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Forum 141

KB KV014 Fuse blowing

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colourstar
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I don't think my everyday-use KB set appreciated me being away for a couple of days over Christmas. It was working fine on Christmas Eve, but when switched on yesterday it audibly blew the 3 amp fuse in the mains plug. The internal fuse, which - rightly or wrongly - is a slow-blow type, remained intact.

I'm left wondering what could have occured to cause this. A resistance check across the pins of the mains plug gives a reading of around 1.1M, which compares to a reading of about 980 ohms on another VC-series set I have.

Where to start?

 

Steve

 

KB.jpg

 
Posted : 29/12/2016 9:48 am
Niall
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Is there a mains suppression cap which could have broken down and blown away a part of the foil as they are designed to do?

Does it power up with a new fuse?

 
Posted : 29/12/2016 11:56 am
Nuvistor
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First thoughts, mains suppression cap blown damaging the on/off switch. Remove the cap and test the switch.

Frank

Frank

 
Posted : 29/12/2016 12:56 pm
colourstar
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Hi Niall and Frank

I should have mentioned that the mains filter cap is no longer present, having been snipped out some time ago.

A new plug fuse holds and a neon screwdriver shows life at the set's internal fuse and at all points on the dropper but there are no valve heaters or other signs of life. I also tested the HT rectifier diode and this read OK.

Steve

 
Posted : 29/12/2016 1:47 pm
Nuvistor
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Is the chassis also live, lights the neon? That would indicate a break in the neutral line, wire, on/off switch etc. But could be an o/c heater.

If you have a multimeter that would be better than the neon.

Frank

Frank

 
Posted : 29/12/2016 1:58 pm
colourstar
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Hi Frank

Yes indeed, the screwdriver lights when touching the chassis.

The multimeter is at hand, fear not! And thanks to your excellent deduction, sure enough there's no continuity on the neutral pole of the on/off switch, so there we have our fault. Time to raid the spares box!

Steve

 
Posted : 29/12/2016 2:09 pm
Nuvistor
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It could have been the switch that blew the fuse but I would carefully check the circuit immediately after the switch in case there is a s/c that could blow the new switch.

Frank

Frank

 
Posted : 29/12/2016 2:21 pm
colourstar
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New switch fitted and with no obvious s/c to be detected I'm happy to say the set is now up and running again.

Thanks to Frank for analysing what seemed (to me) to be some odd symptoms, although they make rather more sense now. Another one to learn from.

Steve 🙂

 
Posted : 29/12/2016 3:41 pm
Nuvistor
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Nice easy one for you to fix.

Some very early radios, especially AC/DC USA ones put the  SPST on/off switch in the neutral line. This had the possibly dangerous effect of leaving the whole set live even though switched off, exactly the same symptoms as you fault.

Frank

Frank

 
Posted : 29/12/2016 3:49 pm
Cathovisor
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nuvistor said
Nice easy one for you to fix.

Some very early radios, especially AC/DC USA ones put the  SPST on/off switch in the neutral line. This had the possibly dangerous effect of leaving the whole set live even though switched off, exactly the same symptoms as you fault.

Frank  

That was usually because it was expedient to do so with sets fitted with line cords, especially tapped ones! Before the war, Bush preferred to fit a double pole switch round the back of the set, separate to the volume or tone control, in the interests of safety.

After the war and with the DAC81 and successors it was a single pole switch in the neutral, even though it wasn't necessary - it could just have easily have gone in the 'live'. Whether this was an attempt to keep 240V of hum away from the volume control or there were simply more SPST switched pots around I do not know, but...

 
Posted : 29/12/2016 5:30 pm
Nuvistor
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Yes and I can understand the switch location when line cords were required.

I found this on http://www.angelfire.com/electronic/funwithtubes/AA5-1.html  that gives a thought about why the US All American Fives  used a SPST switch in the neutral line. These radios were trimmed down as much as possible and still work, anyone who has not come across them, the Angelfire web pages are well worth a read.

quote" Note the location of the switch in the diagram. Placing the switch in the ground side of the line saved them a terminal lug. They were concerned with saving fractions of cents. In the last section on the AA5 I tell how to wire the radio for increased safety."

Placing the switch in the neutral side is an interesting thought about keeping the hum down, not thought of that, I seem to remember some instructions on UK radios suggesting turning the two pin plug around for best hum reduction.

Frank

Frank

 
Posted : 29/12/2016 6:15 pm
colourstar
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Just to conclude, here's the set back in action. Hooray!

 

KB-1.jpg

 
Posted : 29/12/2016 8:09 pm
Nuvistor
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thumb_gif

Frank

 
Posted : 29/12/2016 8:34 pm
PYE625
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That's a smart looking set, glad you got her up and running again welld_gif 

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 29/12/2016 9:20 pm
Focus Diode
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That's a relief! I was reminded of the intermittent trip switch activation on the Regentone/Defiant set I experienced a few years ago. Worth recounting this for newer forum members.

The set worked fine but after a few minutes the household electricity supply trip would activate. The set fuses and plug fuse remained intact. I soon discovered plugging the coax lead into the socket caused the trip to activate.

 

What was happening? I'd recently repaired the dry joint in the coax socket in a not very well lit place. On further examination a splash of solder intermittently shorted to the chassis.

I was using the Dinosaur converter at the time which is 3-wired. The short to chassis resulted in mains Neutral shorting to Earth!

 
Posted : 01/01/2017 5:36 pm
ntscuser
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KB.jpg  

Have to say that set has a wonderful 60s period look. Forum 142

Classic TV Theme Tunes

 
Posted : 07/01/2017 10:10 pm
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