Puzzling Ekco Festival A147 problem

[ukcol]

Originally Posted by McMurdo

The fault occurs on any station preset, I'm using R4 as it's the strongest signal around here. Tuning the slug back and forth during the fault simply makes the signal come and go but at the fault-level.

This suggests to me the instability is in the I.F amplifier or, as you say, the detector. I have heard people say on this forum they have had strange faults with double diode triodes (although not specifying the EBC41 type in particular). So it may be worth trying another EBC41 first if you have one, if only because it is an easy thing to try.

[McMurdo]

thanks colin but Ive already changed valves with my other festival set.

[Keith]

This sort of transient-induced problem has bugged me for years on a wide range of receivers. Obviously, an unstabilised power supply will pass some level of mains transient onto the HT rail and I suspect this can trigger changes in ageing, and hence marginally stable passive components. I had a Pye FenMan II that exhibited a similar problem. Eventually the fault became permanent, a screen resistor finally going completely o/c.

[McMurdo]

I've had an hour on it this morning before real work beckons. I thought I'd found the culprit after finding the AF/detector cathode resistor reduced by half..but a new one made absolutely no difference. Then I tried a 1uF 450V capacitor to decouple the HT line around the RF section. This had the effect of making the fault immune from outside electrical interference, though it's still brought on by a station change and cleared by tapping.

[Wellington]

Hi Kevin

Originally Posted by McMurdo

Instigation of the fault only ever happens by electrical interference.

I'm not sure about this - at around 47s in your video (good work, btw!), you instigate the fault condition by operating the wavechange switch (rather than switching something on and off). I wonder if this is significant - operating the wavechange switch might be generating the same sort of electrical interference as switching another appliance off and on - or it may be instigating the fault condition by some other means.

Originally Posted by McMurdo

The fault occurs on any station preset, I'm using R4 as it's the strongest signal around here. Tuning the slug back and forth during the fault simply makes the signal come and go but at the fault-level.

Originally Posted by ukcol

This suggests to me the instability is in the I.F amplifier or, as you say, the detector.

I'm not sure about that last point either. Moving the slug affects the frequency of the Local Oscillator, does it not? What if the fault lay in the front end of the set, prior to g1 of the hexode? That would still fit in with Kevin's observation of the fault condition persisting as he changed the LO frequency, wouldn't it? (Though it probably doesn't fit in with the AVC voltage remaining unchanged).

So, FWIW, I'd suspect something up that front end, especially as tapping that sub-chassis with the controls on seems to clear the fault. What about jonnybear's suggestion re earthing of the sub-chassis - worth a try with a croc clip lead?

A couple of other thoughts/questions:

• I wonder if the sub-chassis is picking up acoustic vibrations as it is resting on the radio cabinet. If you separate chassis and cabinet, is the fault condition still instigated by switching other appliances off and on?
• This is more for my benefit, but can the AVC line cause instability itself? I wondered if it was feeding something back to the frequency changer and IF amp.

(I hope the above doesn't come across as impertinent. That wasn't my intention. I was just giving the subject some thought and thought I'd put my ideas forward to stimulate discussion! ).

[ukcol]

Originally Posted by Wellington

I'm not sure about that last point either. Moving the slug affects the frequency of the Local Oscillator, does it not? What if the fault lay in the front end of the set, prior to g1 of the hexode? That would still fit in with Kevin's observation of the fault condition persisting as he changed the LO frequency, wouldn't it? (Though it probably doesn't fit in with the AVC voltage remaining unchanged).

I can see Wellington's point here. As this is a pre-set station set and not the usual arrangement with a tuning capacitor gang, tuning away from the station by just changing the local osc frequency doesn't change the gain at RF and so perhaps we do have to allow that the fault could be in the front end.

Originally Posted by Wellington

(I hope the above doesn't come across as impertinent. That wasn't my intention. I was just giving the subject some thought and thought I'd put my ideas forward to stimulate discussion! ).

