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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Jan 2023, 4:03 pm   #1
mpman24
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Default Blowing Radio Fuse help

Hello All:

I have a Korting 1091 that I completely recapped a couple years ago and had been using almost daily. Recently the fuse blew and I'm trying to find out why. All voltage and resistance readings are within tolerances and the fuse has not blown with many on/off cycles during my testing.

It is a stereo unit with an EL84 and ECL86 output tube. The one thing I have noticed is that if I tap on the ECL86 or the small separate circuit board that the ECL86 is on, I will get some noise and static. I know these boards tend to be fragile. Is that board a likely cause of the fuse blowing? If not, any other suggestions?

If that is likely the culprit, is there any harm running the radio without the ECL86 tube if I just keep it in mono mode?

I'm still relatively new to radio restoration and this is a complicated one so any help would be appreciated.

Many thanks,

Mark

Last edited by Station X; 24th Jan 2023 at 4:22 pm. Reason: Please don't post entire documents lifted from radiomuseum.
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Old 24th Jan 2023, 5:24 pm   #2
Techman
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Default Re: Blowing Radio Fuse help

It's quite normal for an old glass type fuse that's been in service for many years to just fail due to old age, so nothing to worry about unless it keeps happening. Just make sure that you replace the fuse with the correct type and rating.

No, it's not a good idea to remove an output type valve permanently. It'll take too much load off the power supply and cause all the voltages in the set to increase above their safe values.

You need to investigate and cure the bad connection crackling, as this can lead to possible arcing and eventual tracking with possibly burnt components, or worse. It could be dirty or oxidised valve base pin and socket connections, or it could be a bad circuit connection or dry joint, it could even be a cracked resistor. Close inspection should find the issue.
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Old 24th Jan 2023, 7:51 pm   #3
Sideband
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Default Re: Blowing Radio Fuse help

Quote:
Originally Posted by mpman24 View Post
The one thing I have noticed is that if I tap on the ECL86 or the small separate circuit board that the ECL86 is on, I will get some noise and static.
As it's an ECL86 I wouldn't be at all surprised if it was the valve itself!

That doesn't mean it's the cause of the fuse-blowing and more than likely it was just the fuse being old (as suggested above). Every time the radio is switched on, the fuse is under some stress and causes a slight 'sag' in the fusewire. After many cycles (years), eventually the sag is enough to just break the wire. No fault, it's just age. Make sure you fit the correct fuse, take note of the exact type, not just the rating. For instance fuses can be fast blow or slow blow and have a high or low breaking capacity. A 'T 1A L' type would indicate 'Timelag' (slow blow) 1 Amp 'Low breaking capacity' and is not the same as 'F 1A L' (Fast blow 1 Amp Low breaking capacity) even though they are both rated at 1 amp......it's a minefield!

It would be worth your while checking the PCB for poor joints and connections particularly near hot-running components and of course the valveholders. Just resolder any dull or poor looking joints. That may cure your crackle when tapped. Even if the ECL86 crackles slightly when tapped, it's not necessarily a sign that the valve is about to die....it might have been like it for years. Some valves are a bit noisy when tapped but are fine in normal use.
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Old 24th Jan 2023, 10:52 pm   #4
mpman24
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Default Re: Blowing Radio Fuse help

Thank you both. I should have mentioned that I did put in a new fuse of the right size so it's not just the fuse being bad. I have tried a new ecl86 and I still get the crackling on that board. So it sounds like I should inspect every component and joint on that board. Would a bad joint on that board be enough to cause that fuse to blow?
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Old 25th Jan 2023, 9:13 am   #5
John10b
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Default Re: Blowing Radio Fuse help

I would treat the crackling on your pcb with some degree of urgency. I have experienced pcbs completely burnt out due to a badly soldered valve holder.
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Old 25th Jan 2023, 9:59 am   #6
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Default Re: Blowing Radio Fuse help

Does the fuse need to be an Anti-Surge/ Time delay type?
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Old 25th Jan 2023, 11:59 am   #7
mpman24
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Default Re: Blowing Radio Fuse help

Thanks Frank. The specs that I have state that it should be a a slow blow which is what I'm using.
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