9 Oct
2007
9 Oct
'07
1:22 p.m.
On 8 Oct 2007 at 23:22, Tony Duell wrote:
True enough. But a neon lamp could develop a hairline fracture and stop
working, for example.
Provided you do the 'test on known live, test on circuit-to-work-on, test
on known live' procedure, I think any voltage measurement/detection
device is equally safe. The chances of it breakign when used on the
circuit-onder-test and then repairing itself when used on the known-live
circuit are pretty remote.
There is 'something to measure', namely
the voltage on the mains wiring :-)
More seriously, I see no reason why a voltmeter (analuge, digital,
whatever), used correctly -- test a known live point, test the wiring you
want to work on, test a known live point again -- is any less safe than
any other method of determining that wiring is dead.
Other than cost and fragility of the old analogue meters, you're
probably better off using something with a low number of points of
failure (a neon lamp is pretty simple). You're not interested in
*measuring* the voltage, just testing for its presence or absence. Back in the
1960s there were many live-chassis valve radios over here.
Seires stringh heaters (normally 0.1A current) and half-wave
rectification to get the HT+ line. WHich meant the chassis of the radio
was connected to one side of the mains.
...as well as hot-chassis television sets. And the US hot-chassis radio goes back to at least the 1940s (12SA7, 12SK7,
12SQ7, 50L6,
35Z5); some of the 50's models used 7xx or 14xx loctal based tubes.
Later, the lineup was usually 12BE6, 12BA6, 12AV6, 50C5, 35W4 in
miniature envelopes.
YEs, I've heard of such sets. They appear over here from time to time,
sometimes with a 'line cord' (mains cable with one resistive wire) to
drop our 240V mains down to 120V for the set.
UK-designed hot-chassis radios of the 1950s and later tended to have
100mA heater chains using U-series vables. Sticking with the B9A (9 pin
miniatures) for the moment, the common ones wrre :
UCC85 : double triode, often used as a FM-band frequency changer.
UCH81 : triode/hexode frequency changer
UF89 IF amplidier pentode
UBF89 : IF amplifier pentode and 2 diodes (detector and AGC rectifier)
UABC80 : Triple diode (AM detector + FM discrimiator) + audio amplifier triode
UL84 : Audio output pentode
UCL82 : Audio triode and output pentode
UY85 : Half-wave rectifier diode.
Be warned there were a few UK AC-only radios using an autotransformer to
supply the heater string -- and thus a live chassis. Worse still, there
wrre a few sets with 6.3V heaters in parallel (E-series valves) but the
HT+ coming from half-wave rectifying the mains (and thus a live chassis).
And just to complete the set (!) there was at least one radio with a
series heater string running off a double-wound mains transformer (so a
'dead' chassis).
TV sets often had 300mA heater chains using P-series valves. I am not
listign all thsoe, but I am sure old-timers will rememebr the PL36 (octal
base) and PL81 (B9A base) line output pentodes.
Hot-chassis phonographs weren't that uncommon either (70L7+12SJ7) or
117L7/M7). But all of the "All American Five" sets here used a 150
AC/DC record players were uncommn, since most of the turntable motors
needed AC [1]. But live-chassis amplifiers were common here too. Often a
UY85 rectifier and a UL84 or UCL82 amplifier valve. The heaters were
often run off a tap on the motor winding, which acted as an autotransformer
[1] There was at least one radiogram with an AC/DC radio/amplifier
chassis and a separate unit, only used on DC mains, containing a pair of
50L6s and some oscillator valve. Its job was to provide 50Hz AC for the
motor.
Oh the joys of having a shelf of radio.TV schematics...
ma heater string. Many of the older ones did not have
polarized
mains plugs, so either side of the line could be connected to the
chassis. If there was a lot of AC hum in the audio, you were advised
to reverse the mains plug.
Some of them, I believe, had the mains switch between one side of the
mains cable and the chassis. Which means if you got the chassis connected
to neutral with the set on, it would be live when you turned it off (the
resistnve of the heater string will not limit the current to a safe
value). Ouch!
-tony