd thought of that. Those philips cores are a real swine to fix! I came up with a similar, but less pretty fix of using the card off my fag paper packet to replace the straw. It more or less works! Moral: don't try and melt wax out in a Philips set!

I usually re-align sets: I've had a good number that have benefited especially on SW and LW and a few (like the Philips) that I've made worse/broken.

Re tuning drift, I noticed that the Ultra set I had went out of true a couple of times, it was running in a factory workshop that was getting very cold at night and then very hot and humid during the day (and the set was on with its ballast pumping out heat).

It's not done it since coming home and sitting in a relatively damp but temperature stable cellar, though it gets pretty light usage.

So I'd conclude extremes of temperature possibly mixed with humidity?

Dom

[Skywave]

Originally Posted by dominicbeesley

.
So I'd conclude extremes of temperature - possibly mixed with humidity?
Dom

'Fraid so!
These variables are probably the most common cause of long-term deterioration in any electro-mechanical device. Repeated mechanical expansion & contraction due to temperature cycling plus the repeated water condensation / evaporation cycle leads to all sorts of troubles.
Now add in corrosive atmospheres: salt (hydrogen chloride), nitrogen-based compounds, ozone. . . plus mechanical vibration of a repetitive harmonic motion . . . and you get some idea why kit for military use is so rugged, chunky and heavy.
In such an environment, it's not just the alignment that drifts . . . it's the whole item!

Al / Skywave.

[Steve_P]

In a lot of cases, leaving a set on for a couple of days and keeping it in a warm place means that the alignment will drift back sometimes.

I've got a few sets that did just that here!

Cheers,

Steve P.