hot chassis really not so hot

[W3RSW]

Seems that many YouTube and net references these days are alarmed that old style, transformerless  radios such as the all American five, cheaper ham receivers, etc. have the possiblility of one side of the old style, two terminal AC line being tied directly to the chassis and common metal work.  Much talk of designs having insulated knobs, plastic or wood cases, insulating grommets, etc. between the chassis and the user.  Yes, many of these precautions are true but not really to prevent full voltage AC electrocution for the user, (and yes when servicing, an isolation transformer is recommended, especially when rummaging around inside the chassis.)  

In virtually all those radios built using octal and later miniature tubes, all the common negative radio circuitry was tied together wire to wire and only connected to the chassis with an approx. 220k to 270k resistor and a 0.05 to 0.1 ufd capacitor in parallel, not directly to chassis.   Depending on plug insertion, this would allow either an AC neutral or voltage/current limited higher voltage AC on the chassis but not the full 110 VAC with any current deliverability, i.e., "first zap time" regulated by the time constant of the resistor/capacitor combo.  

Even the Knight Ocean Hopper had such a 270k and 0.05 cap.  When as a kid I built one with father's help (open chassis/ no cabinet), I remember feeling the buzz occasionally but never realized it was the two wire AC problem. " All our receptacles and plug were two prong."
I grounded the chassis for better reception and got between the connections at times, or touched another metal cabinet or whatever.

Attached is the typical circuitry showing clearly the arrangement, including isolation of even the phono jack.  This is a slightly upscale radio having a tapped volume control for an early loudness compensation design, two light bulbs and even uses both diodes in the 12SQ7. Pretty snazzy.  Hopefully it was placed in a nice large cab with at least a 6 inch speaker.  First time I've ever seen a tapped volume control shown with the resistance division values.

Many AA5's were used in kitchens, commonly used near the sink...  you name it; my mother's was always on the counter about four feet from the sink metal.  I supposed if one was dropped in the bathtub it would be interesting, but so would a transformer fed one.

Yes it's better to err on the side of safety, but when breathless alarm critiquing an ubiquitous design of the '30's through '60's with today's standards, it'd be nice if the design information was accurate.

 

[w4bfs]

hi Rick  ....one thing comes to mind and that is if a gfci outlet has one of these plugged with a 3 wire power cord may give false trips .... need to separate the current returns and chassis grounds to keep current out of the green wire ground .... then remove capacitor and 270k resistor

by the way on another subject  TR7.  thanks for your help on PIN diodes.  there is a SO23 surface mount very close replacement available cheap ($10 for 50 diodes) .... something like Mvn 3401 .... sorry don't have real p/n with me .... if you are curious take a look on eBay and see if you agree ...

[W3RSW]

PIN's yeah I'll check 'em out.  just solder up a couple of leads coming up out of the circuit board and lightly tack the SMD's to them. 

BTW ,  just noticed in the AA5 circuit the neat tapped xfor secondary of the 50L6's output. What's up with that, not just voltage division but probably some NFB seeing how the .01 from plate is attached.  kuhl.

'smoke em if you got em.'

[KD1SH]

While I've gotten many a "tingle" off the chassis of an old rig, I've never yet come across one that was firmly tied to line. The voltage is illusory - disappearing with any real load. But, it's always wise to check whether that voltage on the chassis is "stiff" or you might be, too.

[Scott SWL]

If your cap to the chassis is bad, you could be in for more than a tingle, if plugged in the wrong way

[W3RSW]

Correct point and that's why all the rest of the cabinetry, knobs, etc. insulate an uninformed user from the chassis. -Just in case-
  That cap is for making the chassis look like ground to the common wired system at RF and audio (at the higher freq's a typical AA5 is capable of passing).  Chassis 'grounding' prevents RF etc. strange feedback effects.

Usually the caps were rated for 400 WV, so decent replacements could be 600 V rated.

If one fails closed with hot common you'll have the hot chassis and you'll get a blown fuse in the old fuse box or a tripped breaker these days if the chassis is outside grounded.   If it fails open you still have a 250k ohm bleed for isolation purposes (current depending on your skin/internal resistance, conductivity of what your connected to, etc.) or possibly squirrely feedback or strange operation.  If the resistor fails, similar interest.

Some other interesting stuff about the circuit shown-  The two section main tuning and osc. cap is chassis isolated with insulating grommets since it's tied to the AVC system of a few volts negative and RF grounded to common with a .05ufd cap. (also the AVC time constant cap.)  The IF cans are directly tied to the chassis of course and even shown with the correct gnd. symbol.  All the casual touchable stuff topside are isolated from the internal common wiring.  Even the tube shields are connected to the chassis not the internal common and are shown with the correct gnd. symbol.  

It's the best circuit I could find that illustrated the thought that designers used in eliminating a power transformer along with concomitant lower voltage operating tubes, particulary the AF power amplifier, etc. in those days.