[theboyzz]

I just got done restoring a ten tube zenith chassis #1004 and after about an hour the transformer seems to be getting warmer than it should.I have played it about 10 hrs.and it performs great but concerned about the transformer.I replaced all caps. and most of the resistors out of tolerance.Should i be concerned or is this the nature of the beast.Thanks

[etech]

Zenith transformers have a tendency to run hotter than most. I believe it was due to the fact that Zenith used the least amount of iron to get the job done. What adds to the problem is the higher line voltages in use today. Many times line voltages today can be as high as 125 to 127 volts. These sets were designed to run at about 115 to 117 volts. What most of us do is install a buck/boost transformer or a power resistor to reduce the line voltage to about 110 to 115 volts. The transformer will run much cooler at these values. There is no sense in stressing a 70 year old transformer.

If you decide to use a power resistor in series with the transformer primary, usually a 15 to 20 ohm resistor of about 20/25 watt rating will be sufficient. It would be necessary to mount it away from other components, since it will run warm to hot.

A buck/boost transformer is a more elegant approach. Usually a 12 volt 1 or 2 amp filament transformer will suffice. It should be wired to buck the supply voltage to the primary of the power transformer. Most likely, a quick search of the radio forums will produce a typical wiring diagram.

Regards,

Ed

[theboyzz]

Thanks ed.I will do that. Theboyzz

[etech]

A buck/boost transformer is a more elegant approach. Usually a 12 volt 1 or 2 amp filament transformer will suffice. It should be wired to buck the supply voltage to the primary of the power transformer.

I ran across this drawing on the web. I thought it might help clarify the bucking transformer arrangement. If you are just doing a single radio, a one or two amp transformer will be sufficient.

[Image: http://i135.photobucket.com/albums/q136/...ormer1.jpg]

Regards,

Ed

[theboyzz]

After trying a 25 watt 20 ohm resistor i found that it still ran hot.After talking to a couple of oldtimers in my radio club they said i should actually use a 25w 150 olm resistor to bring down the voltage.Anybody agree.Thanks the boyzz

[MinnesotaHam]

As noted, Zenith transformers tend to run hotter than others. You may not necessarily make it run cold by reducing input voltage, only less warm. Subjective.
Having said that, you should use a voltmeter to see the affect of the 20 ohm resistor on input voltage.
The 150 ohm will reduce input voltage much too much. You can experiment with the 20 ohm value. But you need to measure voltage too and not just heat.

[theboyzz]

Thanks.
I will try that.
THE BOYZZ

[exray]

Those Zenith xfmrs do run hot and 20 ohms should have taken you down to a safer voltage range. Many folks have abnormally high house voltage (125+) but if you can get the voltage down to say 110-115 via the resistor then it shouldn't stand out as being hotter than any other radio. Measurements with a meter will tell the story. Shouldn't take as much as 150 ohms, but... It all depends on your starting point whether or not you need to drop that much.

[codefox1]

I agree with all, but suggest the 50 ohms would probably work out under most circumstances. I would further add that a 50 watt resistor would be better, given the sizew of this set, and it will get hot, so what I would do is locate it away from everything else, use an insulated heat sinking bolt, and play the set for a while. If the B+ is within range, and the performance is good, you're set.

The bucking transformer soloution is more elegant, that is, if you have a suitable filament transformer that you want to donate to the radio. But be careful where you mount it, lest you introduce interference. Been there.