Ever Ready "Sky Countess" - shorting tuning capacitor

[humorist2751]

Greetings:

I have been working on this set all day - it only arrived this morning! I have changed all the usual suspects (all the Hunts Mouldseals and the sole electrolytic), placed the valves in their correct positions (the DK96 and DL96 were swapped), attended to a battery wire having come adrift from the HT battery plug, bridged a broken area of pcb track, and I was finally ready for testing.

Initial results were very promising - the set comes on like gangbusters (and with two of the valves still branded "Ever Ready," yet), but this is only for a few degrees of the tuning. The tuning capacitor then shorts out and stops everything.

Unlike some service sheets, that available for this model (actually for the "Sky Baronet," but it's the same circuit - thanks Paul) does not state the value or swing of the tuning capacitor. I have read online that it is possible to straighten them, but that seems like a very delicate task indeed!

Before I essay this process, is there anything I should know about techniques that have a good chance of success (if anyone knows any)? Incidentally, I have found components in the US that may work, but there are two issues with them: firstly, the supplier will not send them overseas; secondly, they are configured the opposite way round to the one in the set, so even if used, the dial would then be giving the wrong readings!

Any guidance or insights will be very welcome; thank you very much.

[Mike. Watterson]

Sheets of card / thin plastic and careful wiggling.
Some I've fixed and some I haven't.

I can check what capacitance swing on RF and Osc my Sky Countess is and if I have a replacement part if you fail.

Sometime very fine wet & dry works when closed, sometimes a nail board works when open. Check screw & nut at rear that vanes are centred.
Disconnect it and verify which section is shorting. Use lamp and battery or a beeper mode on DMM while working.

[paulsherwin]

You can usually repair them, though not always. You can use anything that will fit - a knife, an old CD, a credit card etc. Sometimes the cap has gone out of adjustment and can be fixed by moving the screw on the back, as Mike says.

I'm sure somebody will have something suitable from a scrap set if necessary.

[sobell1980]

You could try using a set of feeler blades. You could measure the thickness of the vanes where they are not shorted to gain the correct gap. You can then use the feeler blades to straighten out any shorts to the correct gap. Multiple feeler blades can be put together to get exactly the right gap. Worked for me.
Dave.

[paulsherwin]

As a last resort you can actually remove one or more vanes. Obviously this isn't desirable as it will upset the tracking, but it may return an otherwise unusable radio to life.

[Ed_Dinning]

Hi David, in most cases it is the outer vanes that get bent and are easily straightened..
These vanes are meant to be bent to adjust the tracking, but in practice bending them a bit more has vey little effect on the performance of the set.
Sometimes the locknut at the end of the vanes loosens and the set screw needs to be reset.
If you are really unlucky the cap has BEen hit with something and several vanes get distorted.

If it is too bad, remove it and post a pic, I may have one in the spares box, but I'm away at present.


Ed

[SeanStevens]

A small DC voltage can help spot when the vanes touch - one terminal to the moving plates, the other to the static ones.

It is easier with the variable cp out of the radio - in a darkened room. Once a spark is spotted, adjust that vane and test again.

SEAN

[Tractionist]

I've stripped several vane tuning caps [usually because of seizure and corrosion] - cleaning in-between corroded vanes with a very fine file, and then [as mentioned in previous posts] setting then back up using a feeler gauge.

Just one point - are you certain that it's shorting out? Have you cleaned the spindle brush[s] and ensured that this is making effective contact through-out the entire swing of the moving vanes?

[Station X]

Looking at the circuit for this radio, it has a combined OFF/ON/WAVECHANGE switch.

It appears from a cursory glance that in the OFF position the aerial and oscillator coils are disconnected, so it should be possible to use a meter on ohms range to check for shorting plates.

[humorist2751]

Station_X: the data - the only available - is for the "Sky Baronet," which has the same circuit but there are detail differences, the on/off switching being one of them. In this set the on/off is done by a plunger, which is depressed (thus breaking the circuit) when the lid is closed, and comes up under spring pressure (thus making the circuit) when the lid is open.

I removed the tuning cap as required, and it was the oscillator section (the rear) which was shorting out; I could feel the extra friction, and I could even hear the scraping when holding it close to my ear!

Examination under a magnifier showed that the vane gaps were a little asymmetric, so a continuity tester was placed across the rear gang, while the rear adjustment screw was tweaked, in search of a setting that would give no shorts at any point. That ideal setting was found, and the front gang was rechecked in order to see whether that had been compromised; it hadn't.

