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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 12th Aug 2018, 9:02 pm   #1
ukcol
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Default Testing, testing - POP!

Probably as long ago as 15 years I was at a local auction and a Feedback EW604 watt-meter came up for sale. It was a general sale and I shouldn't think anybody there had any idea what it was. Unsurprisingly nobody bid for it and i got it for a couple of quid.

Apart from checking that it worked I have never used it before and it has been sitting on a shelf those 15 years.

Today I had a use for it. I wanted to compare the output from a push-pull amplifier using 2 good output valves compared to 2 valves that were down to about 50% emission.

I started using the speaker as a load and listened to some music just to see if the meter was still alive. I was just about to turn off to lash up a dummy load and connect an audio generator when I heard a "POP" over the music. I looked down at the meter and the needle was still dancing to the music. "Couldn't have been the watt-meter" I thought. Then smoke started to weep out of a joint in the cabinet of the watt-meter. "It is the watt-meter" I thought. (I catch on quickly to these things you know ).

After quickly disconnecting the meter from the mains supply I removed the top half of the case. A lot of dark grey smoke came out and I opened the workshop door to clear the air.

The RF filter capacitor across the mains input had failed (see pictures). The capacitor is after the switch but before the mains transformer primary fuse and so the fuse didn't blow. The 3 Amp plug fuse didn't blow and so it was as well it was not unattended from the point of view of fire risk.

I shall replace the capacitor with a 0.1uF class X type.

I commonly leave vintage equipment running while I am out of the workshop for short periods. Perhaps I need to review that habit.

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Old 12th Aug 2018, 9:08 pm   #2
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Default Re: Testing, testing - POP!

Quote:
Originally Posted by ukcol View Post
I commonly leave vintage equipment running while I am out of the workshop for short periods. Perhaps I need to review that habit.
I recently found one lurking in a Quad amp (or more accurately a Rifa filter - cap with resistor in series).

I tend to leave the amp on for prolonged periods as it sits under the telly and is the general audio system for CD and telly. I've never seen one so crazed and bulged that hadn't actually emitted smoke - reckon I caught it just in time !
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Old 12th Aug 2018, 9:11 pm   #3
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Default Re: Testing, testing - POP!

Yet another nasty see through Rifa cap. They all fail eventually, along with the Wima look alikes.
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Old 12th Aug 2018, 9:33 pm   #4
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Default Re: Testing, testing - POP!

The infamous Rifa cap! I'll bet if you check, the casing is crystalised so it's absorbed lots of moisture during those 15 years.
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Old 12th Aug 2018, 9:52 pm   #5
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Default Re: Testing, testing - POP!

Another B.A.C. !! (and my acronym has nothing to do with BAe's predecessor)

Guy
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Old 12th Aug 2018, 10:25 pm   #6
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Default Re: Testing, testing - POP!

I found a failed one just a few days ago across the mains input of a Goodmans Module 150 receiver.
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Old 12th Aug 2018, 11:11 pm   #7
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Default Re: Testing, testing - POP!

As the blighters are across the mains input, it matters not whether the doodad is turned on or off. It only matters that it is plugged into a live socket.

I tend to turn things off at the wall, and my bench in the radio shack has a general kill switch.

David
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Old 13th Aug 2018, 1:59 am   #8
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Default Re: Testing, testing - POP!

They make a lot more smoke if they go off in a vacuum cleaner.
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Old 13th Aug 2018, 4:56 am   #9
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Default Re: Testing, testing - POP!

I've had plenty of similar capacitors emit clouds of smoke (who hasn't...) but I've never had any further damage as a result, thanfully. Of course I unplug the unit from the mains as soon as the smoke starts.

Has anyone ever had one actually catch fire or do other damage?
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Old 13th Aug 2018, 7:40 am   #10
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Default Re: Testing, testing - POP!

I have repaired 3 Dual 505 record decks recently, they use these see through capacitors as mains droppers for the motor, all 3 had bulging caps, one was smoking gently.
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Old 14th Aug 2018, 12:00 pm   #11
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Default Re: Testing, testing - POP!

Quote:
Originally Posted by Radio Wrangler View Post
As the blighters are across the mains input, it matters not whether the doodad is turned on or off. It only matters that it is plugged into a live socket.