This is the sort of thing I sometimes worry about. Communicating by text takes away all the other clues to what you are trying to convey. Clues such as tone of voice and body language. But you certainly don't come across as impertinent.

[McMurdo]

The chassis is only rested on the cabinet so I can safely probe the underneath..it's been present in other positions. It's an awkward chassis to work on as the frame aerials are on the rear cover and connected with short wiring, and it wont stand up in any way unsupported.

It's true that tapping the bracket with the controls/slugs/rf trimmers is where it's most sensitive..and I've done some tapping recently!! The assembly is very securely bolted to the main chassis at various points and is also earthed in several places with the screens from the volume and gram cables.

Early on I replaced the wire to the tone control as it threads through the RF section over to the audio and the rubber was brittle and I thought it was going down to chassis. Just one of a few false hopes!

Originally Posted by Wellington

can the AVC line cause instability itself?

Yes I guess so, though it appears to work. It's easily disconnected / scoped I suppose. One more for the list!

I'm giving it a break for the moment and will cast a fresh eye over it later on in the week, sometimes you can't see the wood for the trees when one's been staring for hours at a simple yet petulant chassis with an intermittent fault (the curse of my career!).

But keep coming with the ideas as it's motivating and I'm determined to become a festival-set expert by the end!

[McMurdo]

Found the fault!

After pressing the scope into action, (which i suppose I whould have started with) I was rewarded with textbook local oscillator, AVC line and I.F waveforms, all of which were perfect whether the fault was there or not.

The only waveform that changed was the signal on the OP valve grid...which I'd assumed would change anyway.

Whoever said the fault would be on or near the bracket that contains the volume, tone, station selector and tuning slugs was correct. It wasn't an obvious fault, and not a visible one. It was also a fault suffered mainly by transistor radios

If anyone wants to guess what it was before I say, feel free!

[ms660]

Whiskers?

Lawrence.

[McMurdo]

Too easy!


The inside of the 'egen' volume control was a mass of tin whiskers touching the carbon track and altering its resistance to chassis.

A dismantling, cleaning and smearing with silicone grease will hopefully mitigate the problem.

[Herald1360]

Just cleaning them out should be good for another 60 years

Well done!

[Nickthedentist]

Well done, Kevin. Who would have thought that that would be the culprit? Not me, anyway!

[Ian - G4JQT]

I've been following the tread with interest - I too was curious to know what the cause was...

Isn't it satisfying to actually find the cause of a fault like that!

Well done.

Ian

[Keith]

Well done Kevin. Not just AF11x's then. Apparently lead-free solder will also cause this problem in the fullness of time. Would be interested to see a picture of the offending item if you took one.

[Ambientnoise]

Well done, thats one for the memory bank ! I wonder how the volume change was triggered electrically though, unless the audio spike via the speaker moved things.

Ken

[60 oldjohn]

So this is a problem that could potentially affect any set using Egen pots, and possibly other makes eventually. Well done for getting to the cause of the problem.

John.

[McMurdo]

This is a picture of the same model of pot, note it's similar to Erie and Morganite pots but they usually have a bakelite front piece up against the panel. This one has a slightly domed front.

[paulsherwin]

Ekco used that part in a number of their 1950s models. I've dismantled a few to fix crackles and never found tin whiskers, but it's definitely something to be aware of in the future.

[McMurdo]

Paul I dismantled that one after taking the pic and not a sign of a single whisker.

I can't help feeling my symptoms might have been some weird point-contact effect between a whisker and the carbon track. It was so, so repeatable and hence why I went around the houses.

The NASA/Goddard Labs website has a few pics of similarly affected pots:

http://nepp.nasa.gov/whisker/photos/index.html#pot1

[thermionic]

Originally Posted by 60 oldjohn

So this is a problem that could potentially affect any set using Egen pots.....

Just out of interest, EGEN was a company wholly owned by Ekco, the factory was in Canvey Island, Essex, I seem to remember reading somewhere. I think the name EGEN was made up of the directors initials.


SimonT.