Advice needed: working on mains valve radios.

[BakeliteBear]

Well, it’s like this ... I have a good ‘general’ education, I am a graduate in Design and have taught some electronics at GCSE level. I am pretty sound at basic electronics, but obviously nothing close to the standard of you boffins out there. I use a very nice and helpful radio repairman, but would like to be able to test out and make basic repairs. I have a book, which goes through the basics, but I would really like to avoid being killed!!

Any suggestions/advice about the real risks of working on mains valve radios. I am expecting a lot of ‘if you don’t know what you are doing leave well alone’ … well lets see. Thanks in anticipation.

[Brian R Pateman]

Well this (On Paul Stenning's main site)

http://www.vintage-radio.com/repair-...ion/index.html

is a very good starting point. You will find a wealth of information in these forums and of course follow a few simple rules;

Switch off and unplug before fiddling with the innards,
Check that the chassis of "live chassis" sets is connected to neutral,
Keep one hand in your pocket when measuring voltages,

Remember, we all had to start somewhere so don't be afraid to ask.

[Al (astral highway)]

In general, don't do anything unless you can work in uninterrupted concentration for a reasonable period. Think through each little section of your work in advance and identify the hazards and what you can do about them.

I use an RCD (residual current device) on each separate piece of vintage equipment. These are consumer products and you can get them from places like Argos. How they work is OT but have a search on the forum and you'll find past threads.


Be especially aware of turning off power supplies/ disconnecting power supplies when you leave the workbench.

It is easy to get distracted, leave the thing on and then come back later to a live power supply that you think is disconnected. You need to make sure that you have warning systems in place to get round this.

Because there's nothing under the chassis to tell you whether the circuit is complete, I put a neon indicator bulb across the transformer primary side with croc clips and leave it there, on the chassis underside, until the chassis is done with.

Be careful with electrolytic caps in vintage power supplies. These are capable of killing you when charged. In vintage power supplies, they typically operate at a much higher voltage than you may realise (say, close to 400V) as they are charged to the peak voltage at the rectifier cathode.

[BakeliteBear]

Thanks for all good the advice! I have sorted out a RCD and you are definitely 100% right about getting into good working habits. It’s the capacitors that worry me – I really don’t fancy 400V!! Will they discharge in the circuit after a period of time after the supply voltage is removed and by 'be careful' do you just mean 'Don't Touch'?

[wireful3]

Regarding the smoothing capacitors. Yes they should eventually discharge after the supply has been removed but it is wise to treat them as if they don't . Some faults could mean that there is no load on the power supply and if this is the case the charge could remain for a long time.

[chipp1968]

Hello ,You are not the only one on here trying to learn , me too ! not getting far though as to destractions of life . Be carefull you will get a headache heading the wall too long ,Ive already got one

[Skywave]

Smoothing capacitors, discharge of - this might help.

Suppose that you find that you have 400-v dc across the smoothing caps. when the item is switched-on. Switch the set off and leave it for a few hours so that the caps. can slowly discharge. Now with the set switched-off and disconnected from the mains supply, short-circuit the terminals of each cap. with a stout screwdriver. (Make doubly sure that the set really is switched-off and disconnected from the mains before doing this! ) Measure the resistance across each smoothing cap. with an AVO - or other moving-coil ohm-meter. If the reading is substantially above, say, 200 k-ohms, wire a 47 k-ohm (5-watt) resistor across each cap. These become 'bleeder resistors', which at 400-v dc on-load will pass about 8 mA. each and dissipate about 3-watts each. Now when the set is subsequently switched-off, these resistors will rapidly discharge these caps.

Note: If the set uses a pi filter with a series choke, only the final cap. needs the 'bleeder'.

Finally, when all investigations / repairs, etc. are complete, remove these resistors from the caps. and return the set to 'normal'.

For HT values other than 400v, simply scale the R value accordingly. There is nothing sacrosanct about the "8mA" figure - any value of R that produces a drain current of between 4 and 15 mA is adequate, but the current capability of the PSU, its configuration and the magnitude of the HT produced needs to be considered in all cases. The above suggestions are guide-lines only.

Al / Skywave.

[Skywave]

And just to add to Astral's very useful & wise comments above - when working on high-voltage stuff, it's always a good idea to keep one hand in your pocket!
A HV shock across the chest - where the heart is - is NOT a good idea!

Al / Skywave.

[Richard]

Hi

All good advice, but I prefer to use a bulb to discharge caps. I have a 60W bulb in a holder with two leads ending with long probes, conect to the cap and bulb flashes and dies as cap discharges. Gives you visual confirmation of cap discharged and saves you blowing bits out of screw driver blades, and no loud bang to make you jump.

Richard

[Brased]

Originally Posted by BakeliteBear

Any suggestions/advice about the real risks of working on mains valve radios. I am expecting a lot of ‘if you don’t know what you are doing leave well alone’ … well lets see. Thanks in anticipation.

