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Old 10th Nov 2004, 9:45 am   #1
fableglade
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Default Ekco AC86 Fault

Some time back I restored an Ekco AC86 set, I replaced all of the capacitors (except for the mica ones of course), generally gave the set a good clean, cleaned all of the switch terminals etc and the set has worked well for around a year.

Recently a fault has developed where the set keeps crackling when it is in use. Sometimes it will operate perfectly for a while and then it starts again. I had the set to pieces, replaced any old wiring which looked dubious, checked all joints on the capacitors, checked the aerial and earth connections, checked the shielding connection on the valve top cap lead and I've replaced V1, V2 and V5 with brand new (old stock) valves, but it has made no difference. I've cleaned each contact on the switching assembly individually, but this has made no difference either.

I've now run out of ideas. With the volume control right down and on either MW or LW it is the same, similarly if the aerial is disconnected it makes no difference.

Can anyone suggest what the problem may be please? Is there likely to be a problem with V3 or V4 which would cause this? Is there any other fault likely to develop in this set that would cause this?

It operated perfectly for about a year before this started and it just suddenly hapened one day when I switched the set on, occasionally now when the set is on it will be perfectly clear, but usually it starts as soon as the set warms up. You can still tune in to various stations without any trouble, but the crackling does not go away, it is erratic though and doesn't seem to follow any pattern.

Any help or advice would be greatly appreciated.
 
Old 10th Nov 2004, 10:58 am   #2
jjl
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Default Re: Ekco AC86 Fault

If the fault is present with the volume turned right down it is almost certain to be local to the audio output or power supply stages.

Annoying crackling like this can be caused by dirty valve pins or socket contacts. Leakage or even arcing between valve socket contacts is a possibility especially if the socket insulation is made from paxolin, if this is happening you'll need to replace the offending socket. Another possibility is that the rectifier valve is arcing internally, if this is happening you can usually see the effect if you run the set with the back off in a darkened room, of course, take care if you try this!

I'd advise cleaning the pins of the 2D4A, 354V, AC/PEN and IW3 valves and their socket contacts first. Current advice is not to use switch cleaner to do this as it can apparently soak into certain socket insulation materials.

Good luck
John

Last edited by Paul Stenning; 6th Mar 2005 at 12:05 pm.
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Old 10th Nov 2004, 12:22 pm   #3
paulsherwin
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Default Re: Ekco AC86 Fault

Although you say you have changed all the valves, valve faults can cause this sort of crackling, especially in the audio preamp stage. It's just possible that your replacement valve has this fault as well as the original, so it may be worth trying to borrow another set of valves to eliminate this. It's a long shot though

Good luck, Paul
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Old 10th Nov 2004, 12:26 pm   #4
mjizycky
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Default Re: Ekco AC86 Fault

Another thing not mentioned is failure of the output transformer - an intermittent primary winding. I've had this happen to me, I'm afraid to say.

The more likely one is breakdown of a valve holder - again, I have had this happen to me on a Ferranti 145 that had been stored in a damp coal shed for the last fifty years. Although it had been cleaned up thoroughly, I suspect that this was the root cause of the problem - and as Murphy's Law always dictates, it will always be the most difficult valveholder to change; in this case, the frequency changer, which is located above the wavechange switch
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Old 10th Nov 2004, 1:47 pm   #5
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Default Re: Ekco AC86 Fault

Thanks for these tips. I've actually only replaced V1, V2 and V5, I didn't change V3 or V4, these are both the originals. I've cleaned all of the valve pins and holders, unfortunately I've already used Servisol. When the set is on the chassis only has to be tapped very gently anywhere and the set crackles, so I seems as though the suggestion of a valve socket is the most likely cause.

Would you suggest replacing V3 or V4 first, to see if that makes any difference? If not, which is the most likely socket/s to be causing the problem. The problem is still there with the volume right down.

Can anyone suggest anywhere that I could buy the correct type of 5 and 7 pin valve holders please?

Thanks again for the help.
 
Old 10th Nov 2004, 2:45 pm   #6
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Default Re: Ekco AC86 Fault

I don't have the circuit of this radio to hand, but I'd expect V3 to be the audio preamp. This is by far the most likely candidate for replacement. You should check the valveholder and wiring as well, of course.