The tuning capacitor was refitted to the set, and another test was made; the response still died, but not quite as early in the travel as it had before. I decided to check that the IF circuits, at least, were blameless (which I believed them to be - and it would also give me a chance to see if the IF was anywhere near correct).

The adjustment knob on my signal generator is rather coarse in its action, which is why I usually couple it to a frequency counter, but as a quick check, I used it alone; with the result that the IF alignment is definitely in the right ballpark.

This brings me to C14; this is a decoupling capacitor on the edge of the oscillator circuit. The original value was 0.04 uF; this being extinct, I fitted one of 0.047 (that is not a Hunts mouldseal, as the original was).

At the minimum capacitance end of the tuning cap's travel, the inductor L6/L7 does most of the work in determining the oscillator frequency, the capacitance being relatively small (even with an error in it). Yet as the capacitance of the gang increases, the capacitance plays an ever greater part, and it may (if in error) push the resultant frequency beyond the capture of the IF circuits.

That theory depends upon whether or not the change in value of C14 would have any effect upon the oscillator setup; it is, after all, a decoupling capacitor and is not there explicitly to influence the oscillator frequency. Yet, given its position in the circuit, would it do so?

If that is the case, then an RF alignment - preceded by an IF alignment, just to be thorough - should cure the problem. What does the team think?

Any insights and help will be most gratefully received.

[Station X]

Don't overlook the fact that adjusting the bearing screw will change the capacitance range of the tuning capacitor.

As you have a frequency counter you should be able to check LO coverage and check whether the LO stops running at any point. Just use very loose coupling.

[humorist2751]

Going through the IF alignment did noticeably improve the performance in that respect. However, while the low-capacity end of the MW RF alignment could be done, the high-capacity end really didn't work all that well; and the result was, at the maximum end, an audible oscillation from the set.

This may be a layout issue, as when I replaced all the Hunts mouldseals, the only replacements I had to hand were not a similar arrangement - and there was no room for them under the tuning capacitor (where several of the originals were), so I put them on the print side. While that would probably be fine in an audio circuit, in an IF or RF circuit it is probably asking for trouble (which I got)!

So I shall replace those components again, with types that will be able to be fitted where the originals went, and then reassess the situation. I must say that the audio amplifier is pretty powerful in these sets; it's just a pity that it doesn't (yet) pick up any signals.

I did give a squirt of cleaner to the capacitor rotor contact wiper, and it prevented a couple of crackle spots that were there before - but sadly no more than that at this stage.

I will replace these capacitors with more layout-friendly types, and then report back.

[crackle]

Not sure if you have the shorting tuning cap sorted yet, but thought I would mention this anyway.
I have come across a couple of radios where the fixed vanes have become loose on their porcelain posts and hence misaligned. This can usually be rectified with a screwdriver blade gently levered against the bar which links all the fixed vanes together. Then a blob of superglue to lock the cup onto the porcelain post.

Mike

[ms660]

Not sure if this is relevant to the problem but as has already been said in this thread, shifting the rotor plates laterally from a central position will alter the capacitance.

Eg: Suppose a tuning gang has a full capacitance of 400pf per section with the rotor plates set central to the stator plates, suppose there's a gap either side of the rotor plates of 1mm with respect to the stator plates, if the rotor shaft is then adjusted to give a gap of 0.5mm on one side and 1.5mm on the other then the capacitance per section will increase to approx 532pf (33% increase in capacitance)....If I've done the calcs correctly.

Can anyone confirm?

Lawrence.

[humorist2751]

Mea culpa, mea maxima culpa... yes, it was as I surmised; after solving the shorting tuning capacitor problem, the other issues were caused by layout. Having now fitted components that live exactly where their predecessors did - all above board (no pun intended) - the set works.

It does, of course, need yet another realignment - but at least this will be on a firm foundation. Rather than a series of heterodyne whistles and then absolute silence - the silence over much of the band - I now hear stations, albeit perhaps less strongly than the set is capable of when correctly set up. The oscillation at the maximum capacity end has totally gone, obviously the result of some unwanted feedback only made possible by mounting things where they shouldn't have been. A valuable lesson learned, I think.

[Mike. Watterson]

Great.
Align MW first
Coils at low Frequency end, trimmer caps at HF end, but in neither case at the actual end. Typically 600KHz and 1500kHz.
Then LW at 198kHz (coil, or a lower frequency like 170KHz) and 252kHz (cap if there is one!)

You can use suitable off air stations, or 3 to 6 turns loop about 15cm dia on signal generator only close enough to just pick it up. Direct connection pulls off the tuning.

[humorist2751]

Thanks for the information Mike, but in this case the full alignment procedure is in the service sheet, so I followed that. The results are most encouraging!