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Old 29th December 2012, 07:23 PM   #251
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Quote:
Originally Posted by gmphadte View Post
Any potential difference (voltage) that is more than 60volts across two points of your body is unsafe, be it on the secondary of an isolation transformer. Don't think putting one hand in the pocket plus insulating your shoes makes your other hand safe. This is due to the possibility of touching two points by this hand, with a higher unsafe voltage as in circuits with tubes.

Gajanan Phadte
Fluke brand DMM's light up the warning "flash" at 30 volts.
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Old 1st February 2013, 06:30 PM   #252
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Most amps I've come across have transformers powering them, but a few antique radios I collect are the transformerless AC/DC sets, also known as the "All-American Five" design since they typically use the same 5 tubes. I have heard rumors that there are some low-power amps that also use a transformerless design, but they all have the same problem.

The chasses on these are electrically hot depending on how you plug in the line cord. USE AN ISOLATION TRANSFORMER WHEN WORKING ON THESE. I bought one a long time ago, and it has been a blessing. It will not only save your life, but it will keep you from destroying test equipment. An oscilloscope usually has a grounded side to the probe, and if you tried to connect it directly to the chassis of one of these things, it won't be pretty.

I once gave myself a nasty jolt off a 1950 radio that was especially dangerous because it had an all-metal case. I was holding a grounded antenna cable (I'm a ham radio operator), and happened to lean my elbow on this radio that was plugged in but not turned on. OUCH! Getting 120 VAC through your forearm is not fun. I now lovingly refer to that radio as the "little radio of death" and put a polarized plug on it. I still don't keep it plugged in though.
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Old 5th February 2013, 07:28 PM   #253
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I cut my back teeth on Bush Wooden Radio's. They had a live chassis and no permanent magnet in the loudspeaker. Instead there was a coil, used as the HT choke wound around the soft iron and speech coil.
An excellent idea as it rejected most hum. I remember changing the D type valves. They had 2volt heaters!
My first TV was a Pye 11U. Live chassis with 11Kv EHT. To check the EHT we used to draw an arc to the cabinet.
The last Colour TV I worked on, that had valves, was made by Thorn EMI and had a PD500 shunt regulator on the 30Kv EHT over-wind. This was a cause of X radiation.
On the lighter side of things I have repaired successfully, CB Radio aerials have lots of EMF on them when transmitting. Watch your fingers.
Great fun!
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Old 16th March 2013, 12:00 PM   #254
M Gregg is offline M Gregg  United Kingdom
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I would like to raise a point,

Please use grommets when passing cables through chassis or metal work!

If sharp edges cut through cables it does expose you to possible shock hazard it also creates danger with chassis top plates. The idea of passing HT(B+) through unprotected top plates is bad practice..grommets are cheap to buy or find..Please don't cut corners. It is also a good idea to double insulate any B+ cables that pass through metal work.

NB. Any supply and return cables should always run through the same hole in metalwork to stop eddy currents.

Regards
M. Gregg
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Old 16th March 2013, 01:13 PM   #255
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They make some pretty thick sleeves for cables/wire, but yeah a grommet isn't a bad idea, but really leaving anything exposed like that could be bad.
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Old 16th March 2013, 07:15 PM   #256
rif is offline rif  United States
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I just made it all the way through this thread and thanks! There's a lot of really good advice here.

My $64k question - does anyone have any stats on how many diy'ers get seriously hurt or even killed by this hobby?
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Old 28th March 2013, 08:39 PM   #257
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Quote:
Originally Posted by rif View Post
I just made it all the way through this thread and thanks! There's a lot of really good advice here.

My $64k question - does anyone have any stats on how many diy'ers get seriously hurt or even killed by this hobby?
http://www.cpsc.gov/Media/Documents/...eleased-93098/

Gives a breakdown (for 1995) - "only" 3 fatal electrocutions associated with radio/television/stereo. Doesn't break it down into DIY vs other folks.

But for those three, and their families, it was a pretty big deal...


The important thing, IMO, is not to become one of them.

Play safe, play sane.

~ Sam
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Old 28th March 2013, 11:30 PM   #258
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My FIRST goal is not to get a zap from my PSU caps when I build an amp. I read here and other places where folks are "probing" with DMM leads and it makes me cringe to read that when alligator clips are dirt cheap even ones rated to 600v. Connect the clip leads hands off and power on take reading write down voltage for just that one junction in the circuit and power down and so forth, then use a resistor to drain the caps (just in case), if you leave the clips in place you can see the voltage drain off. If you're building just the power supply and don't have any resistors in circuit and no bleed resistor hooked up yet I just drain it with a 10w wirewound high ohm resistor... test voltages on the last cap of the power supply and I'm good. You can actually see the voltage go almost down to zero and also see it climb back up a little if you leave the DMM leads in place.