I tend to turn things off at the wall, and my bench in the radio shack has a general kill switch.

David
Hi David,

In the Feedback EW604 the capacitor concerned is connected across the mains supply after the mains switch, but I take your general point.

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Just in case anyone is interested in the results of the tests.........

I used a dummy load of 3 ohms to match the nominal impedance of the loudspeaker and injected a 400 Hz sine wave at the input of the amplifier. I turned the amplitude of the input signal up until (soft) clipping of the output across the load could be observed on an oscilloscope. I then backed off the amplitude of the input signal to get rid of the clipping.

The output from the good valves was about 5 watts and from the pair with about 50% emission 2.5 watts.

The amplifier is in a Murphy A40C and the output for that model is specified at 12 watts. The rectifiers are low giving only 200 volts at the output stage which is why the output power is down.

There should be 250 volts on the output stage and with 250 volts supplied from my bench PSU an output of 10 watts was obtained. As well as the operating anode and screen voltages for the output valves in this set (AC4/PENs) being 250 volts, the data sheet also gives 250 volts as the maximum rating.

I've got some new rectifiers on order BTW.
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Old 14th Aug 2018, 1:37 pm   #12
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Default Re: Testing, testing - POP!

A recent auction lucky dip purchase contained several dozen of these caps, new. They all went into the bin.
The thought of using them as mains droppers is the stuff of nightmares.
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Old 16th Aug 2018, 4:08 pm   #13
steve1010uk
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Default Re: Testing, testing - POP!

In the 80s and 90s i loved these capacitors, they made me lots of money in my repair business.
Customers feared the worst when they saw all the smoke but it was a a nice easy and profitable repair,
They were also used in many Kenwood food mixers.

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Old 16th Aug 2018, 6:54 pm   #14
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Default Re: Testing, testing - POP!

I was recently at the top of a ladder using my mains powered hammer drill to drill holes in a stone wall to mount a satellite dish and there was a bang, and a small jet of red hot flame/smoke blew out of the trigger assembly and actually left a scorched black streak on my hand. I got a bit of a surprise but no real harm done. I was expecting my drill to stop working, but after my heart rate returned to normal, I tried it out and the drill worked perfectly. On dismantling for examination, there was smoke damage on the one piece enclosed trigger module, but no sign of an external capacitor. I am assuming it was across the mains and had blown open circuit. I will price a replacement module as it looks like I will have to destroy the module to open it up and replace the offending component. The drill was made by a reputable main stream long standing company. I also had to replace a leaky cap across the control switch on my wife's sewing machine. She worriedly told me that the machine had a mind of it's own and would start to run slowly while no one was near it. Two more replaced on an industrial food mixer. Filled the room with smoke. On the subject of leaving equipment switched on at the socket, I always replace the plug top fuse with a 1 amp ceramic to reduce the possibility of fire, in low powered gear which has to be left on unattended.
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Old 16th Aug 2018, 7:46 pm   #15
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Default Re: Testing, testing - POP!

Quote:
Originally Posted by Biggles View Post
I was recently at the top of a ladder using my mains powered hammer drill to drill holes in a stone wall to mount a satellite dish and there was a bang, and a small jet of red hot flame/smoke blew out of the trigger assembly and actually left a scorched black streak on my hand. I got a bit of a surprise but no real harm done. I was expecting my drill to stop working, but after my heart rate returned to normal, I tried it out and the drill worked perfectly. On dismantling for examination, there was smoke damage on the one piece enclosed trigger module, but no sign of an external capacitor.
Alan.
Interesting. I had a drill go bang too. Took out a 16A trip as it was faster than the fuse. When I opened it up there was a capacitor clipped to the trigger module so it looked like part of the trigger. It was clearly a cap as I could see inside where the end cap had gone. Replacement part obtained and fitted it works great again now. Again a well known make though this is their home version rather than trade one.

Less OT, I have told my work colleague about the Rifa Caps in the Farnell AP power supply he is using. He keeps telling me its working fine and they've had it for ages. He doesn't need to tell me how old it is, it was in use when I joined the company back in 1999 (I left and am back there now). I'm going to see if I can find one of the crazed ones I removed from my own identical model.
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