Well yes. That is good advice. However this is mainly a matter of thinking about what you are doing and trying to achieve, and working methodically. There is much safety advice on this site and if you take the precautions suggested against contact with mains or high voltages you are in less danger than in many, if not most, other hobbies. If in any doubt then just ask. There are many experts on this site and all are still alive. Electrocution is a rare cause of death.

Do not fear electricity: just treat it with respect and it will not harm you.

Edward

[Aerodyne]

Lots of useful tips on this thread. I've a couple of points worth making, I think:
1. Many vintage receivers use 'live chassis' technology. It can be a problem to identify which way around the mains is applied and there's a fifty-fifty chance that the twin mains cable connection might make the metal chassis live to 230V mains. Rather than touching it to find out - never a good idea - use a neon screwdriver to test. If it lights, swap the connections to the mains around.
2. Bench surface must NOT be metal or have a metal edging. Melamine kitchen surfacing is useful and non-conductive!
3. A dry floor - lino, rug, wooden duckboarding - plus insulating (dry, of course) shoes. Beware the damp shed or garage.
3. Power your sets through a safety series lamp for initial testing, after checking that no short circuits exist at mains input or from the HT line to chassis.

Discharging capacitors using a screwdriver is a little violent and can damage the component. Better to use a low value resistor or a lamp, as others have suggested. The 'one hand in pocket' rule is important, as already mentioned as it prevents your body making an inadvertent circuit, via the heart muscles, across a high voltage source.
Remember that any set you repair or restore could be used by others.
Good luck. We all have to start somewhere and you've got a good background (from one who also taught design and technology for many years)
If there's anything arising from these observations that might lead off-topic, PM me.
-Tony

[Skywave]

Whilst not wishing to hair-split or sound pedantic - but to avoid any misunderstandings - when I referred to discharging a cap. with a screw-driver, this was under the assumed conditions that the cap. was at least 99% discharged prior to this.

If in doubt, check with a volt-meter first.

Al / Skywave.

[BakeliteBear]

I really appreciate all your helpful comments. I am currently carrying out building work in our house, which should allow me to set up a proper and permanent workshop space, which I used to have until we moved house … I used to love my shed … men need sheds!!

I have done a lot of work on telephones, but next to nothing on radios. I have never known where to turn to for help and advice, but I can already see that this community is really supportive, helpful and encouraging. Thanks to all.

[Skywave]

Originally Posted by BakeliteBear

. . until we moved house … I used to love my shed … men need sheds!!

Absolutely!

An Englishman's home is his . . . . shed - as I keep pointing out to SWMBO. She claims that I'd sleep out there, but for the fact that it's too cluttered with junk (her phrase) to squeeze one in .
Interesting idea though

Al / Skywave.

[RODALCO]

Didn't see anything about an isolation transformer.

Get a 240 / 240 Volts 300 or 500 VA isolation transformer.
Supply the testsocket from the latter.
As long you don't touch both, Phase and Neutral terminals of the secondary side.
It will eliminate getting a zapp from a live chassis.
A leakage breaker is OK but 30 mA's at 230 Volts will still give you a good whack.
Still use the one hand rule during testing.

Regards, Raymond

[paulsherwin]

Originally Posted by RODALCO

Get a 240 / 240 Volts 300 or 500 VA isolation transformer.
Supply the testsocket from the latter.
As long you don't touch both, Phase and Neutral terminals of the secondary side.
It will eliminate getting a zapp from a live chassis.

You will have a similar level of protection if you are very, very careful that the chassis is connected to mains neutral.

You should always check that any mains sockets being used are wired correctly (it's not unknown for live and neutral to be reversed, even with supposedly 'professional' installations). Check the wiring to the plug, and change any non polarised mains cable before doing anything else. Finally, check the chassis with a mains tester when powering up for the first time, and every time you've done significant work. In fact it's a good idea to get into the habit of always checking the chassis with a mains tester each time you power it up outside the cabinet - it only takes a second.

Paul

[Herald1360]

Originally Posted by paulsherwin

You will have a similar level of protection if you are very, very careful that the chassis is connected to mains neutral.

Paul

Even when the radio's own mains switch is turned off- it's not unheard of for said switch to be a single pole that disconnects neutral from the chassis which turns the set off OK but leaves the chassis to float up to line.


There are RCD's which trip out at lower than 30mA- 10mA is available- one of these would be a good investment. They're not used for general household protection because they're more prone to nuisance tripping and don't offer significantly greater safety, but they should reduce the unpleasantness of the inevitable belt!

Chris

[majoconz]

Trouble with an isolation transformer is if you want to use a mains driven 'scope or anything that wants a chassis reference, then a. the scope will pick up a lot of hum, and b. you have effectively removed all the 'safety' the transformer provided. IMHO a low current RCD is better, you only need one. As for bench tops, I bought an 'end of run' piece of carpet and cut it to the same size as my top. Saves scratching cabinets, it's non conductive and can be easily taken outside for a quick beating when it gets dusty (and to take out your frustrations on! )