Cheers,
Bob
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Old 29th March 2013, 11:26 AM   #259
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Quote:
Originally Posted by sparkydave View Post
Most amps I've come across have transformers powering them, but a few antique radios I collect are the transformerless AC/DC sets, also known as the "All-American Five" design since they typically use the same 5 tubes. I have heard rumors that there are some low-power amps that also use a transformerless design, but they all have the same problem.

The chasses on these are electrically hot depending on how you plug in the line cord. USE AN ISOLATION TRANSFORMER WHEN WORKING ON THESE. I bought one a long time ago, and it has been a blessing. It will not only save your life, but it will keep you from destroying test equipment. An oscilloscope usually has a grounded side to the probe, and if you tried to connect it directly to the chassis of one of these things, it won't be pretty.

I once gave myself a nasty jolt off a 1950 radio that was especially dangerous because it had an all-metal case. I was holding a grounded antenna cable (I'm a ham radio operator), and happened to lean my elbow on this radio that was plugged in but not turned on. OUCH! Getting 120 VAC through your forearm is not fun. I now lovingly refer to that radio as the "little radio of death" and put a polarized plug on it. I still don't keep it plugged in though.
The sets that I worked on were made by Philips and they worked on Ac /DC supplies of 235 volts and they did BITE especially if the "earth" capacitor went S/C making the chassis live I hated them! They used the UCH 41 UF 41 UABC42 UL41 and the UY41 tubes The cabinets were bakelite or hard plastic The circuit was virtually the same as the American "Four plus One" receivers only the heater chain were at a higher voltage with a current of 100mA I did service a couple in a radiogram cabinets with a record player. The capacitor on the ground lead went South and had the owner in hospital for some time
There was a circuit from the Mullard publishers that had an AC/DC hi fi amplifier with push-pull output a pair of UL84's it was rated at seven watts per channel. One guy wanted to build it and I asked him why and he said that everybody build the 5/10 and he want to be different This was the guy that landed the guy in hospital . When discharged he dumped it and built then the 5/10 in stereo a novelty in the mid 'sixties.

Last edited by krokkenoster; 29th March 2013 at 11:31 AM. Reason: mysteakes
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Old 11th May 2013, 03:51 PM   #260
opcom is offline opcom  United States
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It does not look like high voltage DMM probes have had too much mention but as I sometimes buy used ones and have built a few I could say they all need inspection before use. It should always be inspected before using and it ought to be made in such a way that it is obvious if something is wrong. By high voltage I mean here anything from 1KV up to 5KV Sadly, some professional ones are not made to be inspected and do not give any indication that something is wrong until it's too late.

I like a probe that has its own ground lead and is not just a string of resistors but a voltage divider. I can only address items made for use in the USA.

Many of the cheaper "TV set" type probes, the ones rated 20-40KV, are just a large HV resistor and meant to be used with a 20K/volt meter. If the 'meter' end of the probe comes loose, then the HV will be there at that end. It will be very current-limited but still enough to give a surprising shock. Some of these same probes also do not have any kind of ground in the handle (with a ground clip) designed to be arced to in case of some failure.

A good probe is a voltage divider having a HV resistor in series with a low value resistor. That resistor should be rated 2-3x the wattage it will ever see, so that there is little question of it failing. It also necessarily has a ground lead as well as a + lead for the voltmeter, and should have a lead with a ground clip. The ground clip should be soldered, everything should be soldered.

Lastly for now, a HV probe of the TV set servicing kind is generally not made to be left in place for more than a few minutes on a 25KV line as the resistor will usually overheat. This should be overcome in any home made probe. It can be left on 2-3KV all day.

A home-made probe, which I consider perfectly safe, can be made for use up to 2-3KV so it ought to help with most audio requirements. Above that, it can also be made to handle higher voltages, just beware that glass can be arced through with enough voltage.

It's already been stated somewhere that the BBS is not responsible for accidents. I am also not responsible for actions undertaken by others, even if I said to do it, which I have not, nor if the action was compelled by the devil or spirits or subliminal messages or visions/voices or any other motivation including being too cheap to buy a special probe brand new (like me). If you don't like this drawing then don't make probes according to it. HV experts are welcome to comment, it won't bother me.

I can post the formulas for calculating the value/watts/number of resistors, need to find it.
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