HTH, Paul
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Old 10th Nov 2004, 3:49 pm   #7
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Default Re: Ekco AC86 Fault

v4 is the pre-amp ( it is 5v +Rectifier according to the Ekco manual) but I'd agree that it is a good suspect.

As you can be reasonably confident that the problem is after the volume control from your observations, I'd trace the signal forward to see if you can pin it down more accurately. If the signal on the input to v5 is much less affected, then you can deduce that v4 is probably okay and the v5 stage or rectifier is to blame

I won't tell you how I'd do the signal tracing in case you electrocute yourself - Mine is not the safest method !!
 
Old 10th Nov 2004, 4:02 pm   #8
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Default Re: Ekco AC86 Fault

When tying to locate this type of faults and also problems with mains hum. I have found it useful to try and isolate the effected section of the radio, This can be done by removing all valves but the output valve. I have also temporarily grounded the control grids to silence various sections. I think I would also try substituting the volume pot. Finally are you certain that its not a simple problem with the mains wiring or an interference problem.

(I should have said that once you have removed the valves put them back in one at a time until the problem starts to occurs).

Regards Chris.
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Last edited by Paul Stenning; 6th Mar 2005 at 12:07 pm.
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Old 10th Nov 2004, 5:18 pm   #9
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Default Re: Ekco AC86 Fault

Well the valve line up is as follows (Trader sheet 656)

V1 - FC4
V2 - AC/VP1
V3 - 2D4A
V4 - 354V
V5 - AC/PEN
V6 - IW3

Further to the advice I've rceived here I've ordered a 'new' V3, V4, V6 and a 250K pot, then we'll see what happens from there.

I don't think it is interference or mains related as there is no hum and none of the other sets are picking anything up like this, also tapping the chassis enhances it considerably. I have completely disconnected the aerial and it does not stop the crackling. I'm really hoping that it is one of the valves as I've never changed a valve holder before, but if it isn't it looks like I'll be going down that avenue.

Thanks again for the help, it's very much appreciated. I'll report back on how things go once the valves and pot arrive.

Last edited by Paul Stenning; 6th Mar 2005 at 12:08 pm.
 
Old 10th Nov 2004, 10:27 pm   #10
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Default Re: Ekco AC86 Fault

Just a thought - the trader sheet shows a connector for the loudspeaker through which the HT current flows. Maybe worth bypassing the pin contacts with croc. leads temporarily to see if the problem persists. Also, I 've known the various earthing tags to be suspect on old EKCO's. Have any worked loose, or are any earths directly soldered to the steel chassis and have become high resistance. As an acid test, it would be worth rigging a mA meter ( ANALOGUE !) in the HT feed from V6 cathode to see what happens when the crackling occurs ? Hope you get there soon, as I find intermittents very annoying after a while !!!

ANDY

Last edited by Paul Stenning; 6th Mar 2005 at 12:09 pm.
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Old 11th Nov 2004, 8:47 am   #11
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Default Re: Ekco AC86 Fault

Thanks for the advice Andy. This sort of fault is indeed frustrating as there is nothing obvious which is causing it. I'll certainly investigate what you've suggested, the earthing points sound like a very susceptable area. Thanks again.
 
Old 11th Nov 2004, 12:20 pm   #12
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Default Re: Ekco AC86 Fault

My suggestion is to check by substitution the screen feed resistors and any cathode resistors in AF and output stages. I've known several resistors in these positions to cause crackling even though they read OK and as it seems you have almost exhausted other possibilities, it'd be worth a try. As already suggested, grounding to chassis the input to the AF amp grid should prove the problem to be in AF or power stages. If it continues when grid is grounded, the fault lies in the output or power stages. If it reduces considerably but does not completely stop even when the output valve control grid is grounded, chances are it's in the output transformer primary winding or in a power supply component.
-Tony
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Old 11th Nov 2004, 2:52 pm   #13
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Default Re: Ekco AC86 Fault

Thanks for these tips Tony, would you happen to know the resistor numbers on the trader sheet at all? I'm not experienced enough yet to know what precisely every component does or identify it without being able to refer back to the numbering given on the trader sheet.

Thanks again.
 
Old 11th Nov 2004, 7:19 pm   #14
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Default Re: Ekco AC86 Fault

That'll teach me to pontificate without looking. This simple Ekco output stage doesn't use a screen resistor for the output valve V5. As for the AF amp, V4, it's a triode so there's no screen to feed! What's left? Possibly the tapped cathode resistor R18. This supplies a bias voltage for V4 and V5. The tapping is at 75 ohms, so you could substitute, say, around 325-330 0hm plus 75 ohms in series to create the tapping point.

I assume you've changed C21. I assume also that you've monitored HT levels under fault and no-fault conditions. If HT changes, a fault perhaps in the wound components could be indicated: smoothing choke (actually the LS field) or the output transformer.

Again, all this assumes you've isolated the fault to the AF and output stages, as described in earlier forum posts. It is worth noting that the volume control itself returns to earth via C16, so a DC component is going to be present at the slider, leaked to earth via R14 - itself a suspect as it's a high value. R17 is suspect, too. Dry joints on any of these components could be the cause.

You've got your work cut out...
-Tony

Last edited by Paul Stenning; 6th Mar 2005 at 12:10 pm.
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Old 11th Nov 2004, 8:20 pm   #15
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Default Re: Ekco AC86 Fault

For an intermittent fault of this nature, one of the first things I would have reached for is that very useful tool in the serviceman's arsenal - the freezer spray can.

Carefully directed onto components, the characteristic changes that freezing induces will often yield up the culprit component without having to change half the bits in the box.

I highly recommend it, and given the amount of time invested so far, I think you should splash out the few quid it will cost. Always worth having one in the "gun cabinet".

p.s. Needless to say, keep the spray off the valves!!

Last edited by Paul Stenning; 6th Mar 2005 at 12:11 pm.
 
Old 11th Nov 2004, 10:30 pm   #16
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Default Re: Ekco AC86 Fault

Thanks for the detailed help Tony, I'm very grateful, I'll follow your advice step by step and see what I can find. I have indeed replaced C21, in fact all bar the mica capacitors have been changed.

Thanks for the tip on freezer spray coherers, that's something I've never used, can it be bought from Maplins? I presume it shows up areas where heat is generated by arcing and the like?

I can see a lot of serious investigation here. I hope to receive the spare valves I've ordered in the next day or so, so I'll concentrate on the set together with a print-out of the advice I've been given here and see what I can find out. Thanks again for the help.
 
Old 11th Nov 2004, 11:29 pm   #17
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Default Re: Ekco AC86 Fault

Quote:
Originally Posted by Doctor Who
Thanks for the tip on freezer spray coherers, that's something I've never used, can it be bought from Maplins? I presume it shows up areas where heat is generated by arcing and the like?
Maplins do a freezer spray, albeit a bit more expensive than I'd like !

Because the aerosol cools the component etc. under test so much, it will often cause the set to either stop crackling or to crackle like mad (more often the latter I have found).

With an intermittent fault, there is something somewhere that can't decide which state it is really in, and the large change in the physical characteristics caused by going from running temp to seriously sub zero often causes it to worsen or to disappear. For instance, on a poor contact the contraction resulting from the cooling could cause it to go more "open" than normal.

Last edited by Paul Stenning; 6th Mar 2005 at 12:12 pm.
 
Old 12th Nov 2004, 8:42 am   #18
fableglade
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Default Re: Ekco AC86 Fault

Thanks for this information Coherers, what a brilliant idea! I'll have to order some and give it a try, it sounds extremely useful in fault-finding. Is the spray itself a conductor and can it be used when the set is actully on or do you spray it and then switch on?

I'll have a hunt for this on the Maplins website.
 
Old 12th Nov 2004, 6:53 pm   #19
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Default Re: Ekco AC86 Fault

You use it with the set running, but as I say keep it 'orf the valves !! You can direct the spray onto the suspect components quite precisely with the thin tube that comes with the aerosol.
I assume the spray is a non-conductive solvent. It rapidly evaporates, removing heat in the process. However, the frost that forms on the component surface will be conductive to some degree but nothing to worry about.

http://www.rapidelectronics.co.uk/rk...0084&XPAGENO=1

http://www.maplin.co.uk/Module.aspx?...dID=&doy=12m11

I should say it doesn't always reveal the problem, but it is worth trying with faults like this because it saves so much effort when it works.

 
Old 12th Nov 2004, 7:02 pm   #20
fableglade
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Default Re: Ekco AC86 Fault

Thanks for that Coherers, I'll get and order some, it sounds very helpful!
 
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