Major Magnavox and Motorola score today

[H2s04]

One worry: so you're saying my chassis isnt isolated?

[Charlie]

That's correct. Your set does not have a power transformer, and your filaments in the tubes are wired in series. Take a look at where your A/C line comes into the set. One side of the line goes to the switch, and the other side of the line goes to a chassis ground. This is why you want to use an isolation transformer.

Also, something else I was noticing looking at the Sams... the speaker in your set uses an electro-magnet, not a permanent magnet. Because of this, you'll need to make sure that all speaker connections are made while powering up this set. If the speaker isn't plugged in, you won't get any B+ voltages.

[H2s04]

hmmmm........120V running through the chassis............MOTOROLA TABLE SET FOR SALE

[peverett]

As mentioned previously, I have a Hoffman made the same way. I also have a Motorola built similarly.

There were some AM radios built in the 1940s that if a knob was missing, 120 volts could be yours if you turned the affected control. Very dangerous. These same radios usually had metal screws attached to the potentially hot chassis sticking out of the bottom of the case. If course the line cord was not polarized.

While quite well insulated when all put together, most 1950s-1960s series string TVs can be a shock hazard when opened up. This is why I always use an isolated variac as a power supply when working on TVs and radios.

[H2s04]

What do you mean by isolated, grounded? Like clipping a ground to the chassis and the other end to the ground port on a variac? my first ever variac is arriving in the mail soon, so i dont really know anything about them; i thought they were all grounded

A variac allows you to slowly apply power, in steps, and also allows you to monitor the ammount of current the device draws as you slowly increase the voltage.I always use a variac when I power up a tv for the first time,this prevents damaging the set in the case that something is shorted.By watching how much current the chassis is pulling you can tell if there are problems.You should hear something from the speaker at around 90 volts.
BTW- Many older televisions did NOT have an isolated chassis.

[peverett]

The isolated variacs that I use are from Heathkit. I built one myself and bought the other. They consist of a 1 to 1 isolation transformer followed by a variac autotransformer. The 1 to 1 isolation takes 120 volts in and outputs 120 volts. The output of the transformer is isolated from the main power lines, greatly reducing the risk of you being shocked by the voltage applied to what you are working on. It also protects your test equipment.

The variac autotransformer allows you to apply power to the TV/radio that you are working on slowly, checking for shorts before any damage is done.

If you buy a variac, be careful, as many of them are not isolated and will not protect you from shock. It that is the case, you will need to purchase a seperate isolation transformer.

[H2s04]

I completely know about the whole variac thing. it being an autoformer and such, but I still dont understand how just adding a 1 to 1 transformer before the variac will significantly reduce my chances of getting shocked? Wont the chassis still be running at 120V?

[peverett]

Without the 1:1 isolation transformer, if you touch the chassis (which could be at 120 volts on your TV) and also touch any other grounded point, you will have 120 volts AC across a portion of your body. Other grounded points include the case of your oscilliscope, the ground of your multimeter (if not battery operated), the case, if metal, of the lighting you may be using, etc. Anything with a ground/neutral connection to the power line.

Even if you do get a shock yourself, connecting a scope ground to the chassis which is at 120 volts will cause a direct short across the 120 volt line, possibly damaging the scope and your shop wiring.

With the 1:1 isolation transformer, a hazard would occur only if you touched both sides of the isolated supply. As one side of this is used to generate your TVs DC power supply , it is usually contained in the TV and an accident is much less likely.

With the 1:1 isolation transformer, touching terminals and surfaces of your test equipment, lighting, etc will not cause an electric shock or damage test equipment.

When working on un-isolated equipment, it is very important to have this transformer. In fact, I believe that it is much more important than the variac.

[Chad Hauris]

If I had this set I would just make an isolation transformer and install it in the bottom of the cabinet. I think it would be unsafe to use without it. I did this with a Magnavox hot-chassis phono console...found a large Heathkit power supply transformer whose secondaries added up to 110 v. when put in series. Also I have taken old computer monitor transformers from the 70's and connected the secondaries together to make an iso. trans. This motorola looks like it does not have the later safety features used on hot chassis sets such as plastic control shafts.

[andy]

...

[peverett]

Andy, you are correct when the item is assembled. However if you are restoring or repairing this type of set(and have it taken apart) it is very important to have the isolation transformer as the chassis (with its possible 120 volts) is fully exposed. In the case of the Hoffman and Motorola that I am working on, the entire metal chass can be hot-the line cord is not polarized.

Of course, when these sets are completely assembled with the correct knobs, etc, they are safe, unlike the 1940's radios that I have